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Small Motor, Gearmotor and Control Handbook
Copyright © 1993-2003 Bodine Electric Company. All rights reserved.
Unauthorized duplication, distribution, or modification of this publication, in part or in
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Electric Motor
Electric motors are designed to convert
electrical energy into mechanical energy to
perform some physical task or work. In
order to understand the types of motors
that are available as well as their performance characteristics, some understanding
of the basic physical principles governing
motor design and operation are required.
Basic electric motor design encompasses the laws of electricity and magnetism.
Motor feedback and control systems involve semiconductor devices, microprocessors and other elements of electronics.
And no discussion of motors would be
complete without a brief understanding of
the mechanical principles governing linear
and angular motion.
This Chapter of the Handbook provides an overview of these fundamentals so
that the reader will have a foundation on
which to build a better understanding of
motor design and performance specifications.

Electric Current (I)
Atomic theory describes matter as an
aggregate of atoms. Each atom consists of
a nucleus containing positively charged
protons and electrically neutral particles
called neutrons. Revolving in orbits around
the positive nucleus are negatively charged
In metallic conductors (such as copper),
one or more electrons from the outer orbits
become detached from each atom and
move randomly from one atom to another.
These are called free electrons. The positive nucleus and the rest of the electrons
remain relatively fixed in position. Insulators, on the other hand, contain virtually no
free electrons.
When an electric field is applied to a
conductor, free electrons will drift under
the influence of that electric field. Drifting
electrons will collide with stationary atoms


causing additional free electrons to drift in
the same direction. This movement of electric charge is called current.
The unit of measurement for current or
rate of charge flow is the ampere. We
speak of a direct current (DC) if the charges always flow in the same direction, even
though the amount of charge flow per unit
time may vary. If the flow of charge reverses its direction periodically, then we have
what is called alternating current (AC). A
more detailed description of direct and
alternating current is presented in Section
1.3 of this Chapter.

Conventional Current Flow:
Before the acceptance of the electron theory, it was assumed that the direction of
current flow was from a positively charged
body to a negatively charged body. This
positive to negative flow of current is called
conventional current flow. However, in a
metallic conductor, it is electrons that carry
the charge from negative to positive. The
flow of current from negative to positive is
called electron flow. We will adopt conventional current flow throughout this
Handbook. In the diagrams, the direction
of current will always be from positive to

Potential Difference (V)
Electrons will move between two points
of a conductor if there is a potential difference (or a difference of “electric pressure”)
between the two points. Voltage is the
measure of the amount of pressure needed
to push electrons through a conductor. It is
analogous to a water pump that maintains a
pressure difference between its inlet and
outlet and results in water flow. Potential
difference and voltage are often used interchangeably.
The unit of potential difference or voltage is the volt. A potential difference of
one volt will be dropped across two points
if a constant current of one ampere flowing

between the two points results in a power
dissipation of one watt.

Resistance (R)
Resistance is defined as the opposition
to current flow. Although electrons may
flow in any substance, different materials
offer different resistance to their flow.
Those which make the transfer of electrons relatively easy are called conductors
(copper, aluminum, steel, etc.), and those
which tend to impose substantial resistance
are called insulators (wood, paper, mica,
glass, etc.). Materials with a level of conductivity between these two extremes are
called semiconductors (germanium, silicon). These “inbetween” materials have
become increasingly useful in the application of electrical energy.
The unit of electrical resistance is the
ohm (Ω). One ohm is defined as the resistance of a conductor which will allow a
current flow of one ampere when a potential difference of one volt is applied. The
resistance of a material is normally dependent on temperature. In general, the resistance of metallic conductors increases with

Ohm’s Law: Ohm’s law explains
the relationship between voltage, current
and resistance. It states that the amount of
current through a conductor is directly proportional to voltage applied and inversely
proportional to the resistance of the conductor or circuit:
I = --R
A conductor obeys Ohm’s law when,
for a given temperature, the current it conducts varies linearly with the applied voltage (Fig. 1-1).

Power: Electricity is used to perform
some type of work or to generate heat.
Power is the rate at which work is done or
the rate at which heat is generated. The unit


Fig. 1-2: Simplified series circuit.

Fig. 1-1: Current varies linearly with
applied voltage in accordance with
Ohm’s law.

for power is the watt. The amount of
power dissipated is directly proportional to
the amount of current flow and voltage
P = VI

curremt flow. There are three rules which
govern series circuits.
1) The total circuit resistance is the sum of
the individual resistances in the circuit:
RT = R1 + R2 + ... + RN

Power Loss: Power can also be

2) Current has the same value at any point
within a series circuit.

expressed as a function of resistance and
current. From Ohm’s law we learned that
V = IR. So if you substitute IR for V in the
power formula you have:

3) The sum of the individual voltages
across resistors in a series circuit equals
the applied voltage:
V = V 1 + V2

P = (IR)I

P = I2R

The windings in an electric motor consist of many turns of copper wires. Although copper is an excellent conductor,
the substantial total length of wire required
in the windings results in measurable power
loss because the resistance of a wire increases with its length. This I2R loss in the
motor is sometimes referred to as the copper loss.

Parallel Circuits: A simple parallel circuit is one that allows two or more
paths for current flow. The resistors in Fig.
1-3 are said to be connected in parallel.
There are also three rules which govern
parallel circuits.
1) The voltage drop across each branch
of a parallel circuit is the same as the
applied voltage:
V = V1 = V2

Horsepower: Electric motors are
rated in horsepower. One horsepower
equals approximately 746 watts. Horsepower and watts are simply two different
ways to express power.
Series Circuits: Figure 1-2
shows a simple series circuit with a voltage
source and resistors R1 and R2. A series
circuit is one that allows only one path for

Fig. 1-3: Simplified parallel circuit.


2) The total current in a parallel circuit is
equal to the sum of the branch currents:
I = I1 + I 2
where I1 and I2 are currents flowingpart of through R1 and R2 respectively.
3) The total resistance in a parallel circuit
is always less than or approximately
equal to the value of the smallest
resistance in any branch of the circuit.

C = -V

Since I = I1 + I2 you can substitute
-- in place of I and arrive at:
-- = --1 + --2
RT R1 R2

Q, measured in coulombs, is the charge
stored in the capacitor. One coulomb has
an equivalent charge of about 6.24 x 1018
The unit of capacitance (C) is the farad
(F). One farad is the capacitance of a capacitor in which a charge of one coulomb
produces a change of one volt in potential
difference between its plates.
One farad is an extremely large unit of
capacitance. Based on the large physical
size needed to produce such a component,
smaller units of more convenient size such
as the microfarad (µF = 10-6F), and
picofarad (pF = 10-12F) are used in most
A simple capacitor can be made by
placing two identical metal plates in parallel
with an air gap between them. See Fig. 14. It is known that the capacitance of a
parallel plate capacitor increases proportionally with the area (A) of the plate and
decreases proportionally with the distance
(d) between them. We may thus write, C =
kA / d, where k is a constant.

Since V = V1 = V2, you can substitute
V( -- + -- ) in the second part of
R 1 R2
the above equation leaving you with:
-- = V ( -- + -- )
R1 R2

of conductors and insulators (dielectric) so
arranged that a large amount of electric
charge can be stored in a relatively small
The capacitance (C) is the measure of a
capacitor’s ability to store a charge on its
plates at a given voltage (V):

-- = -- + -R T R1 R2

Therefore, the reciprocal of the total
resistance is the sum of the reciprocal of
the individual resistances. Solving for R
results in:
R = -------1 1
-- + -R1 R2
In general, for N resistors in parallel,
the equivalent resistance (R) is computed
as follows:
1 1
-- = -- + -- + -- + -R R1 R2 R3 RN

A capacitor is a device that stores electric charge. Almost any insulated body can
hold a limited electric charge, and the
greater the surface area, the greater the
charge that can be stored. In practical use,
however, a capacitor is a compact system


Fig. 1-4: Parallel plate capacitor.

Fig. 1-6: Basic RC circuit.

Fig. 1-5: Increased capacitance with

It is also known that if a dielectric such
as glass is placed between the plates (Fig.
1-5), the capacitance is increased five to
ten times. In varying degrees, putting materials like mylar, mica, wax or mineral oil
between the plates will all result in higher
capacitance. Different insulating materials
(dielectrics) offer different increases in capacitance. The ratio of the capacitance
with the dielectric to that without the dielectric is called the dielectric constant (k)
of the material. A vacuum has a dielectric
constant: k=1.
Dielectrics used in commercial capacitors include air, oil, paper, wax, shellac,
mica, glass, bakelite, polyester and
olypropylene film. Most capacitors are
fabricated with strips of metal foil, as
plates, separated by dielectric strips of the
materials mentioned above. The foil and
dielectric strips are sandwiched, rolled and
encased into a compact form which is then
fitted with terminals.

RC Circuit: The circuit shown in
Fig. 1-5 consists only of a battery and a
capacitor. Theoretically, with no resistance
in the circuit, the capacitor would charge
instantly. In reality however, when an electric potential is applied across an uncharged capacitor, the capacitor will not be
charged instantaneously, but at a rate that is
determined by both the capacitance and
the resistance of the circuit. (The effect of
inductance is neglected here. It will be discussed in Section 1.2 of (this Chapter).


Similarly, when a capacitor discharges it
will not take place instantly. Rather, the
discharge current will diminish exponentially over time.
Figure 1-6 illustrates a basic RC circuit.
The capacitor will be charged if the switch
is closed at the “a” position. If the switch is
then closed at the “b” position, the capacitor will discharge.
With the resistor present in the circuit,
current will not flow as freely. More time
will be required to charge the capacitor.
Likewise, it will take longer for the
capacitor to discharge with the resistor in
the circuit.
With a resistor in the circuit, the voltage
across the capacitor rises more slowly. The
current flow acts directly opposite. When
the switch is first thrown to the “a” position
there is more current flow. As the voltage
across the capacitor reaches the battery
potential, current flow decreases. When
the capacitor voltage equals the battery
voltage level, current flow stops.
Q is the amount of charge on the capacitor and is zero at time t = 0 (Fig. 1-7). Q
will increase as the current flows until it
reaches a maximum value (Q = CV), at
which point the current is zero.
In DC circuits, capacitors oppose
changes in voltage. The time delay for the
capacitor’s voltage to reach the supply
voltage is very useful because it can be
controlled. It depends on two factors:
1) the resistance in the circuit, and
2) the size of the capacitor.
In Section 1.3, we shall see how a RC
circuit functions when AC voltage is

Fig. 1-7: Curves for Q and I during charging.

Fig. 1-8: Curves for Q and I during discharging.

Time Constant: The time it takes
a capacitor to charge to 63% of the supply
voltage is called the capacitive time constant (T). It can be calculated using the
T = RC
A capacitor discharges in a similar manner as shown in Fig. 1-8. The current is
now negative, because it flows in the opposite direction during discharging.
A capacitor is said to be fully charged
or fully discharged after five RC time constants. The figures illustrate that current
varies exponentially with time during the
charging and discharging of an RC circuit
when a DC source is applied.

Electric motors derive their characteristic ability to convert electrical energy to
mechanical energy from magnetostatic
force. Magnetostatic forces result from
electric charges in motion. These charges

may flow freely through space, in a conductor, or exist as spinning electrons of the
atoms that make up magnetic materials.
As early as 640 B.C. certain natural
magnets were known to exist. Nearly 2000
years later, two simple laws governing their
behavior were discovered:
1) Like poles repel each other, while unlike
poles attract.
2) The force of attraction or repulsion is
proportional to the inverse square of the
distance between the poles.

Magnetic Field
An important property of magnets is
that they can exert forces on one another
without being in actual contact. This is explained by the existence of a magnetic field
around a magnetized body. The magnetic
field of the bar magnet (Fig. 1-9) is represented by the lines radiating out from the
north pole and entering the south pole. Any
other magnet placed in this magnetic field
will experience a force. Forces will also be


Fig. 1-9: Flux field pattern of a simple
bar magnet.

Fig 10: Arrangement of: a) electrons
in diamagnetic materials (left), and
b) electrons in magnetic materials (right).

exerted on electrons moving through a
magnetic field.

Flux Density: The magnetic field
lines in Fig. 1-9 are collectively referred to
as the magnetic flux. Magnetic flux density
is the amount of magnetic flux passing
through a unit area plane at a right angle to
the magnetic field. It is a measure of how
concentrated the magnetic field is in a given
area. Magnetic flux density (B) is a vector
quantity. That is, it has magnitude as well
as direction.

Magnetism at the
Atomic Level
While ferrous materials, like iron, are
strongly magnetic, many materials show at
least some magnetic properties. Paramagnetic materials, mostly metals, exhibit very
weak attraction to a magnet. The rest of
the metals and nonmetals are diamagnetic
—very weakly repelled by a magnet. Only
the ferrous materials, some specialized
alloys, and ceramics have sufficiently strong
magnetic properties to be of commercial
No more than two electrons can share
the same electron level or shell of an isolated atom. Diamagnetic materials have two
electrons in each shell, spinning in opposite
directions. See Fig. 1-10a. Since the magnetic response of a material is dependent
upon the net magnetic moment of the atoms, this balanced symmetrical motion
produces a magnetic “moment” of near

zero. Quite simply, the fields produced by
the counterspinning electrons cancel each
For the paramagnetic elements in which
the electron shells are naturally asymmetric
(Fig. 1-10b), each atom has a weak but
significant magnetic field. However, few of
the paramagnetic elements are magnetically
very strong. These are called the ferromagnetic elements.
Ferromagnetism is the result of the
asymmetrical arrangement of electrons in
atoms in combination with a coupling or
aligning of one atom’s magnetic field with
that of an adjacent atom. This results in a
strong magnetic response. This “exchange
coupling” occurs only in materials in which
the spacing between atoms falls within a
certain range.
In iron, cobalt, nickel and gadolinium,
the net magnetic moment is strong enough,
and the atoms close enough, for spontaneous magnetic alignment of adjacent atoms
to occur. Solid ferromagnetic materials
conduct magnetic flux in the alignment

Electric Current and
Magnetic Fields
In 1820, Oersted discovered that an
electric current assing through a conductor
would establish a magnetic field. This discovery of the relationship between electricity and magnetism led to the development


Fig. 1-11: Direction of flux flow with
a) current flowing out of page (left), and
b) flux flow with current flowing into page

of most of our modern electric machines.
The magnetic field around a currentcarrying straight conductor takes the form
of concentric cylinders perpendicular to the
conductor. In Fig. 1-11, the current is
shown emerging from the page and the flux
lines, shown as concentric circles, are
flowing counterclockwise. When the direction of the current is reversed, the flux lines
flow clockwise.
The right-hand rule, shown in Fig. 1-12,
can be used to determine either the direction of the magnetic field or the direction of
current when the other one is known.
When the current-carrying conductor is
formed into a loop as shown in Fig. 1-13,
the faces of the loop will show magnetic
polarities. That is, all of the magnetic field
lines enter the loop at one face and leave at
the other, thus acting as a disc magnet. The
polarities will be more pronounced and the
magnetic field will be much stronger if we
wind a number of loops into a solenoid
(Fig. 1-14).

Fig. 1-13: Direction of magnetic flux
when an energized conductor is formed
into a loop.

Fig. 1-14: Flux characteristics in simple

The magnetic field developed by the
solenoid resembles that of a bar magnet.
The flux lines form continuous loops, leaving the solenoid at one end and returning at
the other, thus establishing north and south
The magnetic flux (Φ) of a given solenoid is directly proportional to the current
(I) it carries. The same holds true for a
straight conductor or a single loop of wire.
For solenoids with different numbers of
turns and currents, the magnetic flux is proportional to the product of the number of
turns and the amount of current.

Properties of Magnetic
Materials: When a ferromagnetic mate-

Fig. 1-12: Right-hand rule: thumb points
in direction of current, palm curls in direction of magnetic field.

rial, like an iron bar, is placed in a magnetic
field, it presents a low resistance path to
the flow of flux. This results in a “crowding
effect,” as flux seeks to flow through it and
flux density increases in the gaps at the
ends of the bar. See Fig. 1-15. Iron, cobalt, nickel, some rare earth metals and a

Fig. 1-15: Effect of an iron bar on a
magnetic field.

variety of other ferromagnetic alloys and
compounds are excellent magnetic conductors with high permeability.

Permeability and Magnetic
Field Strength: Permeability (µ) is a
measure of how well a material will conduct magnetic flux. It is related to magnetic
flux density (B) and magnetic field strength
(H) in the following equations:

B = µH
µ = µr µo

where µo = 4π x 10-7 (in SI units) and µr is
the relative permeability with a value of
unity (1) in free space.
The magnetic field strength (H) is
measured in amperes per meter. The
following formula shows that for a solenoid
(conductor loop) with length (1) and a
number of turns (N), the magnetic field
strength within the solenoid is proportional
to the current (I):
H = --l
For a given solenoid and current, H
remains the same regardless of any material
placed inside the solenoid. However, the
magnetic flux density (B) will be directly
proportional to the permeability (µ) of the

Magnetization, Demagnetization and Hysteresis: If a piece
of iron is used as the core of a solenoid
and the current is increased slowly (increasing the magnetic field strength, H), the

Fig. 1-16: Magnetization curve and
hysteresis loop.

iron will be magnetized and follow the
magnetization curve (abcd) as shown in
Fig. 1-16.
The magnetization curve shows how the
flux density (B) varies with the field
strength (H). And since B = µH, it also
shows how the permeability (µ) varies with
the field strength. When H is gradually increased, the flux density (B) increases
slowly at first (section ab of the curve).
Then, as H is further increased, the curve
rises steeply (bc of the curve). Finally,
magnetic saturation is approached (near d)
where the curve flattens out.
If the current is then gradually decreased, flux density (B) will decrease but
the demagnetization curve will not retrace
the path (dcba). Instead, it will follow a
path de, where at point e, even though the
current has been reduced to zero, there is
some residual magnetism. If we then gradually increase the current in the reverse
direction, creating -H, the iron will be completely demagnetized at point f. By further
increasing the current and then slowly decreasing it, we will go through points g, h, i
and d. The complete loop (defghi) is called
a hysteresis loop and represents a virtual
“fingerprint” for the material being used.
See Fig. 1-16.
As iron is magnetized and demagnetized, work must be done to align and realign its atoms, and this work takes the


Fig. 1-17: a) Flux pattern around an energized conductor (left), b) flux between two
magnetic poles (center), and c) effect of placing an energized conductor in a uniform
magnetic field (right).

form of heat. In alternating current machines (i.e., motors and generators), the
magnetizing and demagnetizing process
takes place many times a second and hysteresis loss (heat) may be considerable,
resulting in lower operating efficiency. The
hysteresis loss for one cycle of alternating
current is equal to the area enclosed by the
hysteresis loop.

the conductor(I):

Motor Action: If we place a current-carrying conductor (Fig. 1-17a) between opposite magnetic poles (Fig. 117b), the flux lines below the conductor
will move from left to right, while those
above the conductor will travel in the opposite direction (Fig. 1-17c). The result is
a strong magnetic field below the conductor and a weak field above, and the conductor will be pushed in an upward direction. This is the basic principle of electric
motors and is sometimes called “motor
The force (F) on the conductor is a
product of the magnetic flux density (B),
the conductor’s current(I) and the length of

Induced EMF

Fig. 1-18: Right-hand rule for force on a
conductor in a magnetic field.

F = BlI
where we have assumed that the conductor
is at a right angle to the magnetic flux
density (B).
An easy way to remember the direction
of motion is to apply the right-hand rule,
shown in Fig. 1-18.

In general, if a conductor cuts across
the flux lines of a magnetic field or vice
versa, an emf is induced in the conductor.
If the direction of the flux lines and the conductor are parallel, there is no induced emf.

Generator Action: If the conductor in Fig. 1-19 is moved vertically up
or down in the magnetic field, an electromotive force is generated in the conductor.
If the conductor is connected to a closed
circuit, current will flow. This is the basic
principle of electric generators and is also
called “generator action.”
The induced emf is a product of the
velocity of the motion (v), the magnetic
flux density (B), and the length of the
conductor (l):
emf = Blv
The relationship is valid only if the motion of the conductor is perpendicular to
the flux lines.
The direction of induced emf depends
on the direction of motion of the conductor
and the direction of the magnetic field. This


Fig. 1-20: Magnetically coupled coils
wound around a steel bar.

Fig. 1-19: Direction of induced emf in a
conductor-cutting flux.

relationship can be shown by Fleming’s
left-hand rule for electromagnetism in Appendix 3.

Faraday’s Law: We have seen
that any conductor cutting across a magnetic field will produce an emf. However,
this is only a special case of the more general law of induction established by Faraday in 1831: “If the total flux linking a circuit changes with time, there will be an
induced emf in the circuit.”
If we were to wind two coils around a
steel bar, as in Fig. 1-20, connecting one
to a battery with a simple on/off switch and
the other to a sensitive galvanometer, the
effect of closing the switch would produce
a change in current and a change in the
field thereby inducing a current in Coil 2.
Similarly, if we were to open the circuit, a
current would again register in Coil 2.
The induced emf in Coil 2 is mathematically related to the change of flux as
emp = -N2 ----dt
Where N2 is the number of turns in Coil 2
and dφ/dt is the rate of change of flux, the
minus sign indicates that the induced current in Coil 2 will flow in such a way as to
oppose the change of flux due to the
change of current in Coil 1.
Since both coils are wound in the same
direction, the induced current will flow in
the direction shown in Fig. 1-20 when the
switch is closed. This induced current in
Coil2 sets up a magnetic field opposes

the sudden increase of flux created by
current flowing in Coil 1. If the switch is
then opened, the current in Coil 2 will flow
in the opposite direction creating a flux that
opposes the sudden decline of flux from
Coil 1.

Inductance (L)
The change of magnetic flux due to
switching in Fig. 1-20 would also produce
a counter emf (cemf) in Coil 1 itself. The
cemf opposes the build-up or decline of
current in the same circuit. The ability of a
coil to store energy and oppose the buildup of current is called inductance.
For a given coil, the change of magnetic
flux is proportional to the change of current. Thus, the cemf may be expressed as
cemf = L --dt
where L is called the inductance of the coil.
A coil or circuit is said to have an inductance of one Henry when a current changing at the rate of one ampere/second induces one volt in it.

RL Circuit: In Section 1.1 we
learned that there is a delay in the rise or
fall of the current in an RC circuit. The RL
circuit, shown in Fig. 1-21, has a similar
When the switch (S) is closed at a, the
current in the resistor starts to rise. However, the cemf presented by the inductor
(L) opposes the rise of the current, thus the
resistor responds to the difference between


Fig. 1-21: Basic RL circuit.

the battery voltage(V) and the cemf of hte
indicator. As a result, the current rises exponentially as shown in Fig. 1-22.
If we allow enough time for the current
to reach V/R and then close the switch at
b, current will continue to flow but diminish
as the stored magnetic field energy is dissipated through the resistor. The current decay curve is similar to the capacitor charging curve in Fig. 1-7.

RL Time Constant: The time
constant is the time at which the current in
the circuit will rise to 63% of its final value
(V/R) or decay to 37% of its initial value. It
is represented by the formula:
τ = -R
The time constant can be controlled by
varying the resistance or inductance of the
circuit. Decreasing the circuit resistance
increases the time constant. Increasing the
inductance will also increase the time constant. Thus, the larger the time constant, the
longer it takes the current to reach its final
value. The current in an RL circuit will rise
or fall to its final value after five time constants (within 99.3%).

Fig. 1-22 Current rise in RL circuit.

Electric motors are generally divided
into DC and AC (induction) types. Each
has its own operating characteristics and
advantages. In this section, a brief review
of direct current vs. alternating current will
be presented followed by discussions of
various AC circuits.

Direct Current: Direct current can
be obtained through the chemical reactions
in primary cells or secondary cells. Primary
cells are batteries that consume their active
materials when releasing electric energy
and hence, are not reusable. Secondary
cells (or storage cells), on the other hand,
can be recharged by applying electricity in
the reverse direction, thus reversing the
chemical reaction.
Direct current is commonly produced
by DC generators in which mechanical
energy supplied by steam turbines, water
wheels, water turbines or internal combustion engines is converted into electric energy. A brief description of a simple DC generator will be presented later.
In addition to the above, direct current
can be generated from thermal energy (i.e.,
thermocouple) and light energy (solar
cells). Furthermore, alternating current can
be converted into direct current through the
use of rectifiers.
Alternating Current: The most
commonly supplied form of electric energy
is alternating current. The main reason for
the widespread use of AC is the fact that
the voltage can be readily stepped up or
down through the use of transformers.
Voltage is stepped up for long distance
transmissions and stepped down for subdistribution. The voltage is stepped down
even further for industrial and home use.


For a given power (VI), stepping up the
voltage decreases the current and consequently reduces the (I2R) power loss in the
power lines.
There are many additional advantages
to AC. For example, AC is used to run
induction motors (which do not require a
direct supply of current to the rotating
member and consequently avoid the
problems associated with brush and
commutator wear in DC motors).
However, there are cases (battery
charging, electroplating, etc.) where DC
must be used. Motor applications in which
adjustable speed control is important are
generally operated from a DC source.
However, in most of these cases, the
energy is originally generated as AC and
then rectified and converted to DC.
Alternating current can be supplied by
generators (which will be discussed next)
and by devices called inverters which convert DC into AC.

AC and DC Generators
Figure 1-23 shows a simple AC generator. In simple terms, a magnetic field or
flux is established between the poles of a
magnet. When a coil of conductive material
is introduced into the air gap perpendicular
to the flux and rotated mechanically at a
uniform speed, it will cut the flux and induce an emf that causes a current to flow in
the closed circuit formed by the slip

rings (X and Y), the brushes and the load
resistor (R). With a full 360 degree revolution of the coil, the current flows first in one
direction and then in the other, producing
an alternating current.
If the coil in Fig. 1-23 were rotated
counterclockwise at a constant speed, the
top of the coil (cd) would cut the flux in a
downward direction, while the bottom (ab)
would cut the flux in an upward direction.
By the right-hand rule of induction, the
resulting current produced in the coil by
reaction with the flux would flow from a to
b and from c to d during the first 180
degrees of rotation.
As the coil continued around to its original position, ab would cut the flux downward and cd upward, causing an opposite
current flow from d to c and b to a. One
360 degree rotation of the coil is equivalent
to one cycle. Since standard available current is 60 Hz (cycles per second), the coil
would be rotated sixty full rotations per
second to deliver standard 60 Hz AC. This
back and forth flow of current can be represented graphically as a sine wave in
Fig. 1-24.

Fig. 1-24: Sine wave characteristic of AC
current during one cycle (360°).

Fig. 1-23: Simple alternating current AC

Without going into the mathematical
details, the wave shape of the induced emf
can be explained by the fact that the rate of
change of flux (Ω) through the surface a-bc-d formed by the wire loop is a sinusoidal


function of time. Since by Faraday’s law
(see Section 1.2) the induced emf is proportional to the rate of change of flux, a
sinusoidal induced emf results.
For DC generators, the same principle
of flux cutting holds true, except that instead of the slip rings, a synchronous mechanical switching device called a commutator is used. See Fig. 1-25.

segments. Each coil is connected to its own
pair of commutator segments. The brushes
make contact with each coil for a short
period of time when the emf in that coil is
near its maximum value. Figure 1-27 illustrates the emf output of a DC generator
with four evenly spaced coils connected to
an eight-segment commutator. The dotted
curves are the induced emfs (eight emfs for
every revolution). The solid line is the output voltage of the generator.

Fig. 1-27: Output of DC generator with
four coils and an eight-segment commutator.
Fig. 1-25: Simple DC generator.

The arrangement of commutator and
brushes allows the connections to the external circuit (in our case, the resistor, R) to
be interchanged at the instant when the emf
in the coil reverses, thus maintaining a unidirectional (although pulsating) current (see
Fig. 1-26).

Two-Phase and ThreePhase AC: In addition to single-phase
AC produced by the generator described
above, alternating current may be supplied
as both two and three-phase. Using the
example of the simple single coil AC generator described before, if we were to add
a second coil with its loop arranged perpendicular to the original (see Fig. 1-28)
and rotate them mechanically with a uni-

Fig. 1-26: Induced emf from the simple
DC generator.

The pulsating emf from the simple DC
generator is not very useful when relatively
uniform DC voltage is required. In practice, a DC generator has a large number of
coils and a commutator with many

Fig. 1-28: Simple two-phase AC


form speed, two-phase voltage would be

Fig. 1-29: Wave shapes produced by
two-phase AC.

The resulting two-phase voltage sequence is shown in Fig. 1-29, where one
phase lags the other phase by 90 degrees.

Fig. 1-32: Delta-connection of three coils
(right), Wye-connection of three coils

produce a rotating magnetic field in the
stator bore, the rotor will follow the field
and result in rotation. This principle will be
discussed further in Chapter 2.

The Delta (∆)-Connection
and Wye-Connection: Although it

Fig. 1-30: Simple three-phase AC

If we were to add one more coil and
space the three at 120 degrees to each
other (see Fig. 1-30), the same generator
would now produce three-phase current
(Fig. 1-31).

Fig. 1-31: Wave shapes produced by
three-phase AC.

Two and three-phase current are used
in both polyphase and induction motor
design. Since both will produce a rotating

is shown in Fig. 1-30 that each coil of the
three-phase AC generator is provided with
its own pair of slip rings and brushes, the
practical design of a three-phase generator
has only three slip rings and brushes. This
is accomplished by either the Delta-connection or Wye-connection of the three
coils (1, 2, and 3) in the generator.
Figure 1-32 shows a Delta-connection
with output terminals (a, b and c). The
three pairs of terminals (a-b, b-c, and c-a)
provide a three-phase output like the one
shown in Fig. 1-31. The line voltage (voltage from any pair of the terminals) is the
same as the coil voltage (voltage across
each coil). The line current, however, is √3
times the coil current.
The Wye-connection shown in Fig. 132 again has terminals a, b, and c. There is
also a common point called the neutral in
the middle (O). Again, the terminal pairs
(a-b, b-c, and c-a) provide a three-phase
supply. In this connection, the line voltage
is √3 times the coil voltage while the line
current is the same as the coil current. The
neutral point may be grounded. It can be
brought out to the power user via a four-


wire power system for a dual voltage
For example, in a 120/208-volt system,
a power user can obtain 208 volt, threephase output by using the three wires from
a, b, and c. Furthermore, single-phase,
120 volt power can be tapped from either
O-a, O-b or O-c.

AC Circuits
While many forms of “alternating current” are nonsinusoidal, the popular use of
the term alternating current, or AC, usually
implies sinusoidal voltage or current. Electro-magnetic devices such as motors consist of ferromagnetic materials with nonlinear voltage/current relationships. Thus,
current will not be pure sinusoidal.

Root-Mean-Square or Effective Values, and Power Factor
in AC Circuits: The voltage (V) and
current (I) in a sinusoidal alternating current
circuit consisting of linear devices are generally written as:
V = Vm SIN (2π ft)
I = Im SIN (2π ft -φ )

average value in one complete cycle is
This result provides no useful information about the magnitude. One useful way
of specifying the magnitude of the AC is to
compute its root-mean-square (rms) value
which is alternatively called the effective
The effective value of alternating current
is that which will produce the same amount
of heat or power in a resistance as the corresponding value of direct current. The
effective value of current (I) is obtained by
first computing the average of the square of
the current and then taking the square root
of the result. Without performing the computation, we will just state that the effective
value of current Ie is:
Ie = ----- = 0.707 Im


Similarly, the effective voltage (Ve) is:
Ve = -----m = 0.707 Vm
Then the average power (P) of the circuit
can be shown to be:
P = Ie Ve COS φ

Here, Vm and Im are the peak values of
V and I respectively, f is the frequency in
hertz (Hz) and φ is the phase angle (in radians) between the current and the applied
voltage. (See Fig. 1-33). Since the positive
portion of the voltage or current is the mirror image of the negative portion, the

The quantity (COS φ) is called the
power factor of the circuit. If the current (I)
and voltage (V) are in phase (i.e., f = 0)
then we have the maximum power (P = Ie
Ve). Stated another way, only the component of Ie in phase with Ve contributes to
the average power. The other component
may be said to be “wattless.”

Pure Resistance
AC Circuit

Fig. 1-33: Vm and Im are out of phase by
an angle φ.

A pure resistance circuit is one in which
there is no significant inductive or capacitive component. In such a circuit, the current and voltage would both be sinusoidal
and in phase ( φ = 0). See Fig. 1-34. Pure
resistance circuits can be treated as if they


Fig. 1-34: Pure resistance (R) circuit. Vm and Im are in phase, φ = 0.

were DC circuits if the effective values of
current and voltage (Ie and Ve) are used:
Ie = ---e

Inductive Reactance: The opposition to the current flow in an inductance circuit is called the inductive reactance (XL), which is given by the formula:
XL = 2πf L

Since the average power:
P Ie Ve COS φ
then for the phase angle f = 0°:
P =IeVe=Ie2R

P = Ve2

Pure Inductance
AC Circuit

where XL is in ohms, f is the frequency in
Hz and L is the inductance in Henrys.
The phase angle (φ) is +90°. Thus, a
pure inductance circuit will not only offer
opposition to current flow but will also
cause the current to lag behind the voltage
by 90° (Fig. 1-35).
The effective current (Ie) and average
power (P) are:
Ie = --XL

In an inductive circuit, the counter emf
(or self-inductance) of the inductor will
offer opposition to any change in the
current. Since an alternating current is one
that is continually changing, there will be a
continual opposition to the flow of current
corresponding in value to the rate of
change of current.

P = IeVe COS φ

Fig. 1-35: Pure inductance (L) circuit. I lags V, φ = 90°.


φ = 90°, COS φ

Fig. 1-36: Pure capacitance (C) circuit. I leads V. φ = -90°.

Therefore, there is no power loss in a
pure inductance circuit.

circuit is called the capactive reactance
(Xc). Its value is given by the formula:

Pure Capacitance
AC Circuit

Xc = -----2πfC

A capacitor placed in a circuit also presents opposition to current flow. This is
due to the limitation that charge will flow
into the capacitor and accumulate only to
the level proportional to the applied voltage. No further charge will flow in or out
until there is a corresponding change in
applied voltage.
Thus, the current in a capacitor
circuit is proportional to the slope of the
voltage curve. The slope is highest for a
sinusoid when V = 0 and the current flow
is at its maximum. The slope is zero when
V is at its peak (positive or negative) and
this corresponds to a zero current flow.

Capacitive Reactance: The
opposition to current flow in a capacitance

where Xc is in ohms, f is the frequency in
Hz and C is the capacitance in Farads.
The phase angle (f ) in this circuit is 90°. Thus, in a pure capacitance circuit,
the current leads the voltage by 90°
(Fig. 1-36).
The effective current (Ie) is:
Ie = ---e
Since φ = -90°, COS φ = 0. There is
also no power loss in a pure capacitance

RL AC Circuit
When R and L are connected in series
in an AC circuit, we have the series RL
circuit shown in Fig. 1-37. Both the resistance (R) are the inductive reactance (XL)

Fig. 1-37: Series RL circuit. I lags V. 0 < φ < 90°.


Since no power is lost in the inductance,
P = Ie2R

RC AC Circuit
Fig. 1-38: Impedance diagram of an RL

of the inductor offer opposition to current

Impedance in RL Circuit: The
combined effect of R and XL is called the
impedance (Z) which is expressed in ohms:

Impedance in RC Circuit:
The impedance (Z) in this case is:

Z = R2 + XL2
The impedance can be represented as
the hypotenuse of a right angle triangle
whose sides are R and XL (Fig. 1-38).
This is also referred to as the impedance
The phase angle (φ) in this circuit happens to be the angle between Z and R (or,
cos φ = R/Z). Since is between 0° and
90°, the current (I) in the circuit lags behind
the voltage by an angle between 0° to 90°
depending on the values of R and XL.
The effective current (Ie) and average
power (P) are:
Ie = --- e= -------Z
√R2 + XL2
P = Ie Ve COS φ

Similar to the RL circuit described previously, resistance (R) and capacitive reactance (Xc) will both oppose current flow in
an AC circuit. Unlike the RL circuit, increasing C or the frequency results in a
decrease in Xc and an increase in current.
See Fig. 1-39.

Z = R2 + XC2
The vectorial representation is shown in
Fig. 1-40, where XC is pointing downwards and represents a “negative” Vector.

Fig. 1-40: Impedance diagram of an RC

The phase angle (φ) is now between 90° and 0°, and:
COS φ = -Z
The current (I) in the circuit lags the
voltage by an angle (φ) between 0° and
90° depending on the values of R and XC.
Refer to Fig. 1-39.

COS φ = -Z

Fig. 1-39: RC circuit. I leads V. -90° < φ< 0.


Fig. 1-41: Basic RLC circuit (left) and vector diagram (right).

The effective current (Ie) is:
Ie = ---e = ————
√R + XC2

COS φ = −−−

Since no power is lost in the capaci-


P = Ie2R

RLC AC Circuit
To further generalize the AC series circuit, we should consider the RLC circuit
shown in Fig. 1-41. The impedance of this
circuit is:

Ζ = √ R2 + (XL - XC)2
The vector diagram of the above relationship is also shown in Fig. 1-41. The
phase angle ( φ ) in an RLC circuit is between -90° and +90° where:
COS φ = -Z
If XL > XC, then the current in the circuit will be lagging the voltage. If XL < XC,
then the current will be leading the voltage.
If XL = XC, the circuit is said to be resonant and will behave as purely resistive.
The effective current (Ie) is:
Ie = ---e = —————
Z √R2 + (XL - XC)2
P = Ie Ve COS = Ie2R
Basic DC circuits involving resistance
(R), inductance (L), and capacitance (C)

have been presented in Sections 1.1 and
1.2. In this Chapter, we have also seen
how these same elements (R, L and C)
work in AC circuits. Understanding these
basic circuits is important, since an induction motor driven by AC power is a system
of resistance, inductance and capacitance.

Until now, we have presented the electrical characteristics of motors to acquaint
you with the fundamentals of motor action
and the effects of direct and alternating
current on motor design and operation.
Electrical characteristics affect a designer’s
decisions on which motor to choose for
any given application.
Equally important in understanding motor operation are the mechanics and performance characteristics of electric motors.
Mechanics encompasses the rules which
govern the motion of objects, in particular:
a) the force which must be applied to start
an object moving or to stop it, and
b) the opposing forces which must be
overcome before movement can begin
or end.
Other factors such as speed, acceleration and amount of displacement all play a
part in determining which motor is best
suited to perform a task. This section is
intended to provide general information on
mechanics. Throughout this Handbook,
other, more specific formulas will be given
as they apply to a particular type of motor


or application. Other mechanical data and
mathematical formulas can be found in the
Appendix Section of the Handbook.

Translational Motion
The movement of a uniform object
in a straight line is referred to as translational motion. The three parameters of
translational motion are displacement, velocity and acceleration.

Displacement: The change in
position of an object is known as displacement. It is a vector quantity with both magnitude and direction and is shown mathematically as:

Acceleration: As an object begins
to move, its velocity changes with respect
to time. This is called acceleration. Like
velocity, acceleration is expressed in average and instantaneous quantities. Average
acceleration equals:
vf - vi
a = --- = -------∆t
t f - ti
where Dv is the difference between the
object’s final and initial velocities, and Dt is
the elapsed time.
The instantaneous acceleration is defined by the following formula:

∆v dv
a = lim ---- = ---∆t→0 ∆t

∆x = xf - xi
where Ox is the total displacement, xf is
the object’s final position and xi is the object’s initial position.

Velocity: The rate at which an
object’s position changes with time is its
velocity. There are two types of velocity:
average and instantaneous. Average velocity is the net displacement divided by the
elapsed time:
∆x xf - xi
ν =--- = -------∆t
t f - ti
where d is the net displacement and t is the
elapsed time to make the displacement, tf
is the final time and ti is the initial time.
At any instant in time the velocity
of an object may exceed the average velocity, so it is sometimes necessary to
know the instantaneous velocity:

∆x dx
v = lim ---- = ---∆t→0 ∆t

Speed: Frequently, the terms speed
and velocity are used interchangeably. Velocity can be positive or negative. Speed is
equal to the absolute value of the instantaneous velocity and is always expressed as
a positive number:
s = | v|

Rotational Motion
Motors can be used to move objects in
a straight line, which is why a brief overview of translational motion was given. But
motor design and application focuses
heavily on rotational motion around an axis.
The same principles of displacement, velocity and acceleration also govern rotational motion. In many motion control applications, it often becomes necessary to
transform linear motion into rotational motion or vice versa.

Angular Displacement: For
rotational motion, displacement is expressed in radians, degrees or revolutions
because the displacement occurs in reference to a rotational axis (one radian =
57.3°, one revolution = 360° = 2π radians.) Angular displacement is expressed as:
∆θ=θ 2-θl

where θl is the object’s initial angular
position relative to the axis and θ2 is the
final angular position.

Angular Velocity: Angular velocity is expressed in radians / second,
revolutions / second, or revolutions /
minute (RPM). It is the rate at which an
object’s angular displacement changes with


respect to time. Like translational velocity,
it can be expressed as an average or instantaneous quantity.
The formula for average angular
velocity is:

θ 2 − θ1
ω = --- = --------∆t
t2 - t1
where ∆θ is the net angular displacement
between the initial position and final position and ∆t is the elapsed time.
Instantaneous angular velocity is expressed as follows:

∆θ dθ v
ω = lim ---- = --- = --∆t→0 ∆t dt r
where v = circumferential linear velocity.

Angular Acceleration: When
an object’s angular velocity changes with
respect to time, it is undergoing angular
acceleration. Average angular acceleration
is expressed as:
ωf −ωi
a = --- = --------∆t
tf - ti

Mass: Mass is the property of an
object that determines its resistance to motion. It is a factor of the object’s weight
(W) and its acceleration due to gravity (g).
Mass is the quantitative measure of inertia.
It is the mass of an object that requires a
force to move it. It is usually expressed in
kilograms or pounds (mass)*.
In a linear system:
M = --g
Momentum: The fundamental
measure of an object’s motion is momentum. In a linear system, it is the product of
the object’s mass and linear velocity and is
expressed in newton-seconds or poundseconds:
Force: The push or pull on an object
that causes it to move or accelerate is
called force. It is directly proportional to
the object’s mass and acceleration:
where M is the object’s mass and a is the

An object’s instantaneous angular acceleration an be calculated as:

Rotational Inertia: In linear motion, the inertia of an object is represented
by the object’s mass:

∆ω dω
a = lim --- = ---∆t→0 ∆t

F = Ma, M = -a

Statics and Dynamics
The previous discussion focused on the
motion of an object either in a straight line
or about an axis. But other factors must be
considered when discussing motion. The
size and weight of an object determine the
amount of force needed to move it or stop
it. Other factors such as friction also play a
role in determining the amount of force
needed to move an object. We will now
center our attention on these other factors.

It is the mass which tells us how large a
force will be required to produce constant
acceleration. The rotational analog of this
formula is:
T = Ia, I = -a
This formula tells us how much torque
(T) is required to produce angular acceleration (a). The moment of inertia (I) can be
defined as the mass of the object times the

* The pound-mass is a body of mass (0.454 kg). The pound-force is the force that gives
a standard pound-mass an acceleration equal to the standard acceleration of gravity
(32.174 ft/sec.).


square of the distance (r) from the rotational axis (see Fig. 1-42):

the driven object or machine. Motor load
in hp can be expressed:
motor load (hp) = ——

I = m1r2 + m2r2 + m3r2 ... + mnr2

where r (in feet) is the radius at which the
force (F, in pounds) is applied and N is
revolutions per minute.
motor load (hp) = ——
Where torque (T) is expressed in lb-ft., or
if T is expressed in oz-in., then:
TN (9.916 x 10-7 )
motor load (hp) = —————————

Fig. 1-42: The moment of inertia of a
hoop containing many small masses on
its circumference.

The moment of inertia can be calculated
for any object this way but calculus is usually needed for the summation. Figure 1-43
shows the values of I for several familiar
shapes used in mechanical systems.
Figure 1-43 shows that the moment of
inertia is always the product of the object’s
mass and the square of a length. For a
hoop, I = Mr2. This leads to a general
where k is the radius of rotation at which
the entire mass of the object should be
concentrated if the moment of inertia is to
remain unchanged. A more standard term
for this length is the radius of gyration.

Motor Load and Torque
The principles we have just discussed
can be applied specifically to motor applications. A motor cannot be selected until
the load to be driven and the torque characteristics are determined.

Motor Load: The term “motor load”
can refer to horsepower (hp) required by

Motor load is best described as the
torque required by the load. The torque
requirement may be dependent upon speed
as well. Various conditions place specific
demands on torque requirements and they
are discussed next.

Breakaway Torque: This is the
torque required to start the shaft turning
and is usually the torque required to overcome static friction:
Accelerating Torque: This
torque may be expressed in percent of
running torque. It is the amount of torque
needed to accelerate the load from standstill to full speed, and to overcome friction,
windage, product loading and inertia.
Peak Torque: Peak torque is the
maximum instantaneous torque that the
load may require. High peaks for brief periods are acceptable, but if an application
requires sustained torque higher than a
motor’s peak rating, a different motor
should be considered.
Constant Torque: A load with a
horsepower requirement that varies linearly
with changes in speed is said to have constant torque requirements.


Moment of
Radius of
Inertia (I) Gyration (k)




-- Mr2




-- Mr2

r -5


--- Ml2



-- (r12 + r22)
Fig. 1-43: Moments of inertia for familiar objects.


(r12 + r22)

AC Motors
Although commutator and brush assemblies may be used in some types of alternating current motors, brushless inductiontype designs are by far the most common
for motors operating on AC supplies.

In an AC motor, the stator winding sets
up a magnetic field which reacts with the
current-carrying conductors of the rotor to
produce rotational torques. The rotor
currents are induced in the rotor conduc-

tors by the stator’s changing magnetic field,
rather than by means of a commutator and
brushes. This induction action is the central
operating principle of AC induction
AC power is commercially supplied in
both single-phase and three-phase forms.
The essential operating characteristics of
AC induction motors will vary according to:
1) winding types (split-phase, shaded-pole,
three-phase, etc.), and
2) the number of phases, the frequency and
the voltage of the power source.
deliberate “skewing” of the slots (position-

Fig. 2-1: Simplified diagram of a two-phase AC motor (left), and cross-section of a
two-phase AC motor showing phase 1 and phase 2 windings (right).


We will consider polyphase motors first,
since their operation is somewhat simpler
and more easily understood than singlephase machines.

The production of a rotating magnetic
field can be simply illustrated by considering a two-phase motor with two embedded
stator windings for establishing the magnetic fields. Each coil, for simplicity, shall consist of a single loop of wire connected to
one phase of a two-phase AC supply. We
shall refer to the coil supplied by phase 1
current as Coil 1, and the coil supplied by
phase 2 current as Coil 2. The two coils
are placed at a right angle to each other in
the stator core, with each coil creating a
two-pole field. See Fig. 2-1.
The output waveform of the two-phase
AC supply is represented in Fig. 2-2. The
voltage in each phase varies sinusoidally in
time and one lags the other by π/2 radians
or 90° (electrical)*.
Let us first consider Coil 1 only. When
the phase 1 current is in its positive portion
of the cycle (current enters Coil 1 from the
right and exits on the left), a magnetic field

Fig. 2-2: Waveforms produced by twophase AC.

Fig. 2-3: Magnetic field set up when
phase 1 is in positive cycle.

Fig. 2-4: Magnetic field set up when
phase 1 is in negative cycle.

is set up which points in the positive (+Y)
direction. See Fig. 2-3. When the current
flows in the opposite direction during the
negative portion of its cycle, the magnetic
field points in the negative (-Y) direction.
See Fig. 2-4. Since the strength of the
magnetic field (H) is proportional to the
amount of current flowing through the coil,
the field strength also oscillates sinusoidally
in time.
Similarly, we can illustrate in Figs. 2-5a
and 2-5b the magnetic field due to current
flowing in Coil 2.
Now we have two perpendicular fields.
Each varies sinusoidally in time, and one
lags the other by π/2 radians. The combined effect (vector sum) of the two fields

*One complete cycle = 2π radians or 360° (electrical).


Fig. 2-5a: Magnetic field set up when
phase 2 is in positive cycle.

Fig. 2-5b: Magnetic field set up when
phase 2 is in negative cycle.

is a rotating resultant field. Figure 2-6 illustrates the progression of the rotation at
eight different points in time. The letters
(A-H) in Fig. 2-6 correspond to the points
(A-H) on the waveform diagram in
Fig. 2-2.
It can also be shown mathematically that
the magnetic field rotates. If we choose the
center of the stator as our reference point,
we can define BY and BX as the magnitudes
of the magnetic flux densities due to the
currents flowing through Coil 1 and Coil 2
respectively. Both BY and BX are functions
of their respective currents* and are functions of time. Also, due to symmetry, their
peak values are the same.
Since BY and BX vary sinusoidally with
their corresponding currents we can express them in the following equations:

By = B COS (2π ft)
Bx = B SIN (2π ft)

B = peak value of either BY or BX
f = frequency of the supply current
(cycles/unit time)
t = time
Let Br be the resultant value of BY and
BX and let ∅ be the angle of B with respect to the axis as shown in Fig. 2-7. For
B SIN (2π ft)
tan ∅ = ------------------ = tan (2π ft)
B COS (2π ft)
∅ = (2π ft)
Hence, ∅ is increasing at a rate of 2πf
radians per unit time. In other words, Br is
rotating with the same frequency as the
supply current.

*This assumes a constant permeability in the ferromagnetic structureture.

Fig. 2-6: Progression of the magnetic field in a two-phase stator at eight different


Fig. 2-8: Aluminum conductors in an
AC induction rotor. The steel laminations
have been removed to illustrate the
"squirrel cage" form of the cast aluminum

Fig. 2-7: The vector sum of BY and BX is
resultant field Br.

can also show that the magnitude of Br
remains constant during rotation, since:
Br2 = By2 + Bx2
Since B is independent of time, the
magnitude of the rotating resultant field (Br)
is constant.
We have demonstrated that a rotating
magnetic field is generated in a two-phase
stator. These basic analyses can be
extended to a three-phase stator and show
that it also has a rotating field. Therefore,
we will not go into detail with three-phase
The rotor of a typical induction motor is
constructed from a series of steel laminations, each punched with slots or holes
along its periphery. When laminations are
stacked together and riveted, these holes
form channels which are filled with a conductive material (usually copper or aluminum) and short-circuited to each other by
means of conducting end rings. The conductors are typically formed by die-casting.
This one-piece casting usually includes
integral fan blades which create a built-in
cooling device. The common term for this
type of rotor is "squirrel cage" (because of
its resemblance to the runway of an oldfashioned squirrel cage). It is an inexpensive and common form of AC induction
rotor. See Fig. 2-8.
As the rotating field sweeps past the
bars in the rotor, an induced current is developed. Since the flow of current in a conductor sets up a magnetic field with a cor-

responding polarity, an attraction will result
between the rotating magnetic field of the
stator and the induced field in the rotor.
Rotation results from the motor's attempt
to keep up with the rotating magnetic field.
The rate of change at which the lines of
flux cut the rotor determines the voltage
induced. When the rotor is stationary, this
voltage is at its maximum. As rotor speed
increases, the current and corresponding
torque decreases. At the point of synchronous speed (speed of the rotating field), the
induced current and developed torque both
equal zero.
The rotor in a nonsynchronous AC
induction motor will always operate at
some speed less than synchronous unless it
is aided by some supplementary driving
device. This lag of the rotor behind the
rotating magnetic field is called "slip", and is
expressed as a percentage of synchronous
% slip =
synchronous RPM-actual RPM
---------------------------------- x 100
synchronous RPM
In designing rotors for induction motors,
the shape and dimensions of the slots have
a demonstrable effect on the performance
characteristics of the motor. This variation
is illustrated in Fig. 2-9.
Another design factor common to most
squirrel cage induction rotors is the


Fig. 2-9: Comparison of speed / torque characteristics for single cage (left) and
double cage (right) integral hp rotor design.

ing the slots at a slight angle to the shaft) to
avoid cogging action and wide variations in
starting torque which may result when bars
are placed parallel to the stator slots.

We have demonstrated in the previous
section that two-phase and three-phase
induction motors will create a rotating magnetic field corresponding to excitation of
the stator windings.
In the single-phase induction motor,
there is only one phase active during normal running. Although it will pulse with intensity, the field established by the singlephase winding will not rotate. If a squirrel
cage rotor were introduced into the air gap
between the stator poles of a single-phase
motor, it might vibrate intensely but would
not initiate rotation. However, the rotor
shaft will start to rotate in either direction if
given a push.
This rotation sets up an elliptical revolving field which turns in the same direction
as the rotor. The “double rotating field theory” and the “cross-field theory” explain
why a single-phase motor will rotate if it is
started by some means. Due to the complexity of the mathematics involved, they
will not be discussed here. What is important to remember is that single-phase AC
motors require an auxiliary starting scheme.

Single-phase motors, without the aid of
a starting device, will have no inherent
“starting” torque. To produce torque,
some means must be employed to create a
rotating field to start the rotor moving. A
number of different methods are used. The
particular method used determines the
“motor type.” An explanation of the various types follows.


Continuous duty
AC power supply
Reversibility normally at rest
Relatively constant speed
Starting torque 175% and up
(of rated torque)
• High starting current (5 to 10
times rated current)

Fig. 2-10: Split-phase (nonsynchronous)


Fig. 2-11: Typical four-pole,
split-phase stator.

Design and Operation: Splitphase motors are perhaps the most widely
used relatively constant speed AC motors
(of appreciable output) employed for driving domestic appliances. Also used for a
variety of industrial applications, motors of
this type are relatively simple in construction and lower in cost than most other
types. Low cost, plus good efficiency,
starting torque and relatively good output
for a given frame size have made the splitphase AC induction motor today’s general
purpose drive. See Fig. 2-10.
Split-phase motors are single-phase
motors equipped with main and auxiliary
windings connected in parallel (during the
start cycle). The auxiliary winding shares
the same slots as the main winding, but is
displaced in space. See Fig. 2-11. To give

the design its unique starting characteristic,
the auxiliary winding is wound with finer
wire and fewer turns (for high resistance
and low reactance) than the main winding,
and the current flowing through it is substantially in phase with the line voltage. The
current flowing through the main windings,
because of their lower resistance and higher reactance, will tend to lag behind the line
voltage in time. This lagging effect will act
to “split” the single-phase of the AC power
supply by causing a phase (time) displacement between the currents in the two
The space and phase displacement of
the main and auxiliary windings produce a
rotating magnetic field which interacts with
the rotor to cause it to start (begin rotating). After the split-phase motor has attained approximately 70% of rated speed,
the auxiliary winding is automatically disconnected from the circuit by means of a
centrifugal switch or current sensitive relay.
The motor will then continue to run on the
single oscillating field established by the
main winding. See Figs. 2-12 and 2-13.

Advantages: Split-phase motors
will operate at relatively constant speed,
typically from about 1790 RPM at no load
to 1725 or 1700 RPM at full load for a
four-pole, 60 Hz motor.

Fig. 2-12: Speed / torque curve for a typical split-phase AC motor.


Fig. 2-13: Example of a centrifugal cutout
mechanism used on split-phase motors.

A standard four-wire split-phase motor
can be reversed at standstill or while
operating at a speed low enough to ensure
that the auxiliary winding is in the circuit.
Split-phase designs can also be reversed at
full speed if a special switching device is
used to connect the starting winding in the
reverse direction sufficiently long to reverse
the motor. This normally is not done,
however, because of the danger of burning
out the starting winding during a long
reversal period.
Perhaps the most important feature associated with split-phase motors is their
relatively low initial cost. The high starting
torque combined with simple, reliable construction make split-phase AC motors ideal for many general purpose applications.
Since the rate at which the motor can be
accelerated is often a primary concern to
the applications engineer, split-phase designs are often specified because of their
ability to come up to speed rapidly (reaching running speeds with normal loads in a
fraction of a second).

Disadvantages: Because of the
high resistance of the starting winding, repeated starting and stopping will heat the
windings (in particular, the starting winding) and result in loss of torque and possible winding damage. This is one of the
reasons why it is not practical to apply
split-phase motors when very frequent

starts are required, or where high inertial
loads must be accelerated.
Split-phase motors have a high starting
current which can range from 5 to 10 times
the current drawn while running. If the
starting load is heavy, the wiring between
the motor and the power source must be of
adequate size to prevent excessive voltage
drop. The low voltage conditions resulting
from inadequate wire size will result in decreased motor starting torque. Frequent
starts, coupled with inherent high starting
current, can also adversely affect starting
switch or relay life.

Cautions: The auxiliary starting
winding in a split-phase motor is designed
for very short duty. If it stays in the circuit
for more than a few seconds, the relatively
high starting current which it draws can
cause overheating of the winding. Should
this happen, a more powerful motor or a
motor having different electrical characteristics should be considered.
Caution should be used when driving
high inertial loads with split - phase motors.
This type of load can prolong the
acceleration and “hang” too long on the
starting winding.

• Continuous duty
• AC power supply
• Reversibility at rest or during rotation,
except split-phase capacitor start which
is normally at rest only
• Relatively constant speed
• Starting torque 75% to 150% of rated
• Normal starting current (3 to 7 times
rated current)


Fig. 2-14: Capacitor (nonsynchronous)

Design and Operation: Capacitor action described in Chapter 1 has
been found to provide specific performance improvements when used with single-phase AC motors. See Fig. 2-14. The
types of capacitors used and the method of
operation varies with motor type (see Fig.
2-15.). The operating characteristics of
each type are quite different and will be
treated separately. In general, there are
three distinct capacitor motor types:
a) Capacitor Start (CS)— motors use
one electrolytic capacitor in the starting
mode only,
b) Permanent Split Capacitor (PSC)—
motors may operate with one permanently-connected, continuous-duty ACtype capacitor for both starting and
running, and
c) Two Capacitor Start/One Capacitor
Run — motors use one continuousduty AC-type and one electrolytic capacitor in the start mode and switch out
the electrolytic capacitor while running.
Capacitor Start (CS): The capacitor start motor is essentially a splitphase motor which has two separate windings: a main or, “running” winding and an
auxiliary or “starting” winding. However, in
the capacitor start motor, an electrolytic
capacitor is added in series with the start
winding during the starting mode to increase starting torque and/or reduce starting current. As in the case of the splitphase design, the starting winding and

Fig. 2-15: Comparison of continuousduty AC-type capacitor and electrolytic

capacitor will be disconnected when the
motor has reached approximately 70% of
running speed.
Like the conventional split-phase motor,
the capacitor start design runs with only the
main winding energized. This “run” winding
sets up a pulsating magnetic field which
interacts with the rotor to develop the necessary running torque and speed. Since the
“run” winding alone has no starting capability, both starting and running windings
are energized while starting. Because of the
high resistance-to-inductance ratio of the
“start” winding relative to the “run” winding, the currents in the two windings (when
energized) are sufficiently displaced (timewise) from each other to produce a rotating magnetic field and the necessary torque
for starting.
The addition of a capacitor, in series
with the “start” winding, can significantly
enhance the starting characteristics by improving the phase relationship between the
“run” and the “start” windings. With the
proper selection of capacitor value, the
starting torque can be increased and/or the
starting current decreased. Of course, capacitor values must be carefully selected to
produce this effect. Because the CS motor’s capacitor is used only when starting,
its duty cycle is very intermittent. Thus, an
inexpensive and relatively small AC electrolytic-type capacitor can be used in CS
designs. The normal, non-polarized, AC
electrolytic capacitor consists of two


aluminum plates separated by a porous paper which is saturated with an electrolyte.

Two Capacitor Start/One
Capacitor Run: A variation on the

Permanent Split Capacitor
(PSC): When split-phase or capacitor

permanent split capacitor design, the two
capacitor motor uses an electrolytic capacitor for starting in addition to the continuous-duty AC-type capacitor used for both
starting and running. The use of two capacitors helps to preserve the efficiency and
quietness of the PSC motor while running
and produces a corresponding improvement in the starting characteristics. If we
increase the value of the capacitor in a
PSC motor, we can normally improve
starting torque, but at the expense of running performance. However, by using two
capacitors (one for running and two in parallel for starting), optimum running and
starting characteristics can be obtained.
To understand how this works, it is
important to realize that the magnitude of
the current flowing in the capacitor winding
changes with the speed of the rotor. The
value of the current in the capacitor
winding is lowest when the rotor is at zero
speed, and highest when the rotor speed is
at its maximum. A capacitor and capacitor
winding combination that is optimized for
“locked rotor” or starting conditions will
not be optimum for normal running
operation. The watt input while running will
be high, and the current in the capacitor
winding will not lead the main current by
the ideal 90 degrees, resulting in inefficient
A capacitor and capacitor winding optimized for running will be correspondingly
less efficient in the starting mode. The use
of two capacitors for starting and one for
running overcomes the compromise made
in the PSC designs.

start (CS) motors are applied in applications which require long or frequent starts,
the motor may tend to overheat and adversely affect the system reliability. In this
type of application, PSC motors should be
The PSC capacitor winding is permanently connected in series with a continuous-duty AC-type capacitor. In contrast to
the split-phase or capacitor start motor, the
“second” winding is energized at all times.
The capacitor used with PSC designs is
rated for continuous duty and consists of
aluminum plates separated by a film
Permanent split capacitor motors operate in much the same way as two-phase
AC motors. The capacitor in the PSC design causes the current in the capacitor
winding to be out of phase (with respect
to time) with the current in the main winding, thus a rotating magnetic field is created. This action gives the PSC motor greater efficiency and quieter, generally
smoother operation than the split-phase
and the split-phase capacitor start designs.
See Fig. 2-16.

Fig. 2-16: Typical performance of a
1/15 hp (50 watt) PSC motor.

Advantages: In addition to the
improved starting torque characteristics
made possible by the capacitor in the capacitor start split-phase design, the reduction of starting current reduces the effect on
other equipment due to line voltage drop


encountered with high starting current splitphase designs. Lower starting current will
also contribute to longer life and greater
reliability in switches and relays.
In general (for a given horsepower
rating), although the permanent split
capacitor motor is more expensive than
split-phase and capacitor start designs, it
produces quieter operation and provides
the frequent start/stop capability essential in
many applications.

Shaded Pole

Continuous duty
AC power supply
Unidirectional reversibility
Relatively constant speed
Starting torque 50% to 80% of rated
• Low starting current

Disadvantages: Since the phase
angle in PSC motors changes with an
increase in load, performance will usually
be less satisfactory while starting. In usual
design practice, a compromise must
therefore be made between the starting and
running modes. Changing the capacitor
value specified by the manufacturer will
affect both running and starting characteristics so that any improvements in starting
will usually result in a decrease in running
Cautions: While an optimum capacitor value can enhance motor performance, an improper value of capacitance
can decrease performance. It is, therefore,
advisable to use the rated capacitor value
recommended by the manufacturer (on the
nameplate). Any change from the rated
value is usually detrimental to the design
and is not encouraged. When a failed capacitor is replaced, it should always be
replaced with a capacitor of equal capacitance and voltage rating. Voltage rating is
important for continued reliability and
It should also be noted that PSC motors
should be run at or near their rated load
points. Unlike other motor types, PSC
designs will tend to run hotter if lightly
loaded or unloaded.

Fig. 2-17: Shaded pole (nonsynchronous) motor.

Design and Operation: A simple and economical drive, the shaded pole
motor (Fig. 2-17) is used in countless consumer and industrial applications ranging
from room air conditioners to advertising
displays. Shaded pole motors have no internal switches, brushes or special parts,
and therefore offer substantial cost savings
in applications requiring relatively constant
speed and low power output.
While split-phase motors make use of a
high resistance auxiliary or “starting” coil
wound similar to the main winding, shaded
pole designs use an entirely different type
of stator lamination which allows for a set
of salient poles* surrounded by the main

*A motor stator has salient poles when its poles are concentrated in relatively confined arcs and the winding is wrapped around these poles (as opposed to distributing
the winding in a series of slots)


Fig. 2-18: Cross-section of a typical
shaded pole motor. Note the larger
salient poles and the smaller shading
poles on one side.

Fig. 2-19: Typical characteristic curve for
a 1/150 hp (5 watt) shaded pole motor.

Salient poles are broad radial projections (equal in number to the number of
poles) distributed around the active surface
of a rotor or stator and around which
windings may be coiled. See Fig. 2-18.
These are full pole pitch windings which
are fractionally distributed in a series of
Embedded in a portion of the face of
each salient pole is a single turn of conducting material, usually copper. These
turns are known as shading coils. The main
winding in a shaded pole motor is connected to the power supply, while the shading
coils form closed circuits on themselves.
The time-varying magnetic field set up
by the alternating current in the main winding induces a current in the shading coils.
This induced current will, in turn, establish
an additional magnetic field in the shaded
part of the pole face. This additional field
lags behind the main winding field in time.
With the main and shading coils displaced
from each other, a moving or revolving
magnetic field is set up in the stator which
interacts with the squirrel cage rotor to
produce rotation in a direction from the
center of the salient stator pole toward the
shaded pole tips.

Advantages: Above all, the shaded pole motor is simple in design and construction, making it readily adaptable to
high-volume, low-cost production. Because there are no internal switches, brushes or special parts, motors of this type can
be extremely dependable. Depending upon
construction, shaded pole motors are relatively quiet and free from vibration. Shaded
pole designs are normally available in sizes
from subfractional to approximately 1/4 hp
(186 W).
The shaded pole motor is classified as a
relatively constant speed machine, and
running efficiency will increase with load.
Variation in applied load will not significantly affect motor speed, providing that
the motor is not overloaded.See Fig. 2-19.
Normal shaded pole designs also offer
the “fail-safe” feature of starting in only one
direction. With split-phase and capacitor
start motors, there is always the remote
possibility that they may start in reverse in
some failure modes (cutout switch doesn’t
operate, open winding, etc.)
Disadvantages: Although a
shaded pole motor is rugged and inexpensive. It typically has low starting torque and
running torque.


Efficiency is also low, making shaded pole
motors impractical beyond fractional
horsepower sizes. Shaded pole motors are
generally used on light-load applications
where heat can be tolerated or supplemental cooling is available.
While efficiency is relatively low, for
applications requiring minimal power
output, this limitation is compensated for by
its lower initial cost. However, with today’s
increased emphasis on energy savings,
shaded pole motor operating costs over
the life of the application should be

The “difference” between the speed of
the rotating magnetic field of an induction
motor (which is always synchronous) and
the speed of the rotor is known as “slip.”
When the rotor design enables it to “lock
into step” with the field, the slip is reduced
to zero and the motor is said to run at synchronous speed. Upon reaching the running
mode, synchronous motors operate at constant speed — the speed being dependent on the frequency of the power supply.
This constant speed feature makes synchronous motors a natural drive for timing
and other applications requiring a constant
speed output.

Design and Operation: There
are two common types of small synchronous motors, classified according to the
type of rotor used:
a) reluctance synchronous motors, and
b) hysteresis synchronous motors.
Reluctance Synchronous: A
variation on the classic squirrel cage rotor,
the reluctance synchronous rotor is modified to provide areas of high reluctance.

This may be done by designing notches
(or flats) in the rotor periphery. The number of notches will correspond to the number of poles in the stator winding. The sections of the rotor periphery between the
high reluctance areas are known as salient
poles. Since these poles create a low reluctance path for the stator flux, they are
attracted to the poles of the stator field.
The reluctance synchronous rotor starts
and accelerates like a regular squirrel cage
rotor, but as it approaches the rotational
speed of the field, a critical point is reached
where there is an increased acceleration
and the rotor “snaps” into synchronism
with the stator field. If the load (particularly
inertial) is too great, the motor will not attain synchronous speed. Motor “pull-in”
torque is defined as the maximum load that
the motor can accelerate and pull into synchronism at rated voltage and frequency.
An applied load greater than the rated
“pull-in” torque will prevent the motor from
pulling the load into synchronism and will
result in rough, nonuniform operation.
The phase relationship between the
poles of the rotating field and the rotor is
known as the coupling angle, expressed in
mechanical degrees. This coupling angle is
not rigid, but will “increase” with an increase in load. At no load, the rotor poles
will line up with the field poles and the coupling angle is considered to be zero.
When a load is applied to reluctance
synchronous motors, the magnetic lines of
force coupling the rotor to the stator field
are stretched, increasing the coupling angle.
If the load is increased beyond the motor’s
capability, the magnetic coupling between
the rotor poles and stator field will break,
and the rotor will “pull out” of synchronism. “Pull-out” torque is defined as the
maximum torque the motor can deliver at
synchronous speed.
Reluctance synchronous motors may be
designed for polyphase operation, as well


Fig. 2-20: Comparison of typical reluctance synchronous rotors (top) and
hysteresis synchronous rotors (middle
and bottom).

as single-phase versions in split-phase, CS
and PSC configurations. These motors
have characteristics comparable to their
nonsynchronous counterparts using the
same types of stator windings. For comparable output in a given frame size, the
polyphase or PSC reluctance synchronous
motor will provide quieter operation and
more nearly uniform angular velocity than
the split-phase or CS reluctance synchronous motor. As shown in Fig. 2-20, the
reluctance rotor can be skewed to improve
smoothness of operation.

Hysteresis Synchronous:
Although the stator in a hysteresis synchronous design is wound much like that of the
conventional squirrel cage motor, its rotor
is made of a heat-treated cast permanent
magnet alloy cylinder (with a nonmagnetic
support) securely mounted to the shaft.
The motor’s special performance characteristics are associated with its rotor design. The rotor starts on the hysteresis principle and accelerates at a fairly constant
rate until it reaches the synchronous speed
of the rotating field.
Instead of the permanently fixed poles
found in the rotor of the reluctance

synchronous design, hysteresis rotor poles
are “induced” by the rotating magnetic
field. During the acceleration period, the
stator field will rotate at a speed faster than
the rotor, and the poles which it induces in
the rotor will shift around its periphery.
When the rotor speed reaches that of the
rotating stator field, the rotor poles will
take up a fixed position.
Like the reluctance synchronous motor,
the coupling angle in hysteresis motors is
not rigid, and if the load is increased beyond the capacity of the motor, the poles
on the periphery of the rotor core will shift.
If the load is then reduced to the “pull-in”
capacity of the motor, the poles will take
up fixed positions until the motor is again
overloaded or stopped and restarted.
The hysteresis rotor will “lock-in” at any
position, in contrast to the reluctance rotor
which has only the “lock-in” points corresponding to the salient poles on the rotor.

Advantages: Synchronous motors
operate at a constant speed fixed by the
number of stator poles and the frequency
of the power supply. Within the limitations
of “pull-out” torque and no variation in line
frequency, the speed can be considered
Hysteresis synchronous motors, with
their uniform acceleration characteristics,
can pull into synchronism any load that is
within their capacity to start and accelerate.
Disadvantages: Synchronizing
characteristics of the reluctance motor require increased acceleration of the rotor at
the critical point when it approaches the
rotational speed of the field. For this reason, it is possible that while the reluctance
motor may easily start a high inertia load, it
may not be able to accelerate the load
enough to pull it into synchronism. If that
should happen, the reluctance motor would
operate as an ordinary induction motor, but
at low efficiency and very irregular angular


velocity (audibly detected as a pounding
noise). It is important, when applying synchronous motors, to be certain that they
will accelerate the loads to synchronous
speed under the most adverse load and
voltage conditions. See Fig. 2-21.
In general, synchronous motors should
only be applied in cases where the load
needs to be driven at an exact rate of
speed. For a given horsepower, synchronous motors are usually larger and more
costly than nonsynchronous motors. In
other words, for a given frame size, synchronous motors (vs. nonsynchronous)
have lower hp ratings and tend to be more
expensive. Stated still another way, a synchronous motor will often be larger than a
nonsynchronous motor to drive a given
application. Because of these factors, synchronous motors tend to be applied only
where the synchronous feature is absolutely

Fig. 2-21: Comparison of typical speed
curves for hysteresis and reluctance synchronous motors of identical frame size.


Commutator (DC) Motors
Although similar in some respects to the
generators described in Chapter 1, motors
have an opposite function in energy conversion. While generators convert mechanical energy into electrical power, motors
convert electrical energy into mechanical
turning force or torque.

When a current-carrying conductor is
placed in, and at a right angle to, a magnetic field, it will experience a force perpendicular to the field and to itself. The direction of the force in relation to the field and
current is shown in Figs. 3-1a and b. The
force on this conductor is proportional to
the flux density, current and the length of
the conductor.
Using the above principle, we can explain the motor action of a simple single
loop armature as shown in Fig. 3-1c,
where DC current enters the right side of
the loop and exits the left. The resultant
forces acting on the single loop armature
generate a clockwise torque. However, the
torque diminishes to zero as the plane of

Figs. 3-1a, b, c, d: Upward and down
ward forces created by interaction of
field and armature flux.


the armature coil becomes perpendicular
to the field as shown in Fig. 3-1d.

Commutation: In order to continue the clockwise motion of our simple single loop armature, we need a commutator
arrangement as shown in Fig. 3-2a. As the
coil becomes perpendicular to the magnetic
field, the direction of current in the coil
reverses, causing the forces acting on the
coil to switch their direction. The coil then
continues to rotate in a clockwise direction.
The torque produced on the armature is

proportional to the sine of the angle between the magnetic field and the plane of
the rotating coil. The torque will produce a
ripple type waveform as shown in Fig. 32b. This figure shows that the resulting
torque reaches zero at the two vertical
positions during the armature (loop) rotation. This simple motor relies on the inertia
of the armature to carry it through the zero
torque points to continue its rotation.
To eliminate this effect and keep a level
of torque always at some point above zero.

Fig. 3-2: Relationship of commutator segments and torque: a) two-segment commutator,
b) two-segment commutator torque curve, c) four-segment commutator, d) four-segment
commutator torque curve, e) 32-segment commutator, and f) 32-segment commutator
torque curve.


Fig. 3-3: Commutator and brush position
in a typical DC motor design.

a four-segment commutator and two
armature coils may be used (Fig. 3-2c).
This arrangement staggers forces to keep
the torque at an acceptable level. The
torque/position curve will then look like
Fig. 3-2d. The more segments added to
the coils and corresponding commutator
armature, the closer the torque curve will
approximate a straight line characteristic.
See Figs. 3-2e and f.
Figure 3-3 shows the position of a commutator in relation to the armature coils of
a typical DC motor.

Counter emf and Armature
Current: When a DC armature is rotating in a magnetic field, there is an induced
voltage produced in the armature which
takes the form of an opposing or counterelectromotive force (cemf). When the flux
field is held constant, this voltage is proportional to the armature speed. Motor
action will continue as long as the voltage
supplied to the commutator is greater than
the cemf. The cemf limits the current flowing in the armature according to the
V = IR + cemf
where V is the source voltage, I is the
armature current and R is the armature
resistance. It is inherent that the current in
the armature is proportional to the load or
torque produced. The current increases
with an increasing load until the motor
stalls, at which point the cemf is equal to

Speed Control: The speed of a
DC motor is easily controlled by adjusting

the voltage either in the field or armature or
a combination of both. This can be accomplished by means of controls, variable
resistors and other devices and will be discussed in detail in Chapter 8.
Having briefly reviewed the fundamental
operation of commutator motors, we will
now consider each electrical type

Series Wound

Continuous or short time duty
AC or DC power supply
Usually unidirectional reversibility
Speed varying with load
Starting torque 175% and up of rated
• High starting current

Fig. 3-4: Series wound motor.

Design and Operation: Series
wound motors are among the most popular
of fractional and subfractional hp motor
types. Capable of operation on either AC
or DC power supplies, series motors deliver high motor speed, high starting torque
and wide speed capability, making them
ideal drives for a variety of applications.
See Fig. 3-4.
The armature and field of a series motor
are connected in series with respect to the


line. This feature allows series motors to be
operated from either AC or DC supplies
between 0 and 60 Hz. Because of their
“dual” capability, series motors are often
called “universal.” The performance difference of a universal motor between 50 and
60 Hz is generally negligible. It should not
be assumed, however, that all series motors are universal. Some may be optimized
for a particular power supply, and perform
poorly or fail prematurely if operated on a
power supply substantially different from
that specified on their nameplates.
Actually, no universal motor has the
same performance on both AC and DC.
Usually, the motor will run slower on AC
than on DC because the windings exhibit a
higher impedance when operated on an
AC supply. The speed difference is most
apparent with higher loads. Sometimes the
AC vs. DC speeds can be more closely
matched if a properly specified resistor is
placed in series with the motor when operated on DC.
At lighter loads, an opposite speed relationship may occur. Since the effective field
strength is lower on AC, the motor may
run faster.

Advantages: In addition to their
versatility, series wound motors have the
highest horsepower per pound and per
dollar of any motor that operates directly
from standard single-phase AC power.
This factor accounts, in part, for the popularity of series motors in household appliances and power tools. The economics are
closely related to the inherent high speeds
of series motors. For example, a typical
AC induction motor rated at 1/10 hp (75
watts) at 1725 RPM weighs approximately
15 lbs. (67 newtons). A series universal
motor rated at 1/10 hp (75 watts) and
10,000 RPM can weigh under 4 lbs. (18
Although there is a dramatic savings in
weight and cost per hp delivered, there

are other aspects to the comparison:
a) At the stated rating point in our foregoing example, the torque of the induction
motor will be 58 oz-in. (410 mN-m),
compared with 10 oz-in. (71 mN-m)
for the series motor.
b) The induction motor will have much
better speed regulation (less change in
speed with variations in load).
c) The induction motor will be significantly
quieter because of its lower speed and
absence of commutating brushes.
d) The induction motor will not have the
maintenance and service life considerations associated with brush commutation.
In spite of these differences, series motors are uniquely suited to a variety of applications. In particular, series motors are
the only small motors capable of more than
3600 RPM operating directly from a single-phase (60 Hz) AC power supply.
Also, the series motor will provide higher
starting torque than any other motor of
equivalent physical size operated from similar power supplies. Used as a DC motor,
the series design is practical up to about
the 5" diameter size range. Above that, PM
and shunt-wound motors become practical
in a cost/performance trade-off.
Although series motors are usually supplied as unidirectional (to obtain greater
efficiency and brush life) bidirectional series
motors can also be produced. One method
accomplishing this is a three-wire design
which can be reversed with a simple single
pole/double throw (SPDT) switch. However, for this arrangement, a split or double
field winding is required, reducing the available hp in a given frame.
An alternative to the three-wire method
is the four-wire series motor which is made
reversible by transposing the armature
leads, usually with a double pole/double


throw (DPDT) switch. With reversible
series wound motors, the application must
be able to tolerate some variations in speed
between one direction and the other, due
mainly to inherent differences in commutation until the brushes seat adequately in
each direction.
In addition to the advantages discussed
above, series motor speed can be adjusted
over a broad range by using a rheostat, an
adjustable autotransformer or an electronic
control. With the application of a mechanical governor attached to the motor shaft, a
series motor can also provide a constant
speed over a wide torque range.
The no-load and operating speeds of
series motors are usually quite high. Noload speeds in excess of 15,000 RPM are
common and are limited only by the motor’s own friction and winding characteristics. Normal operating speeds are from
4000 to 10,000 RPM. The excellent
forced ventilation made possible at these
speeds helps to yield much higher horsepower ratings than “common” induction
motors operating at 1725 to 3450 RPM.

Disadvantages: A series motor
inherently provides poor speed regulation
and is classified as having a varying speed
characteristic. This means that the speed
will decrease with an increase in load and
increase with a decrease in load. The
amount of change will depend upon the
particular motor design. Speed changes are
more pronounced because the armature
and field are connected in series.
As the load is increased, the motor must
slow down to let more current flow to support the load. This increase in current,
however, increases the strength of the field,
and thus the counter emf, which has a limiting effect on current build-up. The result is
a further decrease in speed to compensate
for this change. However, the simultaneous
change in field and armature strength cause
the two to always be matched or balanced

resulting in the excellent starting torque
characteristic of the series motor.
Although high speed is often a significant
advantage, it does not come without a
“price.” Specifically, bearing and brush life
are affected by high speed (household appliance series motors typically have a brush
life of 200 to 1200 hours, depending on
the type of appliance). Centrifugal forces
must also be analyzed to prevent the destructive effects of imbalance at high
speeds. These factors generally limit series
motors to intermittent duty applications.
However, series motors have been successfully applied in many continuous duty
applications where operating conditions are
favorable, or where the nature of the application provides for a moderate amount of

Cautions: Because of the steepness
of the speed/torque curve near the no-load
point, operation at or near no-load is usually discouraged. See Fig. 3-5. If consistent performance between motors or even
in the same motor is desired, series motors
should be operated at some load value
beyond this point. The slope of the speed/
torque curve, along with the point of peak

Fig. 3-5: Typical characteristic curve for
a series type (universal) motor.


efficiency, can be altered slightly by the
motor manufacturer to suit specific
An additional cautionseries motors designed and built for one direction of rotation should never be reversed (extremely poor brush life and
performance can be expected).


Fig. 3-6: Shunt-wound motor.

• Continuous duty
• DC power supply
• Reversibility at rest or during rotation
• Relatively constant and adjustable speed
• Starting torque 125% to 200% of rated
• Normal starting current

Design and Operation: One of
theearliest and most versatile types of DC
motors, the shunt-wound design has always enjoyed considerable popularity as
an excellent electrically adjustable, relatively constant speed drive. With solid state
control circuitry and its inherent relatively
constant speed characteristics, the shuntwound DC motor is a valuable companion
to advanced SCR (Silicon Controlled Rectifier) controls. See Fig. 3-6.
The shunt-wound DC motor has both a
wound field and armature with spring-loaded brushes applying power directly to the
armature by means of a segmented commutator. The term “shunt” is derived from
the connection of the field and armature in
parallel (shunt) across the power supply.
See Fig. 3-7b. The field and armature may
also be separately excited from two independent sources. This allows changes in
armature voltage to vary the speed while
still maintaining a constant field voltage.
Advantages: The shunt motor inherently
provides good speed regulation (changes in
load only slightly affect speed within its
rated torque range).
For example, a 1/4 hp shunt motor operating at a rated speed of 1725 RPM will
generally not vary in speed from no-load to

Fig. 3-7: a) Typical shunt-wound motor performance curve (left), and b) typical shuntwound motor wiring diagram (right).


full load by more than 15%. With modern
feedback-type controls, the speed regulation can be even further improved to ±1%
or less over a defined speed range, without
an add-on tachometer. Tight control over a
wider speed range may require sacrifices in
regulation to compensate for the wide
speed range feature. A tachometer, feedback or closed-loop control may also be
The most common means of controlling
shunt motors is the adjustment of armature
voltage while maintaining constant field
voltage. Armature voltage control is normally used to decrease the motor speed
below its base speed. Regulation and starting torque are generally not affected, except at the very lowest speeds. A totally
enclosed shunt motor can be designed to
operate at rated torque down to zero RPM
without developing excessive temperatures.
Another method, field weakening, may
also be used to vary motor speed. It is,
however, usually used only to increase the
motor speed above its base speed and is
not often recommended unless the load is
decreased to maintain a constant horsepower output. In addition, the percent of
regulation is increased and the starting
torque decreased with the field weakening
Normal NEMA* speed ratings (base
speed) for shunt motors operated from
electronic controls are 1140, 1725, 2500
and 3450 RPM, but a shunt motor can be
wound to operate at any intermediate
speed for special purpose applications.
This same flexibility, within limits, also applies to shunt motor voltage ratings.
Shunt designs are reversible at rest or
during rotation by simply reversing the armature or the field voltage. Because of the
high inductance of the field circuit, reversing the armature is the preferred method.

Disadvantages: If the shunt-wound
motor is operated from a fixed voltage
supply, a decrease in speed will occur as
the motor is loaded. The decreasing speed
with increased load tends to be linear over
a range in which the magnetic characteristics are linear. As load is increased, further
saturation begins to occur, resulting in what
is commonly known as armature reaction
and the resultant abrupt drop in speed, as
shown in Fig. 3-7a. The speed also increases linearly with increasing armature
voltage, making the shunt-wound design
valuable as an adjustable speed motor. The
fact that speed varies proportionally with
armature voltage makes it possible to vary
speed over a wide range with electronic

Cautions: Reversing the armature
while it is rotating is called “plugging” or
“plug reversal.” Because of the counterelectromotive force (cemf) or generated
voltage in the armature, plugging will subject the armature to approximately twice
the rated voltage and therefore should be
used with discretion.
Dynamic braking, while not as severe as
plugging, should also be used with caution.
A shunt motor can be dynamically braked
by “shorting” the armature after it has been
disconnected from the line. Current-limiting
resistors are generally used to reduce the
severity of this operation.
Brush life on a shunt-wound motor is
usually good. However, severe duty cycles, like plugging and dynamic braking,
can adversely affect brush life. Such applications should be carefully studied to prevent excessive stress to brushes and other
motor parts. With direct current, an electrolytic action takes place which causes
one brush to wear faster than the other.
This is a normal condition. The quality of

*NEMA is the national Electrical Manufacturers Association.


• Starting torque 175% and up of rated
• High starting current, relative to full load
running current

the DC wave shape coming from the control will also have an important effect on
brush life. Recognizing these precautions
and using a careful and intelligent approach
to shunt-wound motor application will usually guarantee long and successful brush
and motor life.

Permanent Magnet

Fig. 3-8: Permanent magnet motor.

• Continuous duty
• DC power supply
• Reversibility at rest or during rotation
with current limiting
• Relatively constant and adjustable speed

Design and Operation: Historically, permanent magnet field motors provide a comparatively simple and reliable
DC drive in applications requiring high efficiency, high starting torque and a linear
speed/torque curve. With the great strides
made in ceramic and rare earth magnet
materials, combined with electronic control
technology, the PM motor has taken on a
new importance as an adjustable speed
drive delivering significant performance in a
relatively compact size. See Fig. 3-8.
The single design feature which distinguishes the PM field motor from other DC
motors is the replacement of the wound
field with permanent magnets. It eliminates
the need for separate field excitation and
attendant electrical losses in the field windings. The armature and commutator assembly in conventional PM motors is similar to
those found in other DC types. See
Fig. 3-3.
Advantages: Perhaps the most
important advantage of PM field motors is
their smaller overall size made possible by
replacing the wound field with ceramic

Fig. 3-9: Stators for 1/4 hp (186.5 watt) ventilated shunt-wound field DC motor (right)
and 1/4 hp PM DC motor (left). Note that the inner diameters of the two stators are
the same, while the outer diameter of the PM motor is considerably smaller.


Fig. 3-10: Comparison of shunt and PM
motor curve shapes.

permanent magnets. For a given field
strength, the PM ring and magnet assembly
is considerably smaller in diameter than its
wound field counterpart, providing substantial savings in both size and weight. See
Fig. 3-9. And since the PM motor is not
susceptible to armature reaction, the field
strength remains constant.
Armature reaction can act to weaken
the magnetic field of a conventional shuntwound DC motor at loads beyond approximately 200% of rated value. This characteristic is generally responsible for the
“drop off” in torque associated with shuntwound designs. See Fig. 3-10.
If we examine the field construction of
the wound field and PM field motors, we
can explain the differences in armature reaction and corresponding differences in
speed / torque characteristics of the two
motor types. The armature magnetizing
force in a wound field construction “sees”
a very high permeability (low reluctance)
iron path to follow. In the PM field design,
this armature magnetizing force is resisted
by the low permeability (high reluctance)
path of the ceramic magnet, which tends to
act as a very large air gap. The net result is
that the armature cannot react with the field
in a PM motor, thereby producing a linear
speed / torque characteristic throughout its
entire torque range.
PM motors can be useful in a number of
specific ways:
a) They produce relatively high torques at
low speeds, enabling them to be used

Fig. 3-11: A typical family of
speed / torque curves for a PM motor at
different voltage inputs, with V5 > V4 > V3
> V2 > V1.

as substitutes for gearmotors in many
instances. PM motors operated at low
speeds are especially useful where
“backlash” and inherent mechanical
“windup” of gearing in gearmotors can
not be tolerated.
It should be noted that if PM motors are continuously operated at
the high torque levels which they
can generate, serious overheating
can result.
b) The linear speed / torque curve of PM
motors, coupled with their ability to be
easily controlled electronically, make
them ideal for adjustable speed and
servo motor applications.
c) The linear output performance characteristics of PM motors also make it
easier to mathematically predict their
dynamic performance. See Fig. 3-11.
Since the PM field motor is not affected
by armature reaction, it can produce very
high starting torque. This high starting
torque capability can be a valuable asset in
many “straight motor” (nongearmotor)
applications as well as inertial load applications requiring high starting torque with less
running torque. PM motors function well as
torque motors for actuator drives and in
other intermittent duty applications.


The size reduction in PM motors is generally accomplished without any significant
change in the temperature rise rating for a
given horsepower. In fact, the electrical
efficiency of the PM motor is very often
10% to 15% higher due to the elimination
of field copper losses which occur in
wound field motors. PM motors can be
produced in TENV (totally enclosed nonventilated) construction, eliminating the
need for fans and providing much greater
application flexibility.
With their higher inherent efficiency, PM
motors offer lower current drain for more
efficient battery operation in portable applications. The permanent magnets also provide some self-braking (less shaft coast)
when the power supply is removed. PM
motors require only two leads (shuntwound motors require four). The leads can
be reversed by simply changing the polarity
of the line connection. Dynamic braking is
achieved by merely shunting the two leads
after disconnecting them from the power
source. PM designs also provide similar
performance characteristics to shuntwound DC motors when used with all
common control methods (except field
weakening). See Chapter 8, Section 8.3.

Disadvantages: While ceramic
magnets now have properties which make
them very reliable, certain characteristics of
these materials must be thoroughly understood if proper operation of ceramic magnet PM motors is to be obtained. At lower
temperatures (0°C and below), ceramic
magnets become increasingly susceptible to
demagnetizing forces.
Armature reaction (which is capable of
producing the threshold limit for demagnetization) takes on greater importance at
lower temperatures. Therefore, special
attention must be given to overload current
conditions including “starting,” “locked
rotor” and “plug reversing” when applying
PM motors to low temperature use. Plug

reversing requires current limiting, even at
normal temperatures.
The design of the motor’s power supply
is also important. SCR circuits can be designed to provide current regulating and /
or limiting features to protect the motor at
low temperatures. The actual application
parameters involved vary with each particular PM motor design, since the protection
against demagnetization is part of the motor’s design and must be considered accordingly. It is best to consult the manufacturer if low temperature use or plug reversing is contemplated.
As operating temperature increases, the
residual or working flux of PM motors
decreases at a moderate rate. This flux
decrease is much like the decrease of field
flux strength in wound field motors as copper resistance increases with temperature.
On a motor-to-motor and lot-to-lot
basis, PM motors are sometimes criticized
for having somewhat greater variability in
performance characteristics than wound
field designs. Such criticism may be the
result of greater variations encountered in
both the quality of the raw materials and
the processes employed in the manufacture
of the magnet segments themselves. However, undue variation can be greatly minimized by the motor manufacturer. Proper
magnetic circuit design and calibration of
the magnetic assembly to a predetermined
common field strength value (somewhat
less than full saturation) can do much toward achieving consistent motor performance. Too often, calibration is ignored by
some motor manufacturers because of
cost, and in many cases, the variation in the
level of flux achieved by saturation alone is
considered acceptable.
Another concern is whether a PM motor can be disassembled without loss of
field strength and without having to provide
any additional magnetic circuit keeper. The
answer can be yes and no, depending


primarily upon the characteristics of the
magnetic materials selected for a given
design. Although newer ceramic materials
permit disassembly without loss of magnetic field strength, the user should consult the
manufacturer before attempting to disassemble the motor.

Cautions: Because of their high
starting torque characteristic, care must be
exercised in applying PM gearmotors. A
PM gearmotor application should be carefully reviewed for any high inertial loads or
high starting torque loads. These types of
loads could cause the motor to transmit
excessive torque to the gearhead and produce output torque which exceeds its design limits. SCR controls having current
limiting circuits or overload slip clutches are

often employed to protect gearing used
with PM motors.

In Section 3.1 we discussed how motor
action is achieved in a conventional DC
motor. A segmented commutator rotating
within a stationary magnetic field causes
mechanical switching of the armature current. In a brushless DC motor, the magnetic field rotates. Commutation occurs electronically by switching the stator current
direction at precise intervals in relation to
the position of the rotating magnetic field.
Solid state controls and internal feedback
devices are required to operate brushless
DC motors.

Fig. 3-12: Cross-sections of: a) an AC motor (top), b) a PM DC motor (left), and c) a
brushless DC motor (right).


Brushless DC motors combine characteristics of both DC and AC motors. They
are similar to AC motors in that a moving
magnetic field causes rotor movement or
rotation. They are similar to DC motors
since they have linear characteristics. Figure 3-12 shows cross-sections of AC, DC
and brushless DC motors. The AC motor
has windings in the stator assembly and a
squirrel cage rotor. The PM DC motor has
windings on the rotor assembly and permanent magnets for the stator assembly. The
brushless DC motor is a hybrid of the AC
and DC motors. The rotor has permanent
magnets and the stator has windings.

Brushless DC
• Continuous duty
• DC power supply
• Reversibility at rest or during rotation
with current limiting
• Adjustable speed
• Starting torque 175% and up of rated
• High starting current

Design and Operation: Brushless DC motors consist of two parts: the
motor and a separate electronic commutator control assembly (see Fig. 3-13).

Fig. 3-13: Brushless DC motor.

The two must be electrically connected
with a cable or wiring harness before motor action can take place. See Fig. 3-14.
By energizing specific windings in the
stator, based on the position of the rotor, a
revolving magnetic field is generated. See
Fig. 3-15. Sensors mounted inside the motor detect the position of the permanent
magnets on the rotor. For example, as the
rotor moves through a specific angle or
distance, one of the sensors will detect a
change from a north magnetic pole to a
south magnetic pole.
At this precise instant, current is
switched to the next winding phase. By
switching current to the phase windings in a
predetermined sequence, the permanent
magnets on the rotor attempt to chase the

Fig. 3-14: Schematic diagram of a brushless DC motor and control.


Fig. 3-15: Phase current flow.

current. The current is always switched
before the permanent magnets catch up.
Therefore, the speed of the motor is directly proportional to the current switching
rate. At any instant, two windings are energized at a time with the third one off. This
combines the torques of two phases at one
time, thus increasing the overall torque output of the motor.
The rotor consists of a four-pole permanent magnet and a smaller four-pole
sensor magnet. As the sensor magnet rotates it will activate a series of sensors located 60° apart. The sensors can be photo
sensors, Hall effect devices, magneto resistors or other devices which monitor the

Fig. 3-16: Commutation sequence:
a) clockwise (top), and b) counterclockwise (bottom).

position of the shaft and provide that information to the controller.
The controller logic circuits contain a
binary decoder which interprets the signals
from the sensors regarding the position of
the permanent magnet rotor. The logic circuit outputs a specific address which tells a
drive circuit (Q1 through Q6 in Fig. 3-14)
which windings should be energized.
The rotation of the motor is changed
within the control logic which in turn reverses the phase energizing sequence. A
toggle switch is usually provided to convert
the logic from clockwise to counterclockwise. Figure 3-16 shows the truth tables
for both clockwise and counterclockwise

Trapezoidal vs. Sinusoidal
Torque Properties: Timing of the
"on' and "off" switching of different phase
pairs is determined by the signals emanating from the sensors in the motor's commutation circuitry.
Trapezoidal torque characteristics of the
phase pairs mean that fewer commutation
signals are required than for a motor whose
phases exhibit sinusoidal torque properties.
This simplifies the control design and minimizes its cost, while providing a motor
torque output with low ripple properties.
Commutation circuitry is designed to
switch as the torque output and the back
emf in the individual phase pairs reach their
maximum (and most constant) level. This
produces the least ripple effect on the output torque and the lowest phase current
swing. The resulting smooth output torque
makes it easier to implement digital and
pulse width modulation control techniques.

Advantages: Brushless motors are
more reliable. They do not have commutator or brushes to wear out. The commutation is achieved through reliable solid-state
circuit components, making them ideal for
applications where downtime is critical or
where drive system access is difficult.


ventional DC motors. While electronically
commutated DC motors are now closer to
being competitive with conventional DC /
tachometer feedback units, they are still
costly when compared with conventional
DC / SCR control drives.

Stepper Motors

Fig. 3-17: Typical speed / torque curve
for a brushless DC motor.


Brush sparking and associated RFI are
Brushless motors are not sensitive to
harmonics like AC motors. The brush
noise associated with brushtype DC motors and commutators is also eliminated.
Brushless DC motors provide predictable performance because of their linear
characteristics. See Fig. 3-17. They can
exhibit high starting torques, precise positioning capability and controlled acceleration and deceleration. And more power
can be achieved from a smaller size motor.
Brushless motors can be designed with
low rotor inertia. This means they accelerate more quickly in less time and offer less
power dissipation during the stop / start
cycle. They are also capable of operating
at high speeds since there are no mechanical commutator limitations.

Disadvantages: Unlike conventional DC motors, electronically commutated designs cannot be reversed by simply
reversing the polarity of the power source.
Instead, the order in which the current is
fed to the field coil must be reversed. Also,
due to low friction inherent in brushless
motors, excessive coasting may be a problem after the current has been removed.
Special damping circuits or other devices
may be added to remedy this factor, but
cost will be adversely affected.
From a cost standpoint, the electronics
needed to operate brushless DC motors
are relatively more complex and therefore
more expensive than those used with con-


Continuous duty
DC power supply
Reversibility at rest or during rotation
Adjustable speed
Normal starting current

Fig. 3-18: Stepper motor.

Design and Operation: The
widespread acceptance of digital control
for machine and process functions has generated a growing demand for devices that
translate digital commands into discrete
incremental motions of known accuracy.
The ability to interface stepping motors
with microprocessors and / or mini-computer controls has enhanced their application potential (see Fig. 3-18).
While conventional AC and DC motors
operate from continuously applied input
voltages and usually produce a continuous
(steady state) rotary motion, stepper motors move in discrete steps (increments).
Stepping occurs in strict accordance with
the digital input commands provided. At


very low stepping rates, the stepping action
at the motor shaft may be visible. At high
stepping rates, the shaft appears to rotate
smoothly, like a conventional motor. Step
error is noncumulative. The absolute
position error is independent of the number
of steps taken. Final shaft position is
predictable within a maximum error
determined by mechanical tolerances, and
from the motor’s static torque vs. angular
displacement curve.
Although we refer to the angular position of the stepper shaft as the motor’s
“output,” there are many applications
where this rotation is converted to precise
linear motion, for example, by means of the
lead screw or rack and pinion.
DC steppers are divided into three
principle types, each having its own
unique construction and performance
1) variable reluctance (VR),
2) permanent magnet (PM), and
3) PM hybrid.

Variable Reluctance: Generally
a lower priced drive, the variable reluctance stepper has a wound stator and a
multi-poled soft iron rotor. The step angle
(determined by the number of stator and
rotor teeth) varies typically from 5 to 15
degrees. Unlike the hybrid design, variable
reluctance steppers have relatively low
torque and inertia load capacity. They are,
however, reasonably inexpensive and adequate for light load computer peripheral
applications. Operating pulse rates vary
over a wide range, depending upon the
specific design and construction of a particular motor.
PM Steppers: With step angles
ranging from 5 to 90 degrees, PM steppers
are low to medium-priced units with typically slower step rates (100 steps / second
for larger units and 350 steps / second for
smaller ones). They usually employ a

Fig. 3-19: 1) Hollow laminations,
2) Alnico permanent magnet, and
3) solid laminations.

wound stator with a PM rotor delivering
low torque. Step accuracy is ≥ ±10%.

PM Hybrid: The PM hybrid stepper
combines the construction and performance aspects of both PM and variable
reluctance type stepper motors. Both the
rotor and wound stator are toothed. The
toothed rotor is composed of one or more
elements known as stacks. See Fig. 3-19.
Each stack has both hollow and solid laminations bonded together to form two cupshaped structures. A permanent magnet is
inserted in the space between the two
cups. Rotor stacks are then fastened to a
nonmagnetic (usually stainless steel) shaft.
The perimeter of each lamination has
multiple teeth with a specific tooth pitch
(angle between tooth centers) depending
on the degree of step required. Step angles
vary from 0.5 to 15 degrees. See
Fig. 3-20.

Fig. 3-20: PM hybrid stepper tooth pitch.


Fig. 3-21: Variable stack lengths for PM
hybrid stepper motors.

When the cups are pressed on the shaft
to form a stack, they are positioned in such
a way that the teeth of one cup line up with
the slots of the other cup. The two cups of
each stack are said to be offset from each
other by half of one tooth pitch.
The stack configurations can vary.
When more than one stack is used, nonmagnetic spacers are inserted between
stacks to prevent coupling. See Fig. 3-21.

Without the spacer, the separate magnetic
structures would combine, eliminating the
advantage of multiple stacks. With adequate space between them, magnetic flux
will follow the path of least resistance
through the stator core, multiplying the
available torque by the number of stacks.
This construction gives the PM hybrid
higher torque capacity (50 to 2000 + ozin.) with step accuracies to ±3%. See
Fig. 3-22.
Figure 3-23 shows a cross-section of a
typical DC PM hybrid stepper with
toothed rotor and stator. When the rotor is
inserted into the stator bore, only one pair
of stator poles will line up exactly, toothfor-tooth, with the teeth on a single rotor
cup. The remaining poles will be out of
alignment by some fraction of a tooth pitch.
This misalignment is what makes it possible
for a stepper to develop torque. Most PM
hybrid steppers have four phases which are
bifilar wound, but other phase arrangements and multiples are available.
When phases are energized in a specific
sequence, PM hybrid steppers deliver specific angular output motions (steps) of
known accuracy, provided that system

Fig. 3-22: Flux path through rotor and stator.


Fig. 3-24: Typical torque vs. speed
(steps / second) for a PM hybrid stepper.

Fig. 3-23: Cross-section of a typical DC
PM hybrid stepper with toothed rotor and

inertia and friction do not exceed acceptable limits.
Each angular displacement ends in a
well-defined position of magnetic attraction
called a detent position. These stable equilibrium positions are created by the magnetic interaction between the permanent
magnet rotor and the magnetic field produced by the energized phase windings. As
the motor is stepped, the detent positions
shift around the entire 360° rotation. The
direction of rotation is determined by the
phase energization sequence.
PM hybrid designs offer excellent speed
capability1000 steps / second and higher can be achieved. Because the step angle
is fixed by the tooth geometry and step
error is noncumulative, the shaft position of
a motor loaded within its capacity is always
known within a fraction of one step. This
open-loop operation eliminates the need
for encoders, tachometers and other feedback devices which add to system cost.

Advantages: Steppers are popular
because they can be used in an open-loop
mode while still offering many of the desirable features of an expensive feedback
system. Hunting and instabilities caused by
feedback loop sensitivity and phase shifts
are avoided.

Due to the noncumulative nature of
stepper error, step motors also offer improved accuracy. The replacement of less
dependable mechanical devices, such as
clutches and brakes, with step motors provides considerably greater reliability and
consistency. Predictable and consistent
performance coupled with reasonable cost
make the DC stepper an excellent positioning drive.

Disadvantages: Stepper motors
can be made to produce reasonably high
torques (2000 oz-in. or more). However,
they do have a limited ability to handle extremely large inertial loads. See Fig. 3-24.
Since steppers tend to oscillate (ring) upon
stopping, some sort of damping means is
usually required. Stepper motors unfortunately are also not very energy efficient, but
this is the price that must be paid to
achieve the truly unique performance characteristics available from the stepper motor. Resonance is sometimes a problem
that can be remedied by a specialized electronic control design or by avoiding operation within the step rate ranges prone to
resonance. Refer to Chapter 8, Section
8.5. Most stepping motors are fixed angle
devices (although half angle stepping can
be achieved electronically).


required to get the load to full speed. It can
be expressed as:

Constant Horsepower: This
type of load absorbs the same amount of
power regardless of the speed.

Variable Torque: Some loads
require different torque at different speeds.
Load Inertia: The load inertia is
expressed as:
I = Mk2
where M is the mass of the rotating parts
and k is the radius of gyration.

Wk2 (n2 - n1)
t = ——————
t = accelerating time (seconds)
n2 = final speed (RPM)
n1 = initial speed (RPM)
Ta = accelerating torque (lb-ft.)
available from the drive
(Tdeveloped - Tfriction)
W = weight of rotating system (lbs.)
k = radius of gyration (ft.)

Acceleration Time: The difference between the friction torque required
by the load and the torque delivered by the
drive will affect acceleration time. Greater
accelerating torque decreases the time


Special Purpose Motors
In Chapters 2 and 3 we discussed the
electrical characteristics of AC and DC
motors and the basic methods of achieving
motor action with either AC or DC power.
From this, we determined that each type of
motor offers certain advantages or disadvantages when applied to an application. In
some cases, there is a considerable degree
of performance overlap from one motor to
the next, leaving cost as a criteria for motor
Most applications, if studied carefully,
will have parameters that will be satisfied
more effectively by one type of motor.
There are other criteria such as continuous
operation at very slow speeds, short duty
cycles or high torque requirements within a
limited mounting space, to name just a few,
that can place very unusual demands on
fractional horsepower motors.
To meet these unique design criteria,
motor manufacturers have developed a
variety of special purpose motors that
exceed the specifications of many common
motor designs. In this Chapter we will take
a brief look at some of these special
purpose motors.

For low speed drive applications,
electric motor manufacturers have
developed compact and efficient integral
gearheads. When coupled with common
fractional horsepower (fhp) electric
motors, these gearheads provide speedreducing/torque-multiplying units of
exceptional reliability. In any application
which requires shaft speeds slower than
that of a “straight” motor, fhp gearmotors
can be a highly desirable alternative to
conventional belts, gears and chains.
Gearmotors free the original equipment
manufacturer of the burden of designing
external reduction devices. They also offer
original equipment designers a highlyengineered, field-tested, single-source
drive system.
Because gearmotors are rated and selected
based on both the motor specifications and
the gearhead specifications, they present a
unique situation. Therefore, gearhead design and operation will be discussed in


greater detail in Chapter 6 and the application and selection of gearmotors will be
discussed in Chapter 7.

Some applications require high torque
combined with rapid stop and start characteristics. Low speed AC synchronous motors are appropriate for applications which
require six or more starts per minute. Since
the motor has no significant current rise on
starting, there is no additional heat rise with
repeated starts.
Unlike gearmotors, there is no backlash
associated with low speed synchronous
motors. As a result, they are used in place
of gearmotors in some applications. Most
low speed synchronous motors are designed to start typically within 1.5 cycles of
the applied frequency. Most low speed

synchronous motors will reach full synchronous speed within 5 to 30 milliseconds.
See Fig. 4-l.
Because of their rapid start characteristics, careful attention must be given to
inertial loads especially if the load is to be
coupled directly to the motor shaft. As
inertia is increased beyond a certain value,
the available torque decreases. This inertia
is defined by the “knee” in the torque vs.
inertia curve shown in Fig. 4-2. Also,
operation with less than minimum inertia
can sometimes result in objectionable startup noise or reduced available torque. The
use of gearing can increase the ability of
these motors to move inertial loads. Speed
change gearing produces reflected load
inertia in proportion to the square of the
gear ratio. For example, a 2 to 1 reduction
from 72 RPM at the motor to 36 RPM at
the load reduces reflected inertia 4 to 1,
and conversely, an increase of speed at the
load to 144 RPM increases reflected
inertia 4 to 1.

Fig. 4-1: Typical starting characteristics for a low speed AC synchronous motor.

Load Inertia, oz-in-sec2 x 10-3
Fig. 4-2: Typical torque vs. inertia curves for a low speed synchronous motor.


Resilient couplings can be used in
applications with high inertial loads to
provide some free shaft rotation so the
motor can start the load. A resilient
coupling should provide approximately five
degrees of rotational freedom before full
load is applied. Standard coupling means
include rubber components, timing belts
and slack chains. On the other hand,
adding a resilient coupling in an application,
with less than the minimum rated system
inertia connected to the motor, may reduce
the available torque.
Low speed synchronous motors can
usually withstand stalls without overheating
since the starting, full load and no-load
currents are essentially the same. However,
the motor will vibrate in prolonged stalled
conditions against a solid stop, which could
cause bearing damage over a period of
time. The stall torque cannot be measured
in the conventional manner, because there
is no average torque delivered when the
rotor is not in synchronization with the apparent rotation of the stator magnetic field.
Low speed AC synchronous motors
decelerate faster than conventional motors,
usually stopping within a range of 5° to 15°
after turn-off with no external inertia, depending on the RPM rating of the motor.
Application of DC to one or both motor
windings after removal of AC can produce
deceleration times one-tenth to one-twentieth of those attainable with a conventional
motor. The motor position remains electrically locked after stopping.

prolonged periods, allowing for the controlled tension essential in such applications
as spooling and tape drives.
Torque motors are especially useful in
three general classes of operation:
1) Motor stalled with no rotation
required. Torque motors will operate
like a spring in applications which
require tension or pressure. They can
be easily controlled to change the
amount and direction of force applied.
2) Motor shaft to rotate only a few
degrees or a few revolutions to
perform its function. Torque motors
may be used to open or close a switch,
valve or clamping device. In this sense,
they are used as “actuators.”
3) The shaft must rotate a major
portion or all of the cycle at a speed
much lower than that of a conventional motor. Spooling and reel drives
may require torque motor characteristics. Reel drives may also call for slow
speeds during the “playback” mode,
and higher speeds for short periods in a
rewind or “searching” phase.
AC torque motors are normally of the
permanent split capacitor (PSC) or
polyphase induction type. See Fig. 4-3.
Brush-type motors may also be designed
to operate as torque motors. This would
include shunt and permanent magnet

Torque motors are a variation on conventional induction and DC type motors.
They are designed for duty in slow speed
and tensioning applications. Not only will
they deliver maximum torque under stalled
or “locked rotor” conditions, but torque
machines can maintain a “stall” for
Fig. 4-3: Typical AC torque motor.


Fig. 4-4: Torque motor design vs. high
and low slip motor design

Fig. 4-5: Family of speed / torque curves
for various input voltages

designs which run on DC as well as series
wound torque motors capable of running
on either AC or DC supplies.
Torque motor characteristics are usually obtained by “deviating” from conventional stator winding, rotor winding
(squirrel cage), rotor lamination and air
gap designs. Figure 4-4 shows the substantially different speed / torque curves
achieved in one basic motor design
(frame) by changing one or more of the
above-mentioned design parameters.
Curve A is a motor designed for low
slip, high output running performance and
a high breakdown of torque. By changing
one parameter, we can get performance
characteristics indicated by curve B. By
making additional parameter changes, we
can obtain the characteristics shown in
curve C, which is very nearly a straight
line (curve C approaches the “ideal” for
torque motor service).
Because there is a reduction in the
power input, giving the motor prolonged
stall capability, the locked rotor torque in
curve C must be lower than that in the
other two curves. It is common practice
to operate torque motors at different levels of power input in applications which
have wide variations in torque demand.

.For example, in tape reel drives, high
speed is needed for fast rewind while
relatively low speeds are required for
recording and playback.
Reduced output is usually obtained by
reducing the voltage applied to the motor.
This may be accomplished by a variable
ratio transformer, saturable reactor, silicon
controlled rectifier (SCR) supply, or in the
case of small motors, by a series resistor.
The output of a torque motor will be affected by voltage change in the same way
as conventional motors — by the square of
the voltage, as shown in Fig. 4-5. While
the curves in Fig. 4-5 are for a voltage
reduction across the entire motor winding,
it is sometimes advisable to reduce only the
voltage across the main winding of a PSC
motor. This keeps the full line voltage on
the capacitor and capacitor winding combination so that torque stability at extremely
low operating speeds can be maintained.
When connected in this manner, the torque
can be varied approximately in proportion
to voltage.
Many torque applications require that
the motor be driven against the normal
rotation of its rotating field during a portion
of each cycle. The reverse rotation (resisting) torque is normally never greater than


stalled torque and will decrease slightly as
the reverse speed increases from zero.
A typical tape reel application can be
used to demonstrate this requirement.
When a tape is being wound from one reel
to another, resisting torque is necessary on
one reel motor to provide tape tension.
The voltage is reduced on the motor that
resists being pulled against its normal
rotation to provide the desired tension on
the tape.
There are several specific differences in
rating concepts between conventional induction motors and their torque motor
counterparts. An understanding of these
differences is essential for proper application. In contrast to ordinary induction motors, torque motor input and output are
considered at locked rotor rather than operating speed. While output is normally
expressed as horsepower or watts, torque
motor output is described as torque
(ounce-inches, ounce-feet, pound-feet or
The speed rating of a torque motor is
either its “no-load” speed or the theoretical
synchronous speed if the motor is an induction type.
Duty cycle ratings of torque motors are
also important, and should include two
1) the percentage of the duty cycle during
which the motor may be “stalled” at
rated voltage, and
2) the maximum time duration of the stall.
For example, if a motor has a 40% duty
and 30 minute time rating, the motor can
be stalled for 40% of the entire duty cycle,
and the continuous stalled time cannot
exceed 30 minutes out of a 75 minute duty
During the remaining 45 minutes, the
motor must be de-energized to permit the
heat generated during the stalled period to

Of course, the duty cycle of this motor
could have many other variations. If the
stalled time was only three minutes, the
total cycle could be as short as 7.5 minutes
(the motor will be de-energized for 4.5
minutes). A motor designed with a torque
sufficiently low to permit continuous stall,
and not exceed the maximum acceptable
temperature, would be rated 100% duty
and a time rating would be unnecessary.
In general, the best torque-to-watt ratio
is obtained in low speed induction motors
(six or more poles). The relationship of
motor poles to torque and speed is shown
in Fig. 4-6. Having no commutator or
brushes, induction motors are rugged and
require a minimum of service. The permanent split capacitor (PSC) motor is by far
the most popular in fractional and subfractional sizes. It operates on single-phase AC
and has a torque-output-to-watt input ratio
that compares favorably with the
polyphase motor under locked rotor conditions.
Another advantage of the PSC motor as
a torque motor is that it can be designed
with a three-wire reversible winding which
will permit it to be stopped, started and
reversed by a simple single pole / double
throw switch. The shaded pole design may
satisfy some torque motor applications,
but its torque-to-watt ratio is low, and it

Fig. 4-6: Speed vs. torque for various
numbers of stator poles.


cannot be reversed.
While the output of a torque motor is
usually taken from the rotor shaft directly,
the motor may have a speed reducing
gearhead through which the torque is increased by the mechanical advantage of
the ratio minus the losses in gearing. When
a gearmotor is being considered, the gearing type and ratio are very important and
must be chosen with care. This is especially
true if part of the motor’s function requires
it to be driven by the load, or if the operation requires the motor and load to be
brought to rest by bumping a rigid stop.
The mechanical parts in a gearhead must
be able to withstand the shocks and stresses imposed by the application.
Since the torque motor operates either
under a stalled condition or at speeds too
low to provide self-ventilation, it is important that a motor with a maximum torqueto-watt ratio be used that will also satisfy
all of the other requirements of the application. If the operating temperature of the
torque motor chosen for an application
exceeds safe limits, and there is no available space to accommodate a larger motor, the problem may be overcome by providing additional cooling with a low cost,
motor-blower unit. The use of the smaller
torque motor (with the blower addition)
may even result in a cost savings over the
use of a larger motor.
A “fail-safe” brake may also be used to
reduce temperature in torque motor applications. This would be applicable in cases
where the motor must lift a load to a specific location and hold it for an extended
period. The brake, connected in parallel
with the motor, would be applied by spring
pressure when power is removed from the
motor. This action will keep the load in
position without any heat being generated

Cautions: From the above discussion it is apparent that most torque motor
applications require the use of a sample

motor for tests in the machine before determining final specifications for the optimum motor. Answers to the nine questions
below should give the motor manufacturer
enough information to supply a sample that
is close to “ideal.” The customer could
then adjust the voltage to the sample to
obtain the desired performance with minimum input power. Temperature tests
should also be performed in the equipment
under actual or simulated duty conditions.
Consultation with the motor manufacturer
should determine whether modifications or
resizing will be necessary.
Criteria for determining torque motor
applications are:
1) What is the available power (voltage,
AC or DC, phase and frequency)?
2) What is the torque requirement and
duty cycle?
3) What are the minimum and maximum
speeds and how long will the motor
operate at the various speeds?
4) Will the motor be driven by the load at
any time in the cycle?
5) Is a brake or clutch to be used in the
drive mechanism?
6) Will the motor and load be brought to
rest by bumping a rigid stop?
7) What mounting space is available?
8) Is surrounding air free of dust and
contaminants or should the motor be
enclosed to protect against pollutants?
9) What is the ambient temperature?

The switched reluctance motor is a type
of synchronous reluctance motor. The stator and rotor resemble that of a variable
reluctance step motors. See Fig. 4-7.


Fig. 4-7: Typical switched reluctance
motor design.

The stator of a switched reluctance motor
may have three or four phases as does the
step motor. There are no coils on the rotor
which eliminates the need for slip rings,
commutators and brushes. Both the stator
and the rotor of a switched reluctance motor have salient poles.
The rotor is aligned when the diametrically opposed stator poles are energized.
Two of the rotor poles will align to the stator poles. The other rotor poles will be out
of alignment with the remaining stator
poles. When the next stator pole pair is
energized in sequence, they attract the two
rotor poles that are out of alignment. By
sequentially switching the current from one
stator winding to the next, the rotor continually rotates in a kind of “catch-up” mode
trying to align itself with the appropriate
minimum reluctance position of the energized stator windings — thus the term,
“switched reluctance.”
The switched reluctance motor provides
inherent characteristics and control functions that are directly equivalent to DC
servo motors. The torque of the switched
reluctance motor is equal to the square of
the current giving it excellent starting
torque. Motor direction can be reversed
by changing the current switching sequence
in the stator windings. Like their DC coun-

terparts, the brushless design of switched
reluctance motors simplifies maintenance.
Switched reluctance motors cannot be
operated directly from a three-phase AC
supply or a DC source. They require a
controller which adds to their cost. They
are also capable of four quadrant operation, that is, both speed and torque are
controllable in both negative and positive
directions. For more information on motor
control, refer to Chapter 8.
Switched reluctance motors can achieve
very high speeds which may be limited only
by the type of bearings used. This makes
them ideal for high speed applications.
Ironically, their high speed operation causes considerable noise which is one of their

Conventional rotary motors require
some type of rotary-to-linear mechanical
converter (lead screw, rack and pinion,
etc.) if they are used in applications where
the final stage results in linear motion. The
most obvious advantage of linear induction
motors (LIMs) is that they produce linear
motion directly without the need of a transmission or conversion stage.
The operation of linear induction motors
can be more easily understood if we start
with a conventional rotary squirrel cage
motor, cut the stator and rotor along a radial plane and roll them out flat. See Fig. 48. The rotor equivalent of the linear motor
is called the secondary and the stator
equivalent is called the primary.
Figure 4-9 shows that the primary consists of a core and windings (multiple phases) which carry current and produce a
sweeping magnetic field along the length of


Fig. 4-8: Basic linear motor construction

the motor. The secondary can be a sheet,
plate or other metallic substance. Linear
motors can have single or dual primaries.
The sweeping action induces currents in the
secondary and thus creates thrust in a given
direction depending on the direction of
current flow.
In contrast to a rotary motor, either element can be the moving element in a linear
motor. LIMs can have a fixed primary and
moving secondary or vice versa. This adds
to their flexibility in a wide range of applications. The secondary and primary are
separated by a small air gap, typically
0.015 to 0.045 inches. This gap is maintained by using bearings, wheels or magnetic levitation.

Fig. 4-10: Tubular or round rod LIM.

The flat primary can be rolled in the
transverse direction creating a cylinder into
which a tube or rod-type secondary can be
inserted. See Fig. 4-10. This is referred to
as a tubular or round rod linear motor.

An advantage of this type of linear motor is
that it has no end connections and can be
operated either horizontally or vertically.
One of the factors that determines LIM
performance is the pitch-to-gap ratio of the
primary coils. It affects the input power
delivered to the secondary and the harmonic content of the sweeping magnetic
flux. In general, a larger ratio translates into
better performance since it means less harmonics. Flat LIMs are usually more efficient than tubular LIMs.
The maximum speed of a LIM is directly proportional to the operating frequency
and the pitch-to-gap ratio. Speed is varied
by using a variable frequency controller.
LIMs are ideal for applications such as
computer plotters and read head positioning units, drapery openers, X-ray camera
positioning and a wide variety of conveyor

Servo motors are available in both DC
and AC types. Servo motors are an integral part of a closed-loop feedback control
system consisting of the motor, an amplifier

Fig.4-9: Thrust developed by single (left) and dual (right) primary linear motors.

Fig. 4-11: Block diagram of a closed-loop control system.

which drives the motor, an actuator and a
feedback device.
A block diagram for a closed-loop system using a servo motor is shown in Fig. 411. Any change in a system’s load, amplifier gain or other variable will cause the output of the system to deviate from the expected response. In the closed-loop system, these variations in output are monitored, fed back and compared to a reference or desired input. The difference between the reference and the measured output signal is a deviation. The deviation is
amplified and used to correct the error.
Therefore, the closed-loop system is selfcorrecting. For more information on motion
control systems, see Chapter 8.
Although servo motors show the basic
performance characteristics of the motor
classes to which they belong (AC
induction, PM DC, etc.), they incorporate
special design features which make them
uniquely suited to applications involving
feedback control. Because servo motors
must be sensitive to a relatively small
control signal, their designs stress reaction
to small voltage variations, especially at
Both DC and AC servo motors possess
two fundamental characteristics:
1) the output torque of the motor is
roughly proportional to the applied
control voltage (which the drive
amplifier develops in response to an
error signal), and
2) the instantaneous polarity of the control
voltage determines the torque direction.

AC servo motors are used in the 1/
1500 to 1/8 hp ranges. Beyond this range
AC motors become very inefficient and
difficult to cool. DC servo motors are usually used in higher hp ranges.

Direct-Drive Servo Motors:
In applications where precise positioning
and speed control is required, a directdrive servo motor is often employed. Direct-drive servo motors allow the load to
be directly coupled to the motor which
eliminates backlash and wear associated
with other coupling arrangements. Directdrive servo motors are capable of achieving fast acceleration and have excellent
response times.
Direct-drive servo motors are usually
brushless and provide all of the advantages
of brushless technology. They may also
have built-in resolvers which provide precise position monitoring and feedback control. Position accuracy in the range of 30
arc seconds is typical. For more information on feedback devices, refer to
Chapter 9.

While hardly a new idea (patents were
granted for shell-type armature designs
near the turn of the century), shell-type
armature motors have benefited tremendously from advances in polymer resin
technology. While early armatures were


Fig. 4-12: Basic construction of a shell-type armature motor.

bonded with metal strapping (which contributed to large eddy current losses), more
recent shell-type designs make use of a
variety of bonding methods which do not
contribute significantly to motor inertia.
These innovations have combined to produce motors with extremely low inertia and
high acceleration —characteristics which
are useful in many servo applications.
Shell-type armature motors operate in
much the same way as conventional permanent magnet motors, with an oriented
PM field and commutation by spring-loaded brushes. The feature that makes shell
armature motors unique is the hollow cylindrical armature composed of a series of
aluminum or copper coils (“skeins”) bonded together in polymer resin and fiberglass
to form a rigid, “ironless,” shell. See Fig.
4-12. Because the armature has no iron
core, it has very low inertia and rotates in
an air gap with very high flux density.
The unusual design characteristics of the
shell-type armature motor contribute to
low inductance and low electrical time constant (less than 0.1 millisecond). The absence of rotating iron in the shell-type armature motor results in a very high torqueto-inertia ratio, producing high acceleration
and quick response required in many positioning servo and incremental motion applications. Figure 4-13 shows the typical
speed / torque curves for a shell-type armature motor.

The principal disadvantage to shell-type
armature designs is their thermal time constant (typically 20-30 seconds for armature, and 30-60 minutes for housing).
Without proper cooling and/or sophisticated control circuitry, the armature could be
heated without warning to destructive temperatures in a matter of seconds during an
overload condition.
Another difficulty is the tendency for
shell-type motors to exhibit audio noise
and output shaft “whip” at high speeds.
Like printed circuit motors, shell-type armature motors are of somewhat fragile
construction and should be operated in a
more or less controlled environment. Furthermore, due to the manufacturing techniques and degree of application engineering required for this type of motor, they are
relatively expensive and tend to be employed only where their unique performance characteristics are required.

Fig. 4-13: Typical speed / torque curves
for a shell-type armature motor.


Basic Motor
Electric motor design involves complex
calculations that take into account the
physical laws of magnetism and numerous
empirical factors in order to arrive at an
optimal combination of materials for use in
motor construction. A given motor design
is expected to deliver a range of specified
output torques and speeds while operating
within various physical, environmental and
cost constraints. Since the output of the
motor is determined by the characteristics
of its magnetic circuits, the magnetic materials used in its construction are of primary

Electric machines are designed to convert electrical energy into mechanical energy to perform work. The force necessary
to do this work is typically derived from
two or more magnetic fields set in opposition to each other. The strength of these

opposing fields relative to each other
determines the turning force or torque produced.
In Chapter 3 (Fig. 3-1c), we learned
that if a current-carrying conductor loop is
suspended in an air gap at a right angle to a
magnetic field, and current flows in one end
and out the other, the forces that result
generate a torque. Since the force is partially dependent on flux density, a change in
the permeability of the material used in the
field and armature core can alter motor
Practical motor design requires that
strong magnetic fields be produced and
distributed in a precise fashion across an
air gap which allows the movement of one
member relative to the other (Fig. 5-1).
While current flowing through isolated conductors will produce a magnetic field, the
additional heat generated by the increased
current density needed to produce useful
flux levels results in practical limitations.
The most effective way to produce magnetic fields 15 to 20 times as strong as that
generated by conductors alone is to surround them with a ferromagnetic material.


Fig. 5-1: Cutaway of DC gearmotor
showing magnetic structure.

Electric motors may contain either a
stationary field or a rotating field. The actual configuration depends on several factors:
the supplied current (AC or DC), the type
of commutation (mechanical or electronic),
and the source of the field and armature
flux (wound field or permanent magnet).
In electric motors, magnetic materials
are used in three ways:
1) to form the core around which electrical
conductors are wound,
2) to replace the coil structure as the
source of the magnetic field, and
3) to assist the return of magnetic flux to its
Suitability for these tasks depends on
whether the material qualifies as “hard” or
“soft.” Soft magnetic materials, such as
iron, nickel-iron and silicon steels, magnetize and demagnetize easily with very little
energy loss when cycled. Soft materials
make excellent cores and flux return rings.
Hard magnetic materials, such as ferrite,
alnico and samarium cobalt, require more
energy to magnetize and demagnetize.

Hard materials (also called permanent
magnets) are used to replace wound coils
in many applications.
Motor designs must take into account
all of the practical behaviors of magnetic
materials. In addition to the hysteresis losses described in Section 1.2 (Basic Magnetism), alternating and cyclic magnetization
found in AC and DC motors and gearmotors produces an unwanted by-product
called “eddy current effect” which can seriously impair the performance of medium
and high speed motors.
Eddy currents are induced in the core
material itself and flow in a direction that
counteracts the primary flux change in the
core. To counteract this effect, the core
material can be divided into equal slices
(laminations), bonded together and electrically insulated from one another as shown
in Fig. 5-2. When divided into laminations,
the flux in each represents only a portion of
the total and the maximum induced voltage
is correspondingly reduced. The greater
the number of laminations, the lower the
voltage and corresponding losses. Eddy
current loss becomes more significant in
high speed and high frequency applications,
since the eddy loss is found to increase in
proportion to the square of the frequency
of the cyclic flux. Laminations, materials
selection and techniques which increase the
resistance of the eddy current path all help
reduce eddy current loss.
New magnetic materials offer opportunities for more efficient motor design that
seemed unthinkable a decade ago. Neody-

Fig. 5-2: Half view of DC field and armature laminations (left) and AC stator and rotor
laminations (right)


mium-ironboron and other alloys promise
magnets that are five times stronger than
common ferrite magnets. Amorphous soft
magnetic alloy ribbons can reduce core
losses by as much as 70% when substituted for silicon steel laminations. While both
soft and hard materials deliver a magnetic
flux to the air gap, the effects which govern
the behavior of each type of material make
it practical to treat each separately.

Soft Magnetic
Soft magnetic materials (iron, nickeliron and silicon steels) are very easy to
magnetize and demagnetize, a characteristic which makes them ideal for use in
brush-type armature and field cores as well
as induction rotors and stators. Soft magnetic materials may also be used as structural elements or enclosures that either carry flux between the source and load or act
as shielding.
When specifying soft magnetic materials
in motor design, factors such as mechanical
strength, machinability, corrosion resistance, hysteresis loss, eddy current loss,
permeability and the impact on magnetic
properties of stamping or forming operations must be considered.
Figure 5-3 shows a comparison of the
hysteresis loops for three common soft
magnetic materials. Soft iron (Fig. 5-3a)
provides low hysteresis loss (the area within the loop) with relatively high flux conducting capability (permeability). Hard
steel (Fig. 5-3b) exhibits higher hysteresis
loss, but somewhat lower maximum permeability. Soft ferrites (Fig. 5-3c) have
lower saturation and lower permeability,
but can be magnetized and demagnetized
very quickly, which makes them excellent
for use in equipment requiring quick response time such as computer peripherals.
Figure 5-4 shows a further comparison of
soft magnetic materials.

Note: B = Magnetic Flux Density
H = Magnetic Field Strength
Figs. 5-3a, b, c: Hysteresis loops for
three common soft magnetic materials.

Low Carbon Iron: The popularity of low carbon iron as a core material
can be explained by its combination of very
high permeability, low coercive force, low
hysteresis loss, high saturation and low
cost. The maximum permeability of low
carbon iron ranges from 2 to 7.5 kilogauss
per oersted (kG/Oe). The low carbon level, however, reduces the material’s strength






$ Cost

Low Carbon Iron

2,000 - 7,500



0.30 - 0.40

Ni-Zn Ferrite

2,500 - 5,000

0.2 - 0.5

140 - 280

5.00 - 12.00

Silicon Steels

5,000 - 10,000

0.5 - 1.0


0.40 - 1.00





Amorphous Alloys

Fig. 5-4: Comparison of soft magnetic materials.

and toughness. Iron cores are used primarily in the manufacture of relays.

Iron-Silicon Alloys: Iron-silicon
alloys (silicon steels) contain nominally 1,
2.5 and 4% silicon. They were developed
to enhance both mechanical strength and
magnetic properties, and have been the
most common soft materials used in motor
core laminations. The trend is to minimize
the amount of iron-silicon used because of
cost. Many motor cores are produced
using cold rolled electrical steel with less
than 0.15% iron-silicon content. These
materials can also be optimized for maximum permeability and minimum core losses
by hot rolling, annealing and cooling them
rapidly. Oriented four percent silicon steels
may reach a maximum permeability of 55
Amorphous Alloys: Produced
by cooling molten metals before they can
form crystalline structures, these glass-like
materials combine ease of magnetization
with high strength and low melting points.
Amorphous materials may provide up to
70% reductions in core loss with significant
improvements in efficiency. In spite of their
many advantages, these materials exhibit
much higher hardness (brittleness) than
silicon steels and may require radically different motor lubrication techniques to be
used. Their characteristic brittleness when
annealed also makes them difficult to

Soft Ferrites: The most common
ceramic soft magnetic materials are made
from sintering the powders of iron oxides,
manganese, zinc and also nickel, cobalt
and cadmium. Ferrites may reach a maximum permeability of 600 kG / Oe.

Hard Magnetic
Since permanent magnets provide the
magnetic flux for either the rotating or stationary member of a permanent magnet
motor, they must provide a sufficiently high
flux density to satisfy machine requirements. In addition, they must retain this flux
in the presence of a demagnetizing field at
reasonably high operating temperatures.
Hard materials typically depend on cobalt as an alloying element. Higher concentrations provide both a high energy product
(B x H) and high Curie temperature at
which a material loses its magnetic properties). With the introduction of high energy
rare earth products and neodymium-ironboron alloys, significant savings in motor
size and weight may offset the higher cost
of these materials. Figure 5-5 shows a
comparison of hard magnetic materials.

Magnetic Steels: Cobalt steel
(36% cobalt, 3 to 5% chromium, 3% tungsten, 0.85% carbon) is easily magnetized
and demagnetized. The addition of cobalt






$ Cost

Carbon Steels






2 - 10

600 - 2,000





1,600 - 2,400

2,000 - 4,000



14 - 30

7,000 - 9,000

7,500 - 11,000



26 - 40

9,000 - 15,000



Fig. 5-5: Comparison of hard magnetic materials.

significantly increases both coercivity and
the available energy product. Cobalt steels
are not commonly used due to their expense, lack of a domestic source of cobalt,
and their tendency to react to strong demagnetizing fields.

Alloys (Alnico): Alloys of Al, Ni, Co,
Cu, Fe and Ti, alnico magnets are formed
either by powdered metal processes or by
casting. Alnico (alcomax in England) materials must be cooled at a controlled rate in
a strong magnetic field to develop their
outstanding magnetic qualities. These materials have a high flux density and are relatively easy to magnetize and demagnetize.
Alnico is thermally stable and may be used
at high temperatures. However, it tends to
be extremely brittle and difficult to machine. Alnico is used extensively in stepper
motors and other applications requiring a
high performance coefficient (strength of
the magnetic field vs. breadth of the air gap
between magnetic poles).

Rare Earth-Cobalt Alloys:
Like many newer magnetic materials, rare
earth magnets are produced with powdered metallurgy techniques. Alloys of cobalt and samarium, lanthanum, yttrium,
cerium and praseodymium provide excel-

lent magnetic qualities and temperature
stability. A very high energy product allows
for compact magnet structures, excellent
resistance to demagnetization and good
temperature stability. Typically bonded to
rotor structures in brushless motors, these
materials are extremely costly even in small

(NdFeB): Instabilities in the supply of
cobalt have led researchers to substitute
neodymium in order to obtain an alloy element which is both readily available and
provides the high coercivity of the rare
earth-cobalt magnets. Produced by
quenching molten alloy on the edge of a
rotating substrate disk, NdFeB alloys produce an energy product as high as 40
MGOe with a coercivity of 15 kOe. Although they promise to be important new
materials in magnet design, neodymium
alloys have relatively low Curie temperatures. With the addition of small amounts
(6%) of cobalt, Curie temperatures can be
raised to safe levels.

Ferrites: With more than 40% of the
market for magnetic materials, ceramic
ferrites are the mature entry in the magnet
field. Developed after World War II, these


nonmetallic oxides of iron and other metals
are pressed in powder form to the shape
and size required, and are then heat-treated at temperatures between 1000°C and
1300°C. They are readily available and
inexpensive, exhibit high resistivity to demagnetization and show full magnetic stability at greatest maximum field strength.

In order to meet the often severe conditions of operation, a motor or gearmotor
must be equipped with correct bearings.
Since metal-to-metal contact during rotation causes friction and heat, the type of
bearings used in a drive unit plays an essential role in the life and effectiveness of
any driven machine.
Among the many considerations which
affect the choice of bearings are: speed
requirements, temperature limits, lubrication, load capacity, noise and vibration,
tolerance, space and weight limitations, end
thrust, corrosion resistance, infiltration of
dirt or dust, and of course, cost. Because
of the many factors which enter into bearing selection, it is evident that one bearing
design cannot possibly meet all criteria and
the choice must represent the most desirable compromise.
There are two principle types of bearing
supports used in fractional horsepower
motors: sleeve (journal) and ball. Gearheads use sleeve, ball, tapered roller, needle thrust and drawn-cup full-complement
needle bearings. Figure 5-6 shows a representative sample of bearing types. In addition, the table in Fig. 5-7 outlines the characteristics of ball and sleeve bearings.

Sleeve (Journal) Bearings:
Sleeve or journal bearings are the simplest
in construction and therefore, the most
widely used bearing when low initial cost is
a factor. They are quiet in operation, have

Fig. 5-6: Typical bearing types used in
fractional horsepower motors and gearmotors.

fair radial load capacity, and may be used
over a fairly wide temperature range.
Sleeve bearings also have virtually unlimited storage life if the motor is to remain
unused for extended periods. They show
good resistance to humidity, mild dirt infiltration and corrosion (when made of
bronze). Under light loads, static friction of
sleeve bearings is nearly as low as greasepacked ball bearings (although it is higher
than oil-lubricated ball bearings).
The principle disadvantages of sleeve
bearings are their need for relubrication
and size. They are, by necessity, longer
than ball types, and in general, add somewhat to the overall length of the motor.
Sleeve bearings cannot be allowed to
run dry. An oil reservoir (or felt or similar
oil-retaining material) must also be incorporated into the end shield and the lubricating oil periodically replenished.
A variation of the ordinary sleeve bearing, the graphited self-lubricating bearing, is
made of solid bronze, with graphite-filled
inner recesses (often in the shape of two
figure eights). It may also employ graphitefilled holes to conduct oil between the reservoir and the inner bearing surface. The
bronze body of such bearings provides
strength and resistance to shock or vibration, while the presence of graphite helps to
form a lubricating film on the bearing



Sleeve Bearing

Ball Bearing

Starting and Stopping



Speed Limited by:

Turbulence of oil. Usual
limit 5000 RPM max.

20,000 RPM max.

Misalignment Tolerance

Poor (unless of the selfalignment type)


Starting Friction



Space Requirements:



Damping of Vibration



Type of Lubrication


Oil or Grease

Quantity of Lubricant





Depends upon
quality of bearing and
resonsance of mounting.

Low Temperature



High Temperature

Limited by lubricant

Limited by lubricant


Periodic relubrication

Occasional relubrication.
Greased bearings often
last the life of the
application without

Fig. 5-7: Comparison of ball and sleeve bearing characteristics.

surface, and prevents metal-to-metal contact when the motor is stopped.
The graphite in the bearing will also act
as an emergency lubricant if the oil level is
allowed to run low. It should be noted,
however, that it is not safe to depend on
the graphite and allow the motor to run
dry. Graphited bearings will also usually
withstand higher operating temperatures
than ordinary sleeve bearings.
Oil-lubricated motors or gearmotors
should not be mounted in a vertical shaft


configuration except for right angle
gearmotors designed for this purpose.
When the oil reservoir is mounted above
the motor, gravity may cause oil leakage
into windings, causing subsequent motor
failure and hazards to personnel. Although
generally specified for radial loads, sleeve
bearings can also be designed to cope with
thrust loads or angle mounting. For this
purpose, they may be supplied in the flange
or “spool” configuration. In place of the

flange, thrust forces may also be accommodated by a hardened steel ball and disc
at the end of the shaft (which can also be
adjusted to control lengthwise shaft play
and heavy thrusts with low friction).
Another type of self-lubricating sleeve
bearing is constructed from porous bronze.
The porous bronze sleeve bearing is oilimpregnated and can be used with a felt
washer around its periphery to hold additional oil in suspension (eliminating the need
for frequent relubrication).
Porous bronze bearings are more compact and offer more freedom from attention
than solid bronze bearings. Their porous
feature is achieved by powder metal fabricating techniques. Porous bronze bearings
are often constructed to be self-aligning,
and to reduce friction and shaft binding.
The porous bearing is generally more economical than the graphited or solid bronze
types and given proper design, will carry
loads as well.

Ball Bearings: Ball bearings can
be used for virtually all types and sizes of
electric motors. They exhibit low friction
loss (especially when oil-lubricated), are
suited for high speed operation, and can be
used for relatively wide ranges of temperatures. Ball bearings can also accommodate
thrust loads, and permit end play to be
conveniently minimized. Compared to
sleeve bearings, ball bearings require significantly less maintenance (especially if
On the other hand, ball bearings are
slightly more expensive. Due to the nature
of the rolling action, they will also tend to
be noisier than their sleeve bearing
counterparts. Ball bearing manufacturers
have developed special processing
techniques for ball bearings used in electric
motors. As a result, the difference in noise
levels of sleeve and ball bearings has
become minimal.

Since they are made of steel, ball bearings are more susceptible to rust. However, moisture access to the ball bearings can
be precluded by proper design techniques.
Grease-packed ball bearings may also
have a limited storage life (motors which
have been kept in storage for some time or
exposed to low temperatures may show a
tightening of the shaft due to lubricant hardening). This factor may require that sleeve
bearings be chosen over otherwise more
suitable ball bearings in some instances. In
some cases, simply giving the motor some
warm-up time will “rejuvenate” the ball
bearing grease to a suitable condition. In
recent years, greases which have long storage life have also been developed, but this
advantage has been gained at the expense
of limiting the rating at low temperatures.

Needle Bearings: In many gearheads, full-complement drawn-cup needle
bearings may be used as supports for the
gearshafts. This bearing type has a much
higher length-to-diameter ratio than caged
roller bearings and is generally lubricated
by the lubricant in the gearhead. Compared
with “pure” roller bearings, needle bearings
have much smaller rollers and the highest
radial load capacity of all rolling element
Needle bearings must, however, be
used with a hardened steel shaft because
the shaft becomes the inner race of the
bearing. Maximum operating speeds are
also much lower than those for ball or pure
roller bearings. Their principle advantage
comes with their high-load-capability-tosize ratio, providing the ability to support
relatively severe radial and overhung loads
in high torque, compact gearmotor drives.
Needle bearings are not suitable for motor
shafts chiefly because their noise levels
increase somewhat exponentially with
A variation, the needle thrust bearing, is
also used in gearmotor application (prin-


cipally vertical shaft configurations). They
employ the same type of rolling elements
arranged like spokes emanating from a
central hub. Set in a wafer-like retainer,
needle thrust bearings can operate at reasonably high speeds with high static and
dynamic load capacities.

Thrust Washers: It is common in
small motors and gearmotors for thrust
accommodation and/or tolerance adjustment washers to be used in situations
where the thrust forces are light to moderate. Such washers are made of many materials, some of them having self-lubricating
properties. Steel, nylon and graphite impregnated materials are common. In noisecritical applications, the nonmetallic materials are favored.

Since they form the vital link between
the power supply and the armature coils in
a DC motor, brushes have always been an
important consideration in DC and universal motor design. Viewed as a system, the
commutator and brushes act as a rotary
switching mechanism which distributes current from the power supply to the desired
armature windings at the appropriate time.
Brushes must not only efficiently conduct line current to and from the armature
conductors. They must also resist destruction from voltages induced in the armature
coils undergoing commutation, and have
sufficient bearing qualities to minimize friction and wear at surface speeds which may
exceed 5000 ft/min. Almost all of the important limitations on brush performance
are in some way related to the dynamic
interface of brushes and the commutator.
For example, friction generated at high
speeds can cause sparking and nonconductive films to be formed between the
brushes and the commutator if the brushes
are not properly matched to the motor type
and function.

While there is no magic formula for
selecting the most suitable brush grade for
a particular application, brush and motor
manufacturers work together to narrow the
choices from the many thousands of brush
grades and materials available. Their final
choice is based on the specific motor type
and actual application parameters (since
the commutation characteristics will vary
depending on how the motor is to be
A brush grade is considered to be ideal
for a given application if it meets the
following criteria:
1) long life,
2) minimum sparking,
3) minimum commutator wear,
4) minimum electrical and mechanical
losses, and
5) quiet operation.
Since there are only a few brush grades
that will deliver long life and proper
commutation in any given application,
proper brush selection is critical to motor
To minimize electrical losses, it would
seem reasonable to select brushes with low
bulk resistance and a low voltage drop
(contact drop) between the brush and the
commutator. This approach is appropriate
for low voltage motors where power-robbing voltage drops cannot be tolerated.
However, it can cause excessive sparking
and commutator surface damage in motors
with high armature coil inductance. In these
situations, brushes with high resistance and
high contact drops will improve commutator and brush life by dissipating the energy
in the short-circuited commutator coils and
reducing the short-circuit current during
switching, thus improving the overall efficiency. Mechanical factors such as commutator surface speed, wear properties of
the insulation between the commutator
segments (flush or undercut), and brush


dimensions must also be considered. Dimensions are particularly important because the cross-sectional contact area is
proportional to the amperage-carrying ability of a given brush material.
Other motor design details such as
winding type, current rating, ampere-turns
ratio and type of commutator can affect
brush selection in a number of specific
ways. For example, series motors often
operate more efficiently when designed
with a lower than usual ampere-turns ratio.
But, if a “normal” brush grade is used,
sparking will be more pronounced and the
commutator will become blackened and
burned. For low ampere-turns ratio motors, a harder grade of brush with a slight
cleaning action can be specified which will
effectively counteract this condition.
Application parameters like frequent
starting and stopping (or reversing), overload capacity, need for high efficiency, the
presence of vibration or the minimizing of
brush noise will all influence brush selection. In some cases requirements may be
contradictory, forcing a compromise in the
ultimate selection. For all practical purposes, there are four popular groupings of
brush materials, covered below.

Carbon and Carbon Graphite Brushes: Amorphous carbon
(which is relatively hard) and crystalline
carbon or graphite (which has good lubricating qualities) are used in varying percentages in this brush classification. The
two materials are mixed and bonded together. Hard carbon and carbon graphite
brushes are particularly well-adapted for
use with motors having flush mica commutators (where appreciable polishing action
is required to keep the mica flush with the
copper bars). Their high coefficient of friction, however, generally restricts their use
to slow speed motors having peripheral
speeds below an upper limit of approximately 4500 ft/min (1370 m/min).

In addition, the resistance of the carbon
and carbon graphite brushes limits their
current density to 35-45 amperes/in2 (5.4
to 7.0 amp/cm2). This characteristic generally restricts the application of this brush
type to low current fractional horsepower

Electro-Graphitic Brushes:
The electro-graphitic brush is made by
subjecting carbon to intense heat
(2500°C). The conversion to crystalline
carbon or graphite is a physical (not a
chemical) change.
This group of brush materials has a lower coefficient of friction than the carbon
and carbon graphite class of brush and is
therefore better suited for use at higher
commutator peripheral speeds. The preferred average speed application is about
6500 ft/min (1980 m/min). This material is
less abrasive than carbon graphite. It is
also tougher, and has greater current density capability, with 75 amp/in2 (11.6 amp/
cm2) being fairly standard. The electrographitic group of brush materials is most
often used to solve difficult commutation

Graphite Brushes: Natural
graphite is a mined product. Graphite
brushes, as a class, are characterized by
more polishing action than electro-graphitic
grades. Their frictional properties are usually very low and their characteristic softness gives them good sliding qualities,
adapting them for use at commutator peripheral speeds as high as 8000 ft/min
(2440 m/min).
Due to the ability to orient the flake
graphite during the manufacturing process,
this material’s specific resistance can be
maintained at a very high level in one direction and yet achieve a current density in the
range from 50-65 amp/in2 (7.7-10.0 amp/
cm2) in the other direction. This feature
results in very favorable commutation


characteristics because short-circuited coil
currents are limited during commutation,
while still providing a low resistance path
for the active motor current. Sparking and
noise are generally low with this brush
type. However, the softness, which produces quiet operation, also limits the life of
these brushes.

Metal-Graphite Brushes:
Metal-graphite brushes normally contain
copper and graphite in varying percentages. The two materials are either mixed and
bonded together or the graphite is impregnated with molten metal.
The most important characteristic of this
brush class is its extremely high currentcarrying capacity, varying almost directly
with the percentage of copper content (the
higher the copper content, the greater the
current-carrying capacity and the lower the
contact drop). A brush containing in excess
of 50% copper may have current-carrying
capacity greater than 100 amp/in2 (15.5
amp/cm2). Normal speed limits are 5000
ft/min (1520 m/min).
The life of such brushes is relatively low
because of the wear properties of copper
brushes sliding on copper commutators.
Therefore, copper-graphite brushes are
usually employed only in high-current lowvoltage motors where no other brush
choice is possible.
General brush application guidelines
1) Shunt-wound DC motors generally
exhibit better brush life than series
wound motors due primarily to their
lower average speeds. However, poor
commutation can result even with a
standard brush if resistance is inserted
into the shunt field to weaken the field
strength and increase motor speed. This
additional resistance alters the ampereturns ratio relationship of the field and
armature so that the armature coils are
commutated in a less favorable position

in relation to the magnetic flux. This
factor must be considered in alternate
brush selection.
2) Frequent starting and stopping
imposes challenges on brushes because
of the higher starting currents involved.
This factor has a particularly pronounced effect with high voltage shunt
motors. Also, starting friction considerations play a role in performance.
Selection of a high contact-drop brush
(one with a voltage-drop of one volt or
more) may be more suitable.
3) Quietness of brush operation is
dependent primarily on the maintenance
of uninterrupted, smooth surface
contact between the brushes and the
commutator. Concentricity of the
commutator, brush spring pressure and
fit of the brushes in their brush holders
also relate to quietness. When quietness
is of prime importance, the normallyused brush can be replaced with a
softer grade with enough spring
pressure to ensure adequate commutator contact.
4) Humidity levels affect brush wear.
Low wear rates are dependent upon
the formation of a conductive lubricating
film on the commutator. Applications
that are subjected to an environment of
extremely low humidity (high altitudes)
cause high brush friction and relatively
rapid brush wear because of insufficient
moisture to form the required film.
Special grades of brushes are available
and should be selected for low humidity
applications. High humidity, on the
other hand, may increase the electrolytic action on the brushes. To improve
commutation in high-humidity applications, brushes with a certain degree of
abrasiveness are normally specified.


5) The presence of chemical fumes,
dirt or dust will also be a deciding
factor in brush selection. Recommendations for brush grades to be used in
environments subjected to those
contaminants usually include brushes
with some cleaning action. The use of
totally-enclosed motors also helps to
prevent contaminants from reaching the
commutator and brushes.
6) The nature of the commutator
surface affects brush operation.
Satisfactory service requires that a
smooth surface of uniform finish and
concentricity be maintained. A change
in the character of the commutator
surface, for any reason, is almost
certain to result in a noticeable effect on
brush and commutator system
7) Springs. The pressure exerted by
springs holding the brushes against the
commutator surface is an important
consideration in the total commutation
system. While specific spring composition details will not be discussed here,
there are three basic spring types in
general use:
a) Coil Type—Inexpensive and most
popular, but contact pressure
decreases as the brush wears,
because the spring exerts less
force as it uncoils.
b) Roll Type—Expensive, but
contact pressure is constant
throughout the life of the brush
due to the constant force exerted
by this spring type as it coils or
c) Lever Action Type—The
pressure exerted vs. distance
traveled curve of this spring type
falls somewhere in between the
two previously mentioned types.
8) Preventive Maintenance. The wear
rate of brushes is dependent upon many
parameters (armature speed, amperage

conducted, duty cycle, humidity, etc.).
For best performance, brush-type
motors and gearmotors need periodic
maintenance. The maintenance interval
is best determined by the user.
SAFETY NOTE: Always disconnect power to the motor before
inspecting or replacing brushes.
Follow instructions in motor
manufacturer’s documentation or
contact the motor manufacturer
before attempting preventive
Typical maintenance procedures
〈 Inspecting brushes regularly for wear
(replace in same axial position),
〈 Replacing brushes when their length
is less than 1/4 inch (7 mm.),
〈 Periodically removing carbon dust
from commutator and inside the
motor. This can be accomplished by
occasionally wiping them with a
clean, dry, lint-free cloth. Do not
use lubricants or solvents on the
commutator. If necessary, use No.
0000 or finer sandpaper only to
dress the commutator. Do not use
solvents on a nonmetallic end
shield if the product is so
In conclusion, the motor manufacturer
has considered many factors in specifying
the brushes for a particular motor design
and application. For this reason, it is important to replace worn brushes with the
original type (available from qualified service centers).

An insulation system, as defined by the
National Electrical Manufacturers Associa-


tion (NEMA) Standard MG-1, is “an assembly of insulating materials in association
with the conductors and the supporting
structural parts” of a motor. The stationary
parts of a motor represent one insulation
system and the rotating parts make up another.

Coil Insulation: All of the insulating materials that surround the currentcarrying conductors and their associated
turns and strands and which separate them
from the motor structure are part of the coil
insulation. These include: varnish, wire
coatings, encapsulants, slot fillers and insulators, tape, phase insulation, pole-body
insulation and retaining ring insulators.
Connection and Winding
Support Insulation: All of the insulation materials that surround the connections which carry current from coil to coil,
and which form rotary or stationary coil
terminals or lead wires for connection to
external circuits, as well as the insulation
for any metallic supports for the windings,
are considered part of the connection and
winding support insulation system.

Associated Structural
Parts: Slot wedges, spacers and ties for
positioning the ends of the coils and their
connections, as well as any non-metallic
winding supports or field coil flanges, make
up this insulation system.

Maximum Hot Spot


Over 240



Over 464

Fig. 5-8: Maximum hot spot temperatures 0f insulation systems.

Insulation systems are rated by temperature and divided into classes according to
the maximum operating temperature they
can safely endure for extended periods of
time. The four classes of insulation most
commonly found in motors are Classes A,
B, F and H. The table in Fig. 5-8 shows
the hot spot temperatures for these and
other classes of insulation systems.
The hot spot operating temperature is a
theoretical value. Under normal conditions,
a motor is operated at a temperature less
than the values shown in Fig. 5-8. Various
end-use standards for different types of
motors and controls use different methods
to measure the hot spot temperature for a
given insulation system.

The environmental conditions in which a
motor will operate are critical factors to
consider when selecting a motor for a specific application. Some types of motors are
more suited for specific conditions than
others and some may perform well under a
variety of conditions.
In some applications, the service conditions may constitute a hazard such as areas
where flammable vapors accumulate and
create an explosive situation. Another example would be an application which requires the motor to operate within a high
ambient temperature environment for prolonged periods, increasing the risk of fire or
motor failure.
NEMA has defined usual and unusual
service conditions for motors. They are
categorized by environmental and operating conditions as shown below:

Usual Environmental Conditions:
1) Exposure to ambient temperatures
between 0° and 40°C,
2) Operation at altitudes less than 3300 ft.
(1000 meters),


3) Installation on a rigid mounting surface,
4) Installation in enclosures or areas that
provide adequate ventilation, and
5) Most V-belt, fan belt, chain and gear

Unusual Environmental and
Operating Conditions:
1) Exposure to:
a) combustible, explosive,
abrasive or conducting dust,
b) conditions which could
interfere with normal venti
c) fumes, flammable or explo
sive gasses,
d) nuclear radiation,
e) steam, salt-laden air or oil
f) very humid or very dry
conditions, radiant heat,
vermininfested areas, or
areas conducive to fungus
g) abnormal shock, vibration
or mechanical loading, and
h) abnormal axial or side
thrust applied to the motor
2) Operating:
a) where there is excessive departure
from rated voltage or frequency,
b) where the deviation factor of the AC
source exceeds 10%,
c) where the AC supply voltage is
unbalanced by more than 1%,
d) from an unbalanced rectified DC
e) where low noise levels are required,
f) at higher than rated speeds,
g) in poorly ventilated surroundings,
h) under torsional impact loads,
repetitive abnormal overloads,
reversing or electric braking,
i) in a stalled condition with any winding
continuously energized, and

j) a DC motor at less than 50% of rated
armature current for long periods of time.
Various definitions and classification of
motors have been defined by NEMA in
Standard MG-1 based on a motor’s ability
to withstand environmental conditions. A
brief summary of the environmental
protection classifications for fractional
horsepower motors and gearmotors is
presented here.

Open Motor: One which has ventilator openings so air can flow over and
around the windings for cooling.
Drip-Proof: An open motor with ventilator openings that will prevent liquids and
solids dropped from an angle of 0° to 15°
from vertical, from interfering with its operation.
Splash-Proof: An open motor with
ventilator openings that will prevent liquids
or solids that strike the machine at any angle of 100° or less from vertical, from interfering with its operation.

Guarded: An open motor surrounded by screens, baffles, grilles, expanded
metal or other structures to prevent direct
access to live metal or rotating parts
through the ventilator openings.
Semiguarded: An open motor
with ventilator openings that are partially
guarded, usually on the top half.
Open, Externally Ventilated:
A machine which is cooled by a separate
motor-driven blower mounted on the machine enclosure.

Weather-Protected: An open
motor with its ventilating passages constructed to minimize the entrance of rain,
snow or other airborne particles.
Totally-Enclosed Motor: Motors that prevent the free flow of air from
the inside of the motor enclosure to the


Totally-Enclosed, Nonventilated: A totally-enclosed motor that is
not equipped with an external cooling

not permit sparks or heat generated within
the motor enclosure from igniting dust or
other airborne particles which accumulate
around the motor.

Waterproof: A motor which will
exclude a stream of water from entering its
enclosure from any angle.

Totally-Enclosed, FanCooled: A totally-enclosed motor
equipped with a separate external blower.

Explosion-Proof Motor: A
totally-enclosed motor which will withstand
an explosion of a specific vapor or gas
within its housing, or which will prevent
sparks or flashes generated within its housing from igniting a surrounding vapor or
Dust-Ignition-Proof: A totallyenclosed motor which will not allow ignitable amounts of dust to enter the enclosure
and cause performance loss, or which will

Encapsulated Windings:
Usually a squirrel cage motor with random
windings filled with an insulating resin to
form a protective coating against environmental contaminants.

Sealed Windings: Usually a
squirrel cage motor with an insulation system that is protected from outside contaminants by using a combination of materials
and processes to seal the windings.


Like the shell armature motor, printed
circuit (PC) motors were developed in
response to the need for low inertia, high
acceleration drives for actuators and servo
applications. The ironless armature is again
a feature, this time in the form of a compact
disc-shaped coil operated in conjunction
with a PM field.
The essential element of the PC motor is
its unique disc-shaped armature with
stamped and laminated or “printed” commutator bars. See Fig. 4-14. This nonferrous laminated disc is composed of copper
stampings sandwiched between epoxy
glass insulating layers and fastened to an
axial shaft. Field flux in a printed circuit
motor is provided by either multiple or
ring-type ceramic permanent magnets, with

a flux return plate to complete the magnetic
circuit. The corresponding condensation of
field and armature assemblies gives the PC
motor a somewhat unique “pancake”
The PC motor armature contains no
wound windings, and spring-loaded brushes ride directly on the stamped or printed
conductors (sometimes referred to as facecommutation). This design variation provides relatively low torque ripple (fluctuation in motor torque) and rapid acceleration useful in many servo applications.
The combination of low inertia armature
and resultant high acceleration makes the
printed circuit design a suitable drive in
some intermittent duty applications (positioning servos) where smoothness of
torque is an advantage, and in velocity servo applications where speed control within
a single revolution is a factor. See
Fig. 4-15.

Fig. 4-15: Typical speed / torque curves
for a printed circuit motor.

Since the current flow in a disc armature
is radial, the “windings” are arranged
across a rather large radius. This radius
factor (moment of inertia of a disc increases by the fourth power of disc radius) contributes substantially to the moment of inertia of the armature. In addition, the relatively fragile construction of the thin PC armature usually limits its application to controlled application conditions associated
with data processing and other sensitive
systems equipment.
Fig. 4-14: Basic stator and armature construction of a printed circuit (PC) motor.


Gearhead Construction
The functionality and efficiency of a particular AC or DC type gearmotor is a factor of both the motor and the gearhead.
This Chapter will focus on the mechanical
aspects of the various types of gears and
gear trains, which are employed in fractional horsepower gearmotors to control motor
speed and output torque.


one form to another. They can be
categorized into five basic types: spur,
helical, bevel, hypoid and worm. Gears
facilitate power transmission by providing a
positive means to engage the output of
machine drives. The direction of rotation,
speed of rotation, output torque, environmental conditions and efficiency requirements of a specific application determine
which type of gear should be used.

Over time and because of varying application demands, gears have evolved from

Spur Gears: A typical spur gear is
shown in Fig. 6-1a. Its teeth are cut paral-

Fig. 6-1a: External-toothed spur gears.

Fig. 6-1b: Internal-toothed spur gears.


Fig. 6-2: Spur rack and pinion.

Fig. 6-3: Helical gears.

lel to the shaft axis. Spur gears can be
external-toothed (teeth cut on the outer
edge) or internal-toothed (teeth cut on the
inner edge, see Fig. 6-1b).
The pair of external-toothed spur gears
in Fig. 6-1a makes up a single reduction
stage. The output rotation of such a stage is
opposite the input rotation. When multiple
gear stages are combined, larger speed
reductions can be achieved.
A single stage made up of an internaltoothed “ring” gear and an externaltoothed spur gear produces an output rotation that is in the same direction as the input
(Fig. 6-1b). Ring gears are employed

in planetary gear trains which will be
discussed in the next section.
A special spur gear configuration is the
rack and pinion, where the rack is simply a
flat bar with teeth cut in it, which meshes
with a conventional cylindrical spur gear.
See Fig. 6-2.

Helical Gears: Helical gears are
similar to spur gears except that their teeth
are cut at an angle to the shaft axis. See
Fig. 6-3. Several teeth make contact at any
point in time which distributes the load and
reduces wear. The noise and vibration associated with spur gears is also reduced
with helical gears.

Fig. 6-4: Double helical and herringbone gears.


Fig. 6-5a: Straight bevel gears.

Fig. 6-5b: Spiral bevel gears.

Helical gears have more stringent lubrication requirements because of the inherent
sliding action between the gear teeth.
Thrust bearings may be needed to absorb
the side thrust which helical gears produce.
Double helical gears (two helical gears
mounted side-by-side on the shaft) and a
variation called herringbone gears (Fig. 64) are sometimes employed to eliminate the
net thrust load on the shaft. In both cases,
the side thrusts produced by each gear
cancel each other.

Hypoid Gears: Hypoid gears are
similar to spiral bevel gears with one major
distinction. The shafts to which they are
connected do not intersect as in bevel gear
configurations. This allows end bearings to
be installed on each shaft for additional
support. See Fig. 6-6.

Bevel Gears: Bevel gears are employed in applications where an intersection of the input and output shaft centerlines
occurs. Teeth are cut from a conical or
angular surface and at an angle so that the
shaft axes intersect, usually at 90°. See
Fig. 6-5a.
Bevel gears are available in straight and
angular or “spiral” cut versions. Straight
bevel gears are usually noisier than spiral
cut and create side thrusts which tend to
separate the two gears. Spiral bevel gears
function much like helical gears. See
Fig. 6-5b.

Fig. 6-6: Hypoid gears allow shaft
clearance for additional support.

Worm Gears: Worm gears have
screw-like threads that mesh with a larger
cylindrical gear. See Fig. 6-7. It takes several revolutions of the worm to cause one
revolution of the gear. Therefore, a wide
range of speed ratios can be achieved from
a single stage reduction. The worm is usually the driving member although reversible
worm gears are available. An advantage of
worm gear drives is less wear and friction
due to an inherent sliding action. However,
the same sliding action decreases the overall efficiency of the system.

Fig. 6-7: Worm gear assembly.


Fig. 6-8: Comparison of parallel shaft gearmotors. On the left is an in-line shaft, on
the right is an offset configuration.

The inherent characteristics of gear
types have an overall effect on the power,
efficiency and torque ratings of a drive
when combined in different configurations.
In this section, we’ll take a look at how
various gear trains can be used to adapt
fractional horsepower motors to specific

Parallel Shaft Gear Trains:
The term “parallel shaft” applies to gear
trains with shafts facing the same direction
as the motor shaft. In other words, the axis
of the gear train shaft is parallel to the motor shaft axis.
Although the gear train shafts are parallel they can be either in-line with (concentric with) or offset from (parallel to) the
motor shaft. See Fig. 6-8. The offset configuration is generally more compact than
in-line designs because it eliminates the
axial space needed for the bearing support
of the inboard end of the driveshaft. The
offset output shaft makes it possible to locate the shaft in a 3, 6, 9 or 12 o’clock
position, providing greater versatility in
mounting. The shaft location, however,
may necessitate changing the location of oil
level and oil fill plugs.
Fractional horsepower parallel shaft
gearmotors usually employ spur and/or
helical gearing. Both types provide high
efficiency within a small axial space. Spur

and helical gears commonly provide ratios
up to 6:1 per gearing stage. Spur gearing is
easier to manufacture and is therefore less
Besides slightly higher cost, helical gearing often requires additional constructional
features to accommodate its inherent axial
thrust. The magnitude of the axial thrust
forces is proportional to the load transmitted and the tooth angle of the helical gearing. Because of the greater overlapping or
“load-sharing” of helical gear teeth, the
transmission of power is usually smoother
and quieter with helical than with spur
gearing. Gear quietness is also dependent
upon rotational speed. It is common in
parallel shaft gearmotors for high speed
stages to be helical and slower speed stages to be spur (for economy).
The efficiency of spur or helical gearing
alone is about 97% per stage. Additional
losses result from bearing friction and circulation of lubricant. These losses, in typical fractional horsepower parallel shaft
gearmotors, reduce efficiency to about
92% per stage.

Right Angle Gear Trains: In
right angle gear trains the axis of the output
shaft is at a right angle (90°) to the motor
shaft axis. See Fig. 6-9. They are frequently used in applications where space restricts the use of parallel shaft gear trains of
comparable strength. Right angle gearmotors are especially desirable where a vertical output shaft is required.


Fig. 6-9: Typical single reduction right angle gearmotor.

Right angle gearmotors can be configured with vertical shafts without mounting
the gear train above the motor (an undesirable arrangement due to the risk of gear
train lubricant leakage into the motor).
Various types of worm, bevel and spiral
bevel gearing are used in right angle gear
trains from about 1/100 to 40 hp. The cylindrical worm is by far the most popular
type used in right angle designs. Ratios up
to approximately 72:1 per stage are common in fractional horsepower worm gearmotors. Both single and double stage reductions are possible, and overall reductions of over 2000:1 can be achieved in
two stages (with high single stage reductions). Because of the limited reduction
possible with bevel gearing, it is normally
used only when necessary to provide an
output shaft at a right angle, but not offset
from the motor shaft axis.
Precision, simplicity and reliability are
some of the benefits of using worm and
spiral bevel gearing. However, “self-locking” characteristics can also be achieved.
Self-locking prevents external torque applied to the driveshaft from “backdriving”
the motor, and depends upon tooth angles
and the coefficient of friction between the
worm and gear. Generally, worm gear sets

are self-locking if the lead angle is less than
5°. Gearmotors may start out being selflocking when new, but become non-selflocking as the parts wear in and efficiency
improves. The manufacturer should be
consulted if the self-locking feature is necessary for positioning or hoisting applications over the life of the motor.
Because the worm and gear teeth are
under crushing (rather than cantilever)
loads and many teeth are usually in contact,
worm gears have higher resistance to
shock loads than spur or helical gearing.
The sliding tooth action of worm gears
offers minimal noise in comparison with
spur and helical types. However, sliding
tooth action is more difficult to lubricate
and, as previously mentioned, less efficient
than the rolling action of spur and helical
gearing. The lower efficiency of worm
gearing is more pronounced in the higher
ratios. Worm gear efficiency also decreases with a decrease in speed. It is most critical during starting conditions where the
torque multiplication may be as much as
20% less than under running conditions.
This factor must be considered if the
torques required by the application approach the gearmotor rating.
Thrust loads are always present with
right angle gearing, and many right angle


gearmotors use rolling element bearings for
severe duty conditions. Right angle
gearmotors also impose relatively high
thrust loads on the rotor shaft bearings,
which can be a limiting factor in overall
gearmotor life. Spiral bevel gearing has
different efficiencies depending upon the
direction of rotation. This should be
considered if the torques required by the
application are close to the gearmotor’s
maximum torque rating.

Combination Gear Trains:
Some applications can benefit from a combination of parallel and right angle gear
trains. This is especially true in situations
where large reductions are required and
space is at a minimum. Combination gear
trains accommodate right angle turns in the
drive and can often result in a reduction of
bearings and other system components.
The right angle reduction is usually added
as the first or last stage.

Epicyclic Gear Trains: Anoth-

torque output to control the wing flaps on
an airplane. Epicyclic gear trains are also
used for differential systems and applications where very low reduction ratios are
required. The input, output and auxiliary
shafts can be connected to any of the three
stages to achieve the speed/torque requirements of the application.
Epicyclic gear trains can be configured
in three arrangements:
1) planetary,
2) star, and
3) solar.
See Fig. 6-10. The number of planet
gears required depends on the ratio desired. The ratio also determines the type of
system to be used. Each epicyclic gear
train configuration can be further categorized as:
1) simple,
2) compound, and
3) coupled.
The simple epicyclic gear train has already been described in detail.
Compound versions consist of a common shaft with two planet members connected to it. Coupled epicyclic gear trains
combine two or more simple epicyclic
trains so that two elements of one train are
common to the other train.

er type of gear train is the epicyclic or
planetary gear train. It is comprised of
three stages:
1) a central “sun” gear,
2) several “planets” which engage the sun
gear and rotate around it, and
3) a large ring gear or “annulus” which
surrounds the entire assembly and
engages the planets.
Because the points on the rotating planets trace epicycloidal curves as they turn,
the term “epicycloidal” is used. The term
“planetary” is also applicable because the
rotating action of the entire assembly about
the central sun gear mimics the movement
of a solar system. Epicyclic gear trains are
being used increasingly as actuators in applications where more torque is required
from a smaller drive train package. A typical application in the aviation industry is
where a small motor must produce high

Both metallic and nonmetallic gearing
are used in the gear trains of small multiple
reduction gearheads. A nonmetallic gear is
often used in the first stage for noise reduction and a metallic gear used in subsequent
stages for strength. For reliable service life,
both types of gear materials must be properly lubricated.
Long service life (10,000 hours and up)
requires a fluid lubricant which is circulated
throughout the gearhead. Oils or semi-fluid


Fig. 6-10: Simple epicyclic gear trains: a) planetary (top), b) star (middle), and
c) solar (bottom).

greases provide the best combination of
lubrication properties and is nearly always
used in gearmotors larger than 1/10 hp
designed for industrial applications.
Despite its advantages, oil is not always
used in smaller fractional horsepower gearmotors because of sealing problems. Gearmotors under 1/10 hp do not always have
adequate surface area for gaskets and
more importantly, may not have sufficient
power to overcome the friction of a contact seal on the rotor shaft. Therefore, in
many small fhp gearmotors, grease is used
as a compromise to achieve lubrication
without oil leakage.
Shorter service intervals are required
when grease is used as a lubricant, primarily because of reduced lubrication circulation. The wear rate of gear train parts is
higher when grease is used as a lubricating
agent and the wear rate increases with the
stiffness of the grease. Moderate service
life of approximately 2,000 hours can be
achieved with grease lubrication.

Gearhead inefficiencies (frictional losses) are converted into heat. Because of
their inherent low efficiency, gearmotors
with worm or spiral bevel gearing require
careful attention because their lubricants
reach higher operating temperatures.
Worm gear lubricants generally have high
viscosity and contain “extreme pressure”
additives as well as other additives.


Selection and
Until now we have concerned ourselves
with motor theory, operation and construction. But like any machine, motors never
operate under theoretically ideal conditions. Therefore, when choosing a motor
for an application, specific information
about the tasks it is to perform must be
known and evaluated. Application parameters such as speed, torque, drive train, duty
cycle, operating environment, safety requirements, noise factors and thermal protection must all be evaluated against the
type of motor being considered and its
performance ratings.
This Chapter will focus on how motors
and gearmotors are rated and then discuss
various methods used to select and adapt
motors to meet specific environmental requirements. With this information, the reader will have a better understanding of how
to choose the right motor for a given application in order to assure efficient operation
and required service life.

In Chapter 5 on motor construction, we
discussed the various types of motors and
insulation systems as defined by the National Electrical Manufacturers Association
(NEMA). NEMA has established the rating procedure for the U.S. motor industry
in order to ensure safe optimum operating
conditions for motors and generators.
NEMA standards, in part, conform to other industry standards established by the
American National Standards Institute
(ANSI), the Institute of Electrical and
Electronic Engineers (IEEE) and the National Fire Protection Association (NFPA).


This standardization allows for maximum interchangeability between motor
types produced by different manufacturers.
Conformance to the standards assures the
motor customer that certain minimum
guidelines are in effect for products produced by member companies.
Other organizations have also established standards for motor design to ensure
safe operation and conformance to local
electrical codes. In the United States,
Underwriters Laboratories (UL) develops
safety standards for motor enclosures,

thermal protectors and controls. Similar
standards have been established in Canada
by the Canadian Standards Association
In Germany, national standards are approved by the Deutsche Institute für Normung (DIN) in conjunction with the International Electrotechnical Commission
(IEC). Additional safety test specifications
are also established by the Verband Deutscher Elektrotechniker (VDE).
The International Organization of Standardization (ISO) has also set standards


Standard No.



C22.2 No. 100-M1985

Motors/generators - general


C22.2 No. 77-M1988

Motors with inherent overheating protection



Motor enclosure protection



Motor enclosure protection


IEEE-Std. 1

Temperature limits in rating electric equipment


IEEE-Std. 43

Testing insulation resistance of rotating machinery


IEEE-Std. 112

Test procedures for polyphase induction motors
and generators


IEEE-Std. 113

Guide for testing DC machines


IEEE-Std. 114

Test procedures for single-phase induction motors


IEEE-Std. 115

Test procedures for synchronous machines



SI units and their use



General motor/generator design and applications



Motion/position control - motors and controls



Energy management - polyphase motors



Energy management - single-phase motors


ANSI / NFPA 70-1987

National Electrical Code



Impedence-protected motors



Thermal protection for motors



Electric motors - general

Fig. 7-1: Common industry standards for electric motors.


for international units of weight and measure, called the Systéme International
d’Unites or SI (metric) system.
Figure 7-1 lists various design and safety standards which apply to fractional
horsepower motors and gearmotors.
As mentioned previously, most standards organizations work with others to
assure a level of consistency and continuity
with their standards. It is beyond the scope
of this Handbook to list every standard
that is applicable to electric motors. In
many cases, the ones listed in Fig. 7-1
contain references to other standards on
which they were based. A list of industry
associations and testing organizations is
also provided in the Appendix. Most of
these organizations publish an index of their
respective standards.

An electric motor or gearmotor nameplate is an extremely important source of
information regarding the capabilities and
limitations of the machine. Care must be

exercised to operate electric motors and
gearmotors in conformance with the ratings
expressed on their nameplates.
In other words, the manufacturer will
indicate on the nameplate the conditions
under which it is felt the product can be
operated safely while giving optimum service. See Fig. 7-2. Any variation from
these operating condition specifications
may cause damage to the motor or gearmotor and create potential safety hazards
to personnel.
NEMA defines three basic classes of
electric motors for the purpose of rating:
general purpose, definite purpose and special purpose. We will consider general purpose motors first, since they constitute by
far the largest segment of electric motors.

Rating General
Purpose Motors
A general purpose motor is not restricted to any specific application, but is suitable for “general use” under usual service
conditions. Usual service conditions, as
defined by NEMA, were discussed in
Chapter 5, Section 5.5. General purpose
motors have standard ratings and provide
standard operating characteristics and construction features.

Fig. 7-2: Typical motor and gearmotor nameplates.


The Motors and Generators Standard
(MG-1), published by NEMA, defines the
various physical and performance characteristics which apply to these motors.
A general purpose motor is designed to
develop a certain amount of power while
operating continuously within safe temperature limits. The basis for rating, therefore, is
a rated power output within prescribed
winding temperature limits when operated
for an extended period of time under usual
service conditions.
The rated horsepower and speed
stamped on the nameplate are those values
nominally expected at rated power input.
Likewise, at the rated power input, the
nameplate temperature will not be exceeded when delivering rated load.
The amount of output power that can be
developed in a motor is limited by the losses in the motor, resulting from transforming
the electrical input into mechanical output.
These losses are exhibited in the form of
heat, and any attempt by the motor designer or user to increase the output of a motor
beyond practical limits will produce excessive losses resulting in a temperature rise
beyond safe limits. If the designed rated
load or established safe torque of a motor
is exceeded on an application, higher operating temperatures and / or premature failure will usually result.
Every motor has a maximum temperature limit dictated by the class of insulation
material used in the motor windings, and a
maximum ambient temperature listed on the
nameplate. These maximum limits should
not be exceeded. (See Chapter 5, Section
5.4.) For example, a motor with Class “A”
insulation is designed for a maximum continuous winding temperature of 105°C in a
maximum ambient temperature of 40°C.
Operation for prolonged periods in
overload conditions or high ambient temperatures (above 40°C) will shorten motor
life. The rule of thumb is that for each 10°C
above the rated maximum temperature, the

life of the insulation system will be approximately halved.
Furthermore, prolonged operation at
excessive temperatures will have a detrimental effect on the mechanical components not associated with the windings.
That is, the life of seal materials and lubricants will be similarly decreased.
The output power capacity of a motor is
given on the nameplate in terms of horsepower or watts and is the product of
torque, speed and a constant. The
formulas are:
Power (horsepower) =
torque (oz-in.) x RPM x 9.92 x 10-7
Power (watts) =
power (horsepower) x 746
Power (watts) =
torque (newton-meters) x
RPM x 0.105
Typical standard horsepower (watts)
ratings for fractional horsepower motors
are 1/20 (37.3), 1/12 (62.2), 1/8 (93.2),
1/6 (124.3), 1/4 (186.5), etc. Ratings below 1/20 hp (37.3W) are sometimes classified as “subfractional” and are often rated
in millihorsepower (for example, 2 mhp
instead of 1/500 hp).
In addition to horsepower, the motor
speed is usually shown on the nameplate.
With horsepower (watts) and speed information, the rated torque can be calculated
with the equation(s) above. Some standard
60 Hz fhp AC motor speeds are: 3450,
1725, 1140 and 850 RPM. These are for
relatively constant speed drives. The corresponding synchronous speeds for 60 Hz
AC motors are 3600, 1800, 1200 and
900 RPM.
If a motor has a gearhead, the output shaft
torque rating is usually expressed in terms
of torque and takes into account gearhead
efficiencies and motor and gear train capabilities. With gearmotors, the motor horsepower should be regarded as primarily a


reference parameter, and the nameplate
safe output torque rating should not be
exceeded to assure personnel safety and
gearmotor life.
Generally, both AC and DC general
purpose motors will operate under slight
variations in power source voltage and
frequency (as described by NEMA), but
may not provide the output values defined
at rated voltage and frequency.
For some motors, NEMA also defines
other operating characteristics for each
horsepower and speed rating such as:
breakdown torque, starting torque, locked
rotor current and allowable speed
NEMA standards do not cover all conditions or all motors, especially in the subfractional ratings. In these cases, reputable
manufacturers make a practice of paralleling as closely as possible the standards for
listed NEMA ratings.

General Purpose AC Motors: A
general purpose AC motor, as defined by
NEMA, is an open construction motor
with a service factor rating. The service
factor is a multiplier which is applied to
rated horsepower to establish a permissible
“overload” horsepower under defined conditions (see NEMA MG-1 paragraph
14.36 et al.). The standard fhp motor service factors listed by NEMA range from
1.25 to 1.40. A motor with no service factor indicated on the nameplate is understood to have a service factor of 1.0.
Most U.S. single-phase voltages are
115 and 230 V. Since the standard frequency in the United States and Canada is
60 Hz, this value would be indicated on the
nameplate of all motors sold in those countries. In Western Europe, the nameplate
would list the European standard of 50 Hz,
usually at 220 or 240 V.
General Purpose DC Motors:
The basis for rating fhp DC motors includes a “form factor” (ff) value. See

Chapter 8, Section 8.5. If the direct current supplied to the motor is very close to
pure DC (low ripple), its form factor will
be 1.0. As ripple increases, the form factor
increases. A fractional horsepower DC
motor is not intended to be operated continuously from a power supply that produces a form factor (at rated load) which is
greater than the rated form factor. The user
should also be aware that the form factor
of unfiltered rectified AC and SCR type
power supplies changes as a function of the
output torque and speed of the motor. Operating a motor continuously at rated load
with a form factor greater than rated will
cause overheating and may have an adverse effect on commutator and brush life.
DC motors are often used in variable
speed applications, which means they may
be called on to operate at speeds lower
than rated for extended periods of time.
There is no consensus among standards
organizations that a general purpose DC
motor should be capable of operating at
reduced speeds (particularly if equipped
with a ventilating fan), or at a standstill with
only the field energized, without excess
temperature rise. It is important, therefore,
that the user obtain from the manufacturer
information concerning the capability of the
particular DC motor under the aforementioned conditions.
In the past, common DC voltages were
115 and 230 V for motors operated from
low ripple (1.0 form factor) generator-type
power supplies. With the advent of efficient
solid-state devices, a 90 V armature and
100 V field became popular for motors
operated from an unfiltered, full-wave rectified 115 V supply. Similarly, a 130 V armature and 100 V field are popular for
motors operated from filtered, full-wave
rectified controls. The form factor will depend upon the particular motor and control
combination and may vary by


Rating Definite and
Special Purpose
The basis for rating definite and special
purpose motors is essentially the same as
for general purpose motors. That is, ratings
are based on developing a certain amount
of power while operating within safe temperature limits (on specific power supplies)
to provide long or expected motor life. The
differences that do exist are due to differences in the types of applications.
For example, motor operation for definite or special purpose duty is not necessarily assumed to be continuous, as in the
case of the general purpose motor; the
duty cycle may be intermittent. Also, the
output of a definite or special purpose motor is not necessarily expected to be a certain torque at a certain speed≡starting
torque may be the most important requirement (for example, as in a torque motor).

Definite Purpose Motors: A
definite purpose motor is designed for use
in a particular type of application, or for
use under service conditions other than
usual. In some instances, definite purpose
motors have standard ratings and provide
standard operating characteristics and construction features.
The NEMA Motor and Generator
Standard, MG-1, lists the performance and
construction requirements for certain definite purpose motors (oil burner motors, fan
and blower motors, sump pump motors,
instrument motors, etc.).
Allowable variations in voltage and frequency, and the proper application of
belts, chains and gear drives, are also defined for usual service conditions. Unusual
service conditions like those listed in Chapter 5, Section 5.5 must be considered.
Special Purpose Motors: A
special purpose motor or gearmotor can

be considered a one-customer motor.
Special purpose motors are developed
when an OEM (original equipment manufacturer) defines the operating characteristics or construction features of the required
drive such that a general purpose motor
cannot be used. Therefore, the motor supplier must design a special motor to meet
the OEM design specifications.
A special purpose motor, unlike the
general purpose motor and definite purpose motor, may not have standard operating characteristics or standard mechanical
features. It is designed for a particular customer’s application, which has not evolved
to the point that an industry standard can
be written.
Although special purpose motors are
not usually catalogued, the basis for rating
remains much the same. The motor is again
designed to develop a certain output while
operating within safe temperature and
mechanical limits. Unique circumstances
may exist (for example, operating on an
intermittent basis). When applied intermittently, a motor may be “beefed up” (a
much stronger winding provided without
the danger of overheating the motor). For
example, high starting torques and faster
motor response can be provided for servo
and torque motor applications not
previously obtainable under continuous
duty operation.
It should be noted that NEMA defines
the usual ambient service condition as a
maximum of 40°C. This is why 40° is used
for “maximum ambient” nameplate rating
purposes for general purpose motors. In
the case of definite and special purpose
motors, the maximum ambient may be only
25°C. The permissible temperature rise of
the motor can then be higher without exceeding the maximum recommended insulation temperature. Thus, a stronger motor
can usually be supplied if it is known that
the ambient is less than usual. Conversely,
a higher than normal ambient would restrict


the motor output and may dictate a higher
class insulation system and special lubricant
and seal materials.
A special purpose motor may even be
designed for shorter than normal life, because the motor (as used in the equipment)
need not last longer than the equipment.
Also, it is sometimes more important to
satisfy other requirements such as size and
power output at the expense of long life.
The choice, of course, is determined by the
application after a careful review of all the
parameters with the customer.
IMPORTANT≡ Since definite and
special purpose motors are designed
for specific applications, they should
not be indiscriminately used on other
applications. They usually will give satisfactory service only in the application for
which they were designed.

Rating fhp Gearmotors
Currently, there are no industry standards for fractional and subfractional
horsepower gearmotors. Consequently,
there has been a lack of agreement between manufacturers on gearmotor output
shaft speeds, mounting methods, life vs.
torque ratings and other criteria. Each
manufacturer uses a different set of rating
Before any standard gearmotor ratings
can be established, certain conditions for
satisfactory performance must be set by
the manufacturer. These criteria consist of
application particulars and construction
features which will ultimately affect the life
of a gearmotor.
Duty cycle, ambient temperature, application load characteristics, gear materials,
and bearing and gearing lubricants all contribute to the gearmotor’s actual life. When
comparing manufacturers’ ratings, one of
the most important factors (usually not
published) is expected gearmotor life at
nameplate rated load. Furthermore, the

design of the gearmotor involves material
and component selection that optimizes its
performance properties for a given application. For example, a gearmotor rated at
60 lb-in. of torque output, based on an
expected life of 500 hours, could be totally
unacceptable in an application which requires 40 lb-in. torque load for 2000
Expected life is a function of gearmotor
design, manufacture and loading. However,
tests and experience have proven that the
type of gearhead lubricant is an important
variable in assigning a life expectancy to a
small gearmotor. Typically, grease-lubricated gearmotors are rated to perform satisfactorily (under normal operating conditions) for one year (2000 running hours).
Oil-lubricated gearheads are generally rated for satisfactory performance for 5,000
to 10,000 hours at nameplate torque. Also,
in recent years, the use of greases approaching the consistency of oil have enabled gearheads to have a life expectancy
between 2000 and 8000 hours at rated
Gearmotor Output Torque
Rating: For standard gearmotors, the
torque rating shown by the manufacturer
represents a complete gearmotor rating
and reflects the capacity of the weakest
link or most limiting gearmotor component.
Some of the design limitations considered
are: motor input power, strength or wear
rating of the gearing, radial and / or thrust
capacity of the bearings, and rotor,
armature and shaft strengths. Obviously,
gearmotor torque ratings should not be
For some built-to-order gearmotor
applications, a manufacturer may incorporate nonstandard gear materials to provide
high shock load capacity on an intermittent
basis. In such instances, the nameplate
rating of the gearmotor will usually not be
increased above its rated value for
standard construction since the addition of


nonstandard materials does not always
increase the long-term performance of the

Hazards of Operating
at Other Than
Nameplate Values

Gearmotor Output Speed
Rating: The speed value shown on the
nameplate is established by one of the following methods:
a) For constant or relatively constant speed
motors (generally motors with 6% or
less speed regulation with respect to
load, such as: permanent split capacitor,
split-phase, polyphase or synchronous
types), the output shaft speed is determined by dividing the rated motor
speed by the gear ratio.
b) For variable speed motors (more than
6% speed regulation, such as: series,
shunt and induction motors with high
slip rotors), output speed rating is determined as follows:
Case I: The gearmotor is “motor
limited”. In this case, the gearhead has
more than sufficient capacity to transmit
the rated motor torque. Rated motor
speed is divided by the gear ratio.
Case II: The “package” is “gearhead limited”. In this case, the gearhead cannot transmit the full rated input
torque provided by the motor. The actual speed provided by the motor when
the gearhead is loaded to capacity is
determined experimentally.
Note: Allowance must be made for
seal friction if a seal is used on the
input side of the gearhead. After the
specific motor input speed required to
drive the gearhead at its capacity has
been determined (which will always be
equal to or greater than the motor’s
rated speed), it is then divided by the
gear ratio. It should be understood that
the speed at which a variable speed
gearmotor actually operates in a particular application is a function of the load
and its uniformity.

Nameplate values stipulate the limits at
which a motor or gearmotor can safely
operate. To operate the motor either over
or under the nameplate rated limit can have
adverse effects on motor performance and
safety. Some of the restrictions and associated consequences of ignoring them are
listed below.
1) Do not operate motors at voltages
beyond ± 10% of nameplate rating.
Higher voltages produce adverse
effects on motor temperature, noise and
vibration, operation of current-sensitive
relays, motor life and capacitor life, and
could create nuisance operation of
thermal overload protectors. Lower
voltages create starting problems with
current-sensitive starting relays and
could cause thermal overload protectors, with internal heating coils, to trip at
winding temperatures which exceed the
maximum allowable limits.
2) Do not operate motors on a nominal
power source frequency other than
that specified on the nameplate. With
the exception of brush-type motors,
motor speed will vary directly with
frequency. While it is understandable
that original equipment manufacturers
would seek to design a machine that
operates on several different frequencies, any decrease in speed due to
lowering frequency may have an
adverse effect on temperature and on
the proper operation of centrifugal
cutout switches and relays. At higher
frequencies, the torque capability is
reduced, and starting relays may fail to
engage the auxiliary winding.
Motor laminations (and the windings
installed in them) are specifically designed
for operation at nameplate frequency. For


example, the laminations for 60 Hz motors
are considerably different than those used
for 400 Hz motors. Moreover, motor manufacturers usually do not laboratory test at
frequencies more than 5% from that shown
on the nameplate. Since the amount and
type of noise and vibration emanating from
a motor will change directly with frequency, undesirable hum and other resonance
effects are quite likely with deviations from
nameplate frequency.
Dual frequency (50/60 Hz) motors can
be provided by manufacturers, usually at
output ratings lower than the standard for a
given frame size.
3) Do not drive a load in excess of
nameplate rating. Where nameplate
rating is in horsepower or watts, the
rated torque can be readily computed
by mathematical equations (relating
speed, torque and power). Overload
limitations also apply to gearmotors
where maximum gearhead torque is
shown. Technical assistance should be
requested from the manufacturer if
overloads are anticipated. Operation at
higher torque loads can result in lower
speeds, higher winding temperatures,
reduced life of windings, gears and
bearings, and nuisance operation of
thermal overload protectors. In many
cases, overloads can create hazards to
personnel. Noise and vibration also
increase with excessive loading.
4) Do not operate permanent split
capacitor motors at light loads. An
inherent characteristic of permanent
split capacitor motors is that they
generally run hotter at very light loads
than at rated loads. To prevent PSC
motors from running “too hot”, they
should be matched to the application
with respect to load.
5) Do not exceed nameplate ambient
temperature. Lack of air intake,
obstructions to the ventilation flow, and

excessive deviations from the nameplate
parameters will result in excessive
motor temperatures. Operating at
excessive temperatures will reduce the
motor life, and in general, result in
decreases in motor torque and speed.
High temperatures may also result in
nuisance operation of thermal overload
protectors, and motor start failures
where current-sensitive relays are
employed. These hazards can be
avoided by ensuring that the application
provides adequate ventilation for the
6) Do not indiscriminately change the
value of capacitance. This parameter
applies mainly to permanent split
capacitor motors. Motor start capacitors, used with split-phase motors, are
normally specified to achieve maximum
starting torque and / or minimum locked
current and deviations are not usually
made by the user. Changing to a higher
value of capacitance will increase the
starting torque and in some cases,
speed. It can also introduce hazards
such as: higher winding temperatures,
shortened motor life, nuisance operation
of thermal overload protectors, and
increases in the level of noise and
vibration. The voltage rating of the
applied capacitor must also be capable
of handling the voltage it experiences
during operation.
Problems may be encountered with
safety testing laboratories (UL, CSA, etc.)
if the applied capacitor differs from the
value specified on the nameplate. Always
obtain assistance from the motor manufacturer when evaluating the proposed deviation and explore the possibility of changing
the nameplate rating or developing a more
satisfactory motor design.
7) Do not subject the motor to duty
cycles for which it was not designed.
Continuous (cont.) or intermittent (int.)
duty, as stamped on the nameplate,


indicates the designed mode of
operation for the motor and is generally
based on the motor’s insulation system
class and the power (watts) that the
motor must dissipate as heat when
energized. Adverse effects can develop
from operating a continuous duty motor
in an application requiring a high rate of
starts and stops, or from operating an
intermittent duty motor continuously.
Generally, an adverse deviation in duty
will result in higher winding temperatures
with a shortened motor life and the possibility of nuisance operation of thermal
overload protectors. Increased frequency
of starts could result in failure of electrolytic
motor start capacitors and a reduction in
the life of motor starting switches or relays.
In summary, a motor is designed to provide satisfactory operation and long trouble-free life when operated in accordance
with its nameplate specifications. The motor user should develop an awareness of
the hazards that could result from any deviation from these performance characteristics, and if deviations are anticipated, the
motor manufacturer should be consulted.

Noise, quite simply, is objectionable
sound. The human ear responds to two
different characteristics of noise≡
(loudness) and frequency (pitch). The noise
characteristic of most concern in motor
operation is frequency, since motor noise
can be very annoying (even at low volume)
when its frequency is irritating to the ear.
Objectionable vibration and noise differ
only in the way they are transmitted. Vibration is transmitted by the motor structure to
surrounding parts while noise is transmitted
by the surrounding air. The causes of
motor noise and vibration can be separated

into two general groups: mechanical and
electrical. We will discuss mechanical causes first, since their effects are more

Mechanical Noise
Mechanical noise is usually a result of
bearings, fans or gear trains. Some of the
noise is inherent and can be minimized but
not eliminated.
Another source of noise is the result of
unbalanced rotation. Most motor manufacturers take precautions to balance internal
rotating parts during production. The end
user must take precautions to assure that
motor loads are balanced. Besides noise,
unbalanced rotation can cause premature
wear of bearings and shafts which can
shorten motor life.

Dynamic Unbalance: Dynamic
unbalance is caused by the nonsymmetry of
the rotating member with respect to mass.
Lack of uniform wire spacing in a wound
armature, nonuniformity of rotor material or
attached fan assembly, or eccentricity of
the shaft can all cause relatively noticeable
unbalance. In fractional horsepower motors, balance can be corrected to within
thousandths of an ounce-inch by dynamic
balancing. Standard balance limits are established by manufacturers based on motor
type, weight of the rotating member and
motor speed.
Special tolerance balancing is also possible, but seldom necessary, after other
noise and vibration-causing factors are
checked and corrected. An easy way to
check for dynamic unbalance, in some motors, is to bring the motor up to speed and
then disconnect it from the power source.
If vibration is still present during coasting,
the problem is likely to be mechanical dynamic unbalance.
Ball Bearings: Bearing noise is
very closely related to bearing speed and
preload. Preload refers to an axial force


Fig. 7-3: Typical noise level vs. ball
bearing preload of a fhp motor.

applied to a ball bearing to eliminate “rattling” of unloaded balls. This is commonly
achieved with spring washers of various
configurations which act as the bearing’s
outer race. The inner race is constrained
axially by the shaft shoulder. The amount of
preload necessary to produce minimum
noise levels is amazingly low (below two
pounds for most fhp motor ball bearings).
Refer to Fig. 7-3. Noise-critical applications may require a preload feature consisting of an adjustment screw to transmit the
axial force (preload) to the outer race of
the ball bearing. See Fig. 7-4. A locking
nut maintains the factory-set adjustment
screw position.
Even with carefully manufactured and
electronically inspected ball bearings, motor noise levels below 40 db are very difficult to achieve, and noise levels approaching 60 db are not uncommon. The slightest
variations in ball bearing manufacture can
have significant effects on noise level. For
this reason, pronounced variations in noise
levels (10 db or more) between seemingly
identical motors is common.

Sleeve Bearings: Sleeve bearings have much lower inherent noise levels,
making them the first choice if their load
and service limitations can be met. The
most frequent problem with sleeve bearing
construction is control of thrust washer

Fig. 7-4: End shield showing preload

noise. The intermittent scraping sound
from thrust washers is very difficult to control, and the use of a ball/thrust arrangement is often specified where absolute minimum noise is required.
Since sleeve bearings require clearance
for proper operation (in contrast to preloaded ball bearings), they are sensitive to
radial vibration, which is often experienced
with a powerful motor operated at or near
electromagnetic saturation, or with a high
degree of dynamic unbalance. Under these
conditions, and especially if high temperature thins the bearing oil film, “knocking” or
“pounding” will occur in the bearing. The
motor manufacturer will control shaft-tobearing clearance tolerances more closely
than normal when this condition is likely to

Fans: Fans can be a major source of
noise, even in low speed motors. Noise
from air movement is usually very low in


frequency, at a point where the human ear
is less sensitive. However, the swish or
rumble of air passing through an exhaust
opening can be very annoying. High speed
fan design requires special attention to
avoid a siren effect, and the fan blades
must not be brought in close proximity to a
stationary surface.
NOTE: Noise-measuring equipment
should not be placed in direct line with
substantial air flow, to avoid erroneous
noise level readings.

Gear Trains: Gear trains may or
may not contribute to overall noise levels,
depending on the type of gearing and the
precision with which they were made.
Worm-type gearing, with its sliding contact
action, is normally considered noiseless. If,
however, it has a numerically low ratio with
high input speed, even slight deviations
from print tolerances can cause noise.
Helical gearing is also quiet because its
overlapping teeth produce a smooth transfer of load from tooth to tooth. Spur gearing noise is usually the most difficult to control, especially if maximum ratio per stage
of gearing is used. Under these conditions,
the small number of teeth in contact at any
one time causes a rather abrupt load transfer and resulting noise. This type of noise is
worse under load, and generally increases
in intensity as the load is increased.
An important factor with all types of
gearing is the “backlash chatter” that can
occur at very light loads. At light loads,
even the slightest tolerance deviations in
precision-made gearing will cause very
slight momentary speed changes and resulting noise. Loading the gearing more
heavily can eliminate the noise. Backlash
noise in very lightly loaded gear trains, especially in numerically low ratios, should be
considered normal. (In most cases the applied load is sufficient to load the gearing
beyond the backlash noise point.)

Electrical Noise
and Vibration
Although less obvious than their mechanical counterparts, electrical sources of
noise and vibration can be just as disturbing. Most of the electrical sources of noise
must be minimized at the manufacturing
stage since they are directly related to the
construction and design of the motor rather
than its application.

Saturation: Over-saturation of
magnetic circuits is one of the most frequent causes of excessive electrical noise
and vibration. The magnetic path of any
motor is designed to carry a certain amount
of flux without undue magnetic stress. If the
flux becomes excessive, it will not only
result in increased flux leakage, but sets up
excessive vibration-inducing stresses on the
weakest portion of its path (usually the
stator teeth) with a resultant increase in
electrical noise and vibration.

Distribution of Ampere
Turns: The quietness of motor operation is dependent not only on the strength
of the field flux, but also on how it is distributed in the air gap. The ideal distribution
is sinusoidal, with the windings (of induction motors) placed around the teeth of a
slotted stator so as to produce a sinusoidal
flux configuration. More stator teeth produce a more sinusoidal distribution pattern.
Permanent split capacitor type motors,
which employ two windings for a more
even flux distribution and a true rotating
field, are inherently quieter in operation
than split-phase start motors, running on
one winding with a pulsating field.

Air Gap: The radial length of the air
gap in induction motors has an influence on
motor noise. The air gap in some motors
can be increased to reduce noise. In general, larger air gaps are not desirable, since


Fig. 7-5: Half-view of field and armature
laminations of typical brush-type motor.

they will have an adverse effect on motor
efficiency. Larger air gaps for the purpose
of noise reduction are restricted, therefore,
to applications that can tolerate less motor
output for a given motor volume.
Quieter operation of brush-type motors
can be achieved by increasing or tapering
off the air gap at the tips of the field poles.
See Fig. 7-5.

Fig. 7-6: Half-view of stator and rotor
laminations of typical induction motor.

Number of Stator Teeth and
Rotor Conductors: There are only
certain ratios or combinations of stator
teeth and rotor conductors that will produce a quiet running motor. See Fig. 7-6.
However, combinations which are optimum for quiet operation tend to sacrifice
motor efficiency or torque output. For this
reason (unless quietness is the most important factor), motor designs will always be a
compromise between desirable motor
noise and necessary output and efficiency.
Salient Pole Effect: Reluctance
synchronous rotor cores are normally
flattened or “notched out.” The areas
where ferromagnetic material remains at
the outer diameter of the rotor are called
salient poles. During motor operation,

these poles become areas of relatively
concentrated magnetic force. The
concentrated magnetic force in the salient
poles makes such rotors more susceptible
to magnetic imbalance, and closer
tolerances must be maintained with regard
to rotor position, concentricity and other
magnetic symmetry considerations, in order
to maintain quiet operation of reluctancetype synchronous motors.
By comparison, hysteresis synchronous
motors are inherently quieter because of
their nonsalient pole construction.

Number of Stator Poles: A
basic stator lamination design is usually
employed for all induction winding types of
a given fractional horsepower motor frame,
regardless of the specific operational speed
desired. This is dictated by the number of
stator poles wound into the stator lamination. The stator lamination geometry establishes the magnetic path for all winding
types and is usually optimized for the most
popular operational speed. Four-pole operation is most common. For a given
horsepower output, when such a lamination
is employed, the magnetic noise is usually
less with a two-pole winding. When a fourpole stator lamination is used for six-pole
operation, the higher flux density in the air
gap generates increased magnetic noise per
given hp output.
Frequency of Applied Voltage: Higher harmonics (multiples) of the
line frequency are generated by all induction motors and are taken into account
during lamination design. Conditions of
near saturation or over-saturation magnify
the harmonics and produce unwanted electrical noise. In general, the higher the line
frequency, the more objectionable the
electrical noise generated by the harmonics. At very low frequencies (below 25
Hz), harmonics may cause resonance effects in the motor frame, making it


mit an even commutator film build-up on
the commutator and a resultant reduction in

Armature Slots: The number of
armature slots of a brush-type motor has a
direct relationship to the motor’s noise level during operation. A large number of armature slots is considered preferable, with
an even number of slots being more conducive to smooth and quiet operation.

Noise Control
Fig. 7-7: Typical rotors: a) reluctance
synchronous (top), b) nonsynchronous
(middle), and c) hysteresis synchronous

necessary to use resilient mounting to
dampen the vibrations.

Skewing of Armature or Rotor Cores: Quieter operation can be
obtained when the rotating core is skewed
as shown in Fig. 7-7. This permits the rotor
conductors or armature winding to enter
the magnetic field at an angle, reducing
sudden variations in the circuit reluctance
and minimizing vibration of the stator and
rotor teeth. There are, however, practical
limits to the angle of skew that can be used
because of difficulties encountered in rotor
or armature assembly. Consideration must
also be given to the fact that skewing
somewhat reduces the speed regulation
and efficiency of a motor.

Commutation and Ampere-Turns Ratio: Quiet operation
of a brush-type motor is dependent upon
good commutation. To assure good commutation in wound field motors, a proper
ratio of field ampere-turns to armature ampere-turns must be maintained. Motor
brushes must be designed to ride smoothly
and quietly, and hold sparking to a minimum. Good commutation also depends on
the correct grade of brush material to per-

In addition to measures taken by the
manufacturer to ensure that motors run at
minimum noise and vibration levels, there
are several noise reduction procedures that
can be followed by the motor user. The
general approach to noise reduction can be
divided into reduction of noise at its source
and reduction of the airborne noise level.
The overall study of motor noise and
vibration shows that in addition to the motor design itself, its use or application, its
mounting and the presence or absence of
sound absorbing or reflecting surfaces near
the motor, each affect the measurable level
of sound at the various frequencies generated by motor operation.

Reduction of Noise at Its
Source: Before attempting to reduce
noise “at the source” it is important that we
understand the relationship between frequency and noise or vibration. This is
probably the most overlooked aspect in
noise reduction studies.
Low Frequency Disturbances≡
Mechanical low frequency disturbance is
confined to rotor or armature unbalance
which occurs at the rotational frequency of
the motor. In the case of a 60 Hz, 1800
RPM motor, the rotational frequency is 30
Hz. This frequency is actually below the
normal hearing range. However, vibrations
generated by this frequency can excite audible resonant frequencies in other parts of


Fig. 7-9: Recorded vibration trace of a
typical fractional horsepower motor.
(1800 RPM at 60 Hz.)

Fig. 7-8: Motor frame is coupled to
mounting brackets via resilient material.

the motor unless preventive measures are
The most effective approach to minimizing the effects of low frequency disturbances is to use resilient mountings and couplings. See Fig. 7-8. Resilient elements
such as rubber, felt, cork or springs can be
placed under the feet or between the base
and body of the motor. The ideal mounting
is soft enough so that the natural frequency
of the motor and the support system is
lower than the minimum disturbing frequency. Because of other considerations (such
as deflection of the mounting under load),
the ideal mounting condition is not always
obtainable. In general, it is best to use the
most resilient mounting possible.
In those cases where vibration still presents a problem after resilient mounting,
adding weight to the motor assembly may
effectively reduce the vibration. For example, doubling the weight of a motor assembly can reduce the amplitude of the vibration by half.
An additional problem, often present in
portable equipment, is the use of thin sheet
metal panels as mounting surfaces. Thin
walled structures can act as diaphragms
with resulting “soundboard” effects. Some
trial and error in the addition of stiffening
members, or crimping, may be necessary
to solve problems of this type.

Generally, electromechanically sourced
disturbances for a 60 Hz induction motor
are stronger at 120 Hz and usually negligible above 500 Hz.
High Frequency Disturbances ≡
The major sources of high frequency disturbances (in the range above 500 Hz) are
caused by ball bearings and cooling fans.
Brush noise can also be a factor in brushtype motors.
Ball bearing noise is usually the most
troublesome noise disturbance in induction
motors and almost always occurs in the
1000 to 4000 Hz range. See Fig. 7-9.
Usually selecting motors with sleeve bearings will eliminate these problems provided
it is compatible with the load requirements.
Changing brush materials will help reduce
brush noise but this should not be done
without consulting the motor manufacturer.
See Chapter 5, Sections 5.2 and 5.3. Airborne noise in this frequency range can be
effectively lessened by the use of acoustic
deadening materials.

Reduction of Airborne
Noise Level: An increase in distance
between the noise source and the listener,
or merely changing the relative position of
the source with respect to the listener, can
serve to decrease the noise level.
Acoustical absorbing materials can be
used to control and reduce the noise level.
Such materials are very effective in


reducing high frequency noise. However,
when acoustical absorbing materials are
used, care must be taken to ensure that
motor ventilation is not obstructed.
Almost any degree of reduction of airborne sound can be achieved through the
use of a “total enclosure” or a combination
of several enclosures. Although not as effective as total enclosures, barriers may be
used to shield high frequency sound.
It is important to note that motor heating
usually requires that total enclosures incorporate some means of ventilation. Carefully
designed ventilation ducts, lined with
acoustical material, will assure that the
sound reduction provided by the enclosure
will not be lost by sound transmission
through the ducts while motor heat is being

Since motor overheating and possible
“burnout” of winding insulation materials is
a major cause of motor failure, the effects
of heat on motor parts have long been an
important consideration in the design and
construction of electric motors. No matter
how carefully they are designed and applied, temperatures over the maximum allowed for a given insulation system may
occur under abnormal conditions (see Fig.
7-11). Therefore, in applications where the
load, line voltage, ambient temperature,
duty cycle, form factor, etc., are likely to
change and result in excessive motor temperature, the addition of some type of thermal protection device is advised.
Thermal protectors are available in a
wide variety of designs for specific functions, but all employ some type of sensing
device which monitors motor temperature
and automatically switches the machine off
when a designated temperature level has
been reached. These temperatures are

based on the class of insulation used in the
motor. See Chapter 5, Section 5.4.

Thermal Protection
A motor properly designed for the maximum normal load requirements of a specific application will provide the user with the
desired motor life, safety and reliability, as
long as no abnormal condition arises to
increase motor heating. While the causes
for abnormal conditions such as increase in
the motor load, low or high line voltage,
contamination of lubricants, jamming of the
driven device, etc., are numerous, the end
result is the same≡overheating and possible motor insulation breakdown. While the
breakdown of the motor insulation system
may result in immediate failure of the motor, the underlying cause≡overheating≡ is
less detectable. This is especially true with
fractional horsepower motors, which are
usually “buried” or mounted within an external machine enclosure. Overheating for
prolonged periods will create degradation
of the insulation system, and bearing and
gear reducer lubricants as well. Both types
of degradation result in a reduction of normal motor life.
The National Electrical Code (NEC) is
one basis for determining whether thermal
protection is required. (UL, CSA, VDE
and other safety regulatory agency requirements are also factors.) The NEC dictates
that a separate overload device (thermal
protector) integral with the motor≡or motor impedance protection≡shall be provided for a continuous duty motor (one hp
or less) if the motor is:
a) automatically controlled,
b) manually started out-of-sight of
the motor,
c) manually started and permanently


d) manually started and over
125 volts, and

Intermittent duty motors are treated
separately. The reader should refer to the
latest edition of the Code to avoid any misunderstanding of the subject. Other safety
controls are also considered in the Code.
As indicated by the NEC, there are
various means by which the motor can be
prevented from operating at excessive temperatures. Current-sensitive fuses (usually
selected by the appliance or machine manufacturer), special motor design to provide
high impedance (commonly referred to as
impedance protection), and the use of devices that are sensitive to motor temperature or a combination of motor current and
temperature, can be used to give this additional protection.
Temperature-sensitive protectors or
thermostats commonly consist of a bimetallic disc, which will cause a normally closed
set of contacts in series with the motor
winding circuit to open if temperature exceeds a specified level. The difference in
the rate of expansion between the two
metals, when exposed to heat, causes the
disc to change from a concave to a convex
shape with a snapping action (opening the
contact and de- energizing the motor).
These thermostats are capable of being
calibrated to specific temperatures, usually
within ±5°C.

For motors operated from controls, the
bimetallic contacts will activate a logic circuit which disables the motor. The control
circuit may provide braking and may even
prevent the motor from being automatically
re-energized after cooling.
The type of thermostat commonly referred to as an “in-the-winding” or “onthe-winding” protector is shown in Fig. 710a. These types may be located in the
stator winding slot or winding end-turns.
The “on-the-winding” thermostat will automatically reset when the motor has cooled
sufficiently. Certain appliances could result
in a safety hazard to the operator if automatically re-energized. Therefore, they
should not be equipped with automatic
reset-type protectors.
A manual reset-type protector,
equipped with a reset button that must be
depressed before the motor is re-energized
(even though the motor has cooled), can
be mounted to the motor enclosure. The
primary limitation of temperature sensitive
protectors is that the mass of their enclosures causes a “thermal lag” which prevents the following of rapidly rising temperatures found under locked rotor conditions
in some motor types.
Motor manufacturers also employ protectors which are sensitive to both the motor current and temperature. These protectors (Fig. 7-10b) are designed for placement in the motor enclosure and are available in both manual and automatic reset
construction for single or three-phase motors. Basically, these protectors are similar

Fig. 7-10a: In-the-winding type thermal

Fig. 7-10b: In-the-enclosure type thermal

e) manually started and operated on a
branch circuit where branch circuit
protection exceeds 20 amperes.


from protector manufacturers has greatly
simplified the proper mating of protector to
A successful mating is accomplished
through analysis of motor, application and
protector characteristics. Premature or
“nuisance” trip-outs of the protector during
normal operation are as intolerable (though
less damaging) as failure to prevent the
motor from reaching destructive temperatures. It should be obvious that the proper
matching of a protector and motor is a
tailoring process involving a significant
amount of testing.
UL standards UL-519 and UL-547
define the locked rotor and running

to the thermostats, except that a heater coil
is placed in the proximity of the bimetallic
disc and connected in series with the disc
and motor circuit to rapidly activate the
protector under high motor overloads and
locked rotor conditions.
Therefore, in the application of a current-temperature sensitive protector, it is
essential that consideration be given to the
motor operating current and temperature
(with respect to the ultimate trip temperature of the protector) and the locked rotor
current of the motor (with respect to the
short trip time of the protector). The availability of ultimate and short trip time curves

UL Requirements
1. Maximum acceptable overload and locked rotor temperature limits
(thermocouple method).
A. Thermally Protected Motors (UL-547)
Class A

140 C

1. Running Overload:
Max. Temp.
2. Locked Rotor:

Class A

Class B

a. Automatic Reset:
1) During 1st hour



2) After 1st hour



b. ManuallyReset:
1) During 1st hour
or 10 cycle
(whichever is



2) After 1st hour



Class B

* Max. Ave. Temp.
Class A

Class B





*Multiple windings individually monitored.
B. Impedance-Protected Motors (UL-519)
1. Locked Rotor:

Class A

Class B

1) During 1st 72 hours

150 C


2) During 15 day test



Fig. 7-11: UL-acceptable overload and locked rotor temperature limits for thermally
protected and impedance-protected motors.


overload temperature limits for impedanceprotected and thermally protected motors.
Refer to Fig. 7-11. These temperatures,
which represent maximum limits for motors
employing thermal protection, are higher
than those normally allowed for a particular
insulation class because they are only expected to occur for short durations under
abnormal conditions.
A motor properly designed to meet the
load requirements of an application would
normally operate under much lower temperatures (based on its class of insulation).
The maximum acceptable continuous duty
temperatures are specified in either UL1446 for the type of insulation system employed or in the applicable end use standard for the specific product in which the
motor is being used.
Although we are still faced with the
threat of abnormal conditions attributed to
the causes mentioned earlier, plus the never-ending uniqueness of machine operators
in creating “improbable situations,” the use
of thermal protectors in motors will provide
greater assurance of safe, reliable operation and long life of electric motors.

Proper selection, application and maintenance of electric motors is essential to an
effective energy management program.
With increasing shortages and higher costs,
energy management is becoming increasingly important. It is crucial to mankind
from the standpoint of conservation of natural resources, energy independence and
energy availability. As part of a system,
electric motors play a significant role in
total energy consumption. However, they
cannot be considered alone and are only
one of many factors in the analysis of an
entire system.

Users and specifiers of electric motors
must now, more than ever, understand the
proper selection, application and maintenance of drive components. Reprinted below are excerpts from the NEMA Energy
Management Guide for the Selection
and Use of Polyphase Motors (NEMA
No. MG-10) and the NEMA Energy
Management Guide for Selection and
Use of Single-Phase Motors (NEMA
No. MG-11). Contact NEMA for more

The efficiency of a motor is the ratio of
its mechanical output to its electrical input.
It represents the effectiveness with which
the motor converts electrical energy into
mechanical energy. The efficiency of a motor is a function of the load, horsepower
rating and speed, as indicated below.
1) A change in efficiency as a function of
load is an inherent characteristic of
motors. Operation of the motor at loads
substantially different from rated load
may result in a change in motor
2) Generally, the efficiency of motors, as
measured at rated load, increases as the
motor horsepower rating increases.
That is, large motors are inherently
more efficient than small motors.
3) For the same horsepower rating,
motors with higher speeds generally
have a higher efficiency at rated load
than motors with lower rated speeds.
This does not imply, however, that all
apparatus should be driven by high
speed motors. Where speed changing
mechanisms, such as pulleys and gears,
are required to obtain the necessary
lower speed, the additional power
losses of the mechanisms may reduce
the efficiency of the system to a value
lower than that provided by a directdrive lower speed motor.


A definite relationship exists between
the slip and efficiency of an induction motor
(the higher the slip, the lower the efficiency)
because slip is a measure of the losses in
the rotor winding. Under steady load conditions, squirrel cage induction motors with
less slip should be used, if the application
Slip of an induction motor is expressed
(approximately) in the following equation:
% Slip = ———— x 100
where: NFL = Full load speed
NNL = No load speed
The efficiency of a multi-speed motor at
each operating speed is somewhat lower
than that of a single-speed motor having a
comparable rating. Single-winding multispeed motors are generally more efficient
than two-winding multi-speed motors. Significant energy savings may be possible by
operating at low speeds where possible,
and at high speeds only when necessary.
Motors which operate continuously or
for long periods of time provide a significant opportunity for reducing energy consumption. Examples of such applications
are processing machinery, air-moving
equipment, pumps and many types of industrial equipment. A small change in motor efficiency can make a significant change
in total energy consumed per annum, due
to the lengthy operating time.
While many motors operate continuously, some motors are used for very short
periods of time and for a very low total
number of hours per year. Examples of
such applications are valve motors, dam
gate operators and industrial door openers.
Thus, a change in motor efficiency would
not substantially change the total energy
consumed since very little total energy is
Viewed from a motor losses standpoint,
a modest increase of a few percentage
points in motor efficiency can represent a

significant decrease in percentage of motor
losses. For example, for the same output,
an increase in efficiency from 75% to
78.9%, from 85% to 87.6% or from 90%
to 91.8% may each represent a 20% decrease in motor losses.
For two similar motors operating at the
same specified load but having different
efficiencies, the following equation can be
used to calculate the savings in operating
costs when using motor A rather than
motor B:
100 100
S = (0.746)(hp)(C)(N)(— - —)
Ea Eb
S = savings (dollars per year)
hp= horsepower rating of the specified
C = energy cost (dollars per kilowatt
N = running time (hours per year)
Ea = efficiency (in percent) of motor A at
the specified load
Eb = efficiency (in percent) of motor B at
the specified load
The equation applies to motors
operating at a specified constant load. For
varying loads, the equation can be applied
to discrete portions of the cycle where the
load is relatively constant for a reasonable
increment of time. The total savings are the
sum of the savings for each load-time
period. This equation is not applicable to
motors operating on pulsating loads or on
loads which cycle at rapidly repeating

Motor Losses
An electric motor converts electrical
energy into mechanical energy incurring
losses which are described here in general
terms (for a more accurate explanation of
losses, see IEEE Test Codes 112 and
115). These losses are converted into heat,
causing the temperature of the windings
and other motor parts to rise.


Electrical Losses (vary with
load): Current flowing through the motor
winding produces losses which are approximately proportional to the current
squared times the winding resistance (I2R).
Similar losses result from current flowing in
the squirrel cage of an induction motor.

Iron Losses (essentially independent of load): These losses
are confined mainly to the laminated core
of the stator and rotor. The alternating
magnetic field, essential to the production
of torque in the rotor, causes hysteresis
and eddy current losses that increase with

Mechanical Losses (independent of load): Mechanical losses occur in the bearings, fans and brushes
(when used). In open, low-speed motors,
these losses are small. However, they may
be appreciable in large, high-speed or totally enclosed, fan-cooled motors.

System Efficiency
Since the system efficiency is the
combination of the efficiencies of all of the
components of the system, good energy
management requires a consideration of the
total system of which the motor is a part.
Typical factors to be considered are
covered below.

Motor Rating: The optimum motor
rating necessary to handle the load should
be determined. Where the load is constant,
the appropriate motor rating is readily indicated. A close matching of motor and load
generally optimizes the economic considerations. Moreover, the selection of a motor
rating adequate for the load is important to
avoid unnecessary losses which consume
energy and might overheat the motor. The
use of motors having an output rating excessively greater than the load causes a
reduction in the system power factor, with
resultant added losses in the distribution

Application Analysis: When
the driven machine provides a widely varying load involving a number of stops and
starts, a careful analysis of the application
can result in savings in energy. Operating
conditions such as starts, plug stops, reversals, some forms of braking, etc., all consume energy at rates much higher than
when the motor is operating continuously at
a rated load. When variable duty cycles
are encountered, two actions can be taken
to minimize energy usage. The first is to
reduce the mass of the moving parts wherever possible, because energy used to accelerate these parts is proportional to the
mass or inertia.
Secondly, all aspects of the load should
be carefully analyzed. This should involve
consultation with the motor manufacturer
for recommendations. Motors which are
designed for high full-load efficiency may
not be suitable for applications involving
frequent starting, intermittent duty operation and repetitive pulse loading.
Process and Machinery: The
most efficient process and machinery
should be selected. Frequently, alternate
means are available for doing a job, and a
variety of machines often exist that are capable of performing the task. Once these
determinations have been made, the appropriate motor rating and design type
consistent with system economics can be
First Cost vs. Long-Range
Energy Costs: For variable and
multi-speed drives, the first cost and longrange energy costs should be carefully
evaluated because such systems vary
widely in first cost and in operating efficiency, (i.e., the choice of multi-speed or adjustable speed motors as compared to
throttling control), or the choice of a highspeed motor with speed reduction as compared to a low-speed motor.


Because the electric motor generally
needs little maintenance, it is often neglected. Proper care of the motor will prolong
its life and will conserve the material which
would be needed for replacement if it fails
prematurely. A basic motor maintenance
program requires periodic inspection and,
when encountered, the correction of unsatisfactory conditions. Among the items to be
checked during inspection are: lubrication,
ventilation and the presence of dirt or other
contaminants which form a heat transfer
barrier, alignment of the motor and load,
possible changing load conditions, belts,
sheaves, couplings, and the tightness of the
hold-down bolts.
Sometimes, additional friction develops
within the driven machine as a result of a
dust build-up on the fan, wearing of parts,
misalignment of gears or belts, or insufficient lubrication in the driven machine.
These conditions cause the driven machine
to become less efficient by making the motor work harder, thus reducing system efficiency and increasing energy consumption.
All motors should be provided with
proper overload protection at the time of
their initial installation. If the protective device should trip, the cause should be determined immediately. Increasing the trip rating of the protective device should be
avoided because it may:
1) conflict with the National Electrical
2) permit overheating of the motor,
3) waste energy,

In order to determine the size of a
motor or gearmotor to optimally drive a
given machine, a host of variables must be
known. Perhaps the most significant of
these is the torque or turning force needed
to rotate the machine shaft from standstill
through the different stages of its operating
Torque requirements may vary depending on the machine. In some inertial load
devices, maximum torque is required at the
start to bring the machine up to speed,
while the necessary running torque is a
fraction of the starting requirement.
Other machines such as a printer may
start with no load applied, and at some
point later in the cycle, clutch in the maximum load. See Fig. 7-12. In this application, the average torque must be sufficient
to drive the machine without noticeable
decreases in drive speed when peak loads
are seen by the drive. If the machine can
stop at peak load, the drive starting torque
must be sufficient to start the peak load.
Because these kinds of variations exist, one
must know starting and running torque as
well as peak loads occurring in the machine
cycle. In some cases it is not practical to
measure peak requirements, and average
running torque must be given.
Whenever possible, it is extremely useful for machine designers to supply the motor manufacturer with load diagrams like
that illustrated in Fig. 7-12. Such load vs.
time graphs are valuable in selecting a

4) mask the problem, and
5) create hazards to personnel.
To ensure continued efficient operation
and long motor life, a regular schedule for
inspecting motors and driven equipment
should be established.

Fig. 7-12: Load diagram for a machine
that starts at essentially no load, with
peak loads occurring later in the cycle.


Fig. 7-13: Simple string and pulley torque
measurement method. (Torque = force
reading on scale x radius of pulley.)

motor with the best set of performance
characteristics for a given application.
In making a final load requirement diagram, it is important to consider not only
the load cycle itself, but any anticipated
changes that may occur over the life of the
machine. Most machines will tend to “loosen up” after a break-in period, while some
(particularly those in hostile environments)
may actually “tighten.” Obviously, the load
diagram should reflect the most demanding
torque condition of the machine.
NOTE: This discussion concentrates on the determination of torque
requirements. Other factors are important in final drive selection, and the
Application Guidelines outlined in Section 7.8 should be reviewed before the
final selection is made.
There are three principle means by
which torque can be measured:
1) the “string and pulley”
2) the torque wrench method, and
3) the “test” motor method.

The String and Pulley Method:
Affix a pulley to the shaft of the machine to
be driven. See Fig. 7-13.

Fig. 7-14: Typical torque wrench.

Secure one end of a cord to the outer surface of the pulley and wrap the cord
around it a few times. Tie the other end of
the cord to a spring scale (like those used
to weigh fish). Pull on the scale until the
shaft turns. The force, in pounds indicated
on the scale, multiplied by the radius of the
pulley (in inches) gives the torque or twisting effort in pound-inches (if the scale is
read in ounces, the result will be in ounceinches).
Depending upon the application and if
used carefully, this method is often successful in determining both starting and
running torque. The spring scale reading,
when the pulley begins to turn, indicates
starting force. If a long enough string can
be used, an indication of the average running torque can be obtained. When the
torque characteristics of the machine vary
in different parts of the operating cycle, the
starting torque must be determined at the
point where the motor or gearmotor will
“see” the highest resistance (torque) to

Torque Wrench Method: A
simple torque wrench can also be applied
to the shaft of the machine to be driven.
See Fig. 7-14. Turn the wrench as you


Fig. 7-15: AC motor or gearmotor with
adjustable autotransformer.

would an ordinary pipe wrench, and when
the shaft begins to rotate, read the value (in
ounce-inches or pound-inches) on the
torque wrench gauge. The observed value
represents the torque required to start the
This method is generally limited to
measuring starting torque or peak
torque since it is unsafe and difficult to
continuously rotate a torque wrench.

“Test” Motor Method: Both
AC and DC test motors or gearmotors can
be used to measure a machine’s starting
and running torque. This method requires
more time and instrumentation, but can be
well worth the expense in the long run. It is
the best way to optimally match the machine and drive unit, and is popularly used
for all high volume OEM applications.
Whether AC or DC drives are used, the
method is basically one of experimenting
with an “oversize” drive at reduced power
levels, recording the experimental readings,
and then bench-testing the drive to determine the torque that was being produced at
the recorded readings. The method is actually a variation of dynamometer testing a
machine (the test motor is, in reality, a substitute dynamometer).
AC Method: Use a torque wrench
or “string and pulley” to find the approximate size of the test motor or gearmotor

needed. An AC motor or gearmotor
whose rated output speed is close to the
desired “final” speed of the machine should
be obtained. Next, connect the AC drive,
powered by a variable autotransformer to
the load as shown in Fig. 7-15.
With a voltmeter connected to the line,
increase the voltage supplied by the autotransformer until it starts and accelerates
the load up to speed. (To check the speed,
use a tachometer or stroboscope.) Record
the starting voltage at all possible starting
locations of the device. Next, back off
slowly until the motor breaks down. Read
the voltage and supply the data and the test
motor (gearmotor) to the manufacturer.

DC Method: The DC method, utilizing a permanent magnet DC motor, provides the experimenter with more latitude in
that the speed of the device can be varied.
This can be an advantage if the “final”
speed of the machine has not yet been decided and experimentation is desired for
The DC method requires the measurement of the test motor input voltage and
current once the desired operation of the
load is achieved. Speed of the DC motor is
proportional to voltage while torque is proportional to the current. For maximum accuracy, the actual test motor should be sent
to the manufacturer with the voltage, current and speed information for dynamometer testing. The minimum starting torque
should also be supplied.

While determining the maximum torque
requirement for a potential application is
important, many other performance characteristics may affect machine operation at
different stages of the operating cycle.
The motor speed / torque curve should
be examined to determine if the load can
be started and accelerated to running
speed. When the time accelerate the load


Fig. 7-16: How to read a speed / torque curve.

is a specified requirement, additional acceleration torque must be available in excess
of the needs to overcome friction. It is also
important to be sure that the motor selected can cope with peak load requirements.
The curve shown in Fig. 7-16 contains the
basic speed / torque information for a typical AC squirrel cage, nonsynchronous

Proper application of any motor or
gearmotor requires careful preliminary
planning. The factor which most often determines the success or failure of a motordriven device is the initial care exercised in
matching the load characteristics of the
machine to be driven with the performance
characteristics of the motor to be used as
the driving member. A motor too large or
too complex is unnecessarily expensive to
purchase and operate, while a motor too
small may fail to drive the load under all
conditions to be met in the normal course
of the application.
The characteristics chart shown in Fig.
7-17 provides a good general guide to the
selection of a proper motor with respect to
electrical type, but many other factors must

be taken into consideration before the final
selection is made.
Unfortunately, some of the more important factors are not always apparent and
may be recognized only by an applications
engineer having years of small motor design

Supplying Application
Unnecessary communications, loss of
time, excessive development and experimental costs, and repeated trial and error
can often be avoided if a machine designer
supplies the motor manufacturer with
complete application data before the
design of a driven machine reaches the
detailing stage. Figure 7-18 shows a typical
application data sheet provided by motor
manufacturers to assist product designers
in supplying all information necessary for
motor selection. Since this selection
process is critical, we will consider each
point individually.
1) Product to be Powered? What kind of
machine is it and what kind of work will
it be expected to do? (For example,
main drive for an office copier, reel
drive for a magnetic tape deck, etc.)
2) Estimated Quantity? Is the
production run to be large or small? This


question is asked because the feasibility of
some alternative solutions may depend
upon the quantity projected.
3) What Does the Motor Drive? The
first question defines the end product.
This question determines how the motor
or gearmotor is related to the operation
of the machine. The function of the motor may take on many forms. In its simplest form, the motor may be directly
coupled to the load (as in a grinding
wheel in a lathe attachment). On the
other hand, the motor may be the main

source of power for several functions in
a machine via chains, gears, belts, etc.
4) Power Supply? Since the power available to a plant has, in most cases, already been installed, this is a fixed factor. Here it must be known if AC or
DC is to be used, and the line voltage
or voltages available. Furthermore, if
the source of power is AC, the frequency and number of phases must also
be known. If the source of power is not
pure DC, the form factor must be
known. Sometimes there is a choice of

Fig. 7-17: Motor characteristics chart.



Company _____________________________________________
Address _______________________________________________________


City ____________________________state_______Zip


Phone Number________________________Date____________

This form has been prepared to assist you in supplying us with the basic information required to propose a trial
motor for your application. The success of the motor selected will depend upon the accuracy and completeness
of the information you supply.
1. Product to be powered: ____________________________________________________________________
2. Estimated quantity requirements: Initial order __________ First year ________________________________
3. What does motor drive?____________________________________________________________________
4. Power supply: 115 VAC, 60 Hz ( ). _________________ Other ____________________________________
5. Fixed speed ______________ RPM. Allowable variation _______________________________________ %.
6. Variable speed (universal or DC motors only) ____________ to ____________________________ RPM.
7. Direction of rotation viewing drive end of motor or gearmotor:
CW ( ) CCW ( ) Reversible ( ) Optional ( )
8. Load requirements and conditions: Load data obtained from present practice ( ), estimated ( ), determined by
actual test ( ). If equipment was successfully driven by a Bodine or competitive motor, give complete
nameplate data.
a) Continuous load _____________________________ torque.
b) Intermittent load _____________________________torque.
1) Maximum length of time at full load ____________________________
2) No-load running time ____________________ Average time at rest ______________________________
3) Maximum momentary or peak torque______________________________________________________
c) Reversing service:
1) Maximum reversals per minute __________________
2) Must motor reverse while rotating? ( ) Or from rest? ( )
d) Shock loads, if any. Describe _____________________________________________________________
e) Radial loading:
1) Directly applied type: Indicate (by sketch on next page, No. 20) magnitude, direction, and point of
concentration of loads such as initial belt tension, supported weight, etc. Show front and side views.
2) Reaction type: Indicate (by sketch on next page, No. 20) how motor is coupled to driven load, giving
pitch diameter of pinion, worm, sprocket or pulley, location on shaft, and direction of load. Show front
and side views.
f) Axial loading: What is magnitude and direction of load? (Show by sketch on next page, No. 20). If worm
drive is contemplated, include complete worm data.
g) Direct drive: If load is coupled directly to shaft, describe type of coupling employed __________________
h) Is motor started under load? ________ If so, what is starting torque required? _______________________
i) Is load of inertia (flywheel) type? ___________________________________________________________
j) Is time a factor in bringing load up to speed? __________________________________________________
9. Life expectancy of motor _____________ hours. (Motor life varies with operating and load conditions, and
duty. Normal duty is considered to be 8 hours per day, 5 days per week, or 2000 hours per year.)
10. How frequently will motor be serviced? (annually, quarterly, monthly, never)
b) brushes________________________________________________________________________________
c) general cleaning _________________________________________________________________________

Fig. 7-18: Application data sheet (continued on next page).


11. Space and weight limitations, if any ________________________________________________________
12. Motor mounting: Standard Floor ( ), Other ( ). Show by sketch (in space below, No. 20) if other than
standard floor mounting.
13. Temperature surrounding motor: Max.
______________________ °F, Min.
______________________ °F
14. Is equipment designed to provide adequate ventilation to motor? __________ How? _____________
15. What is the condition of the air surrounding the motor? (dusty, gritty, humid, acid, explosive, etc.)
16. Shaft end play restrictions _________________________________________________________________
17. Shaft dimensions if other than standard _____________________________________________________
(If shaft features are complex, show by sketch.)
18. Electrical leads:
a) Bodine standard acceptable ( ).
b) Special material or length ( ). Describe _________________________________________________
c) Cord ( ). Describe, including type, length, plug or switch specifications, etc. __________
d) Terminal box ( ).
19. Give additional requirements not covered by the above data such as UL, CSA, sanitary, municipal or
military, braking, overload protection, degree of quietness, etc. (Describe fully)__
20. Use this space for sketches as required.

Fig. 7-18: Application data sheet (continued from previous page).


there is a choice of currents and voltages. In situations involving unusual
voltages or voltage fluctuations, high
form factors, or unusual and varying
frequencies, special care must be exercised in selecting a motor. The power
source, therefore, must be fully defined
and understood before proceeding.

Most motor manufacturers have
adopted this designation, but some,
including the Bodine Electric Company,
have historically considered the direction of rotation of motor and driveshafts
to be that which is seen when looking at
the end of the shaft, and so indicate in
their literature.

5) Fixed Speed? Allowable Variation?
The answer to the first half of the question will usually establish whether a motor or a gearmotor is required. The
variation allowable will establish the
speed constant required; that is, if the
motor is to be of synchronous or nonsynchronous type, or if tachometer
feedback or openloop control is

Since there is inconsistency between
motor manufacturers, there is always
the possibility of misunderstandings
which can result in motors being wound
for the wrong direction of rotation. To
avoid this, when specifying the direction
of rotation of unidirectional motors or
gearmotors, always include a point of
reference. For example, in the case of a
single-shafted motor, a typical specification might read: “Rotation clockwise,
facing end of shaft,” or in the case of a
single-shafted gearmotor: “Rotation
counterclockwise, facing the end of the
driveshaft extension.”

6) Adjustable Speed? Universal (series
wound), brush-type DC or brushless
DC motors are usually indicated if adjustable speed is required. Brushless
DC motors offer excellent speed regulation plus less maintenance and greater
torque-per-motor frame size than
brush-type DC motors. Series motors
can be adjusted over a wide speed
range by means of a rheostat, adjustable autotransformer or an electronic
speed control. However, due to loss in
torque with decrease in voltage, the
practical speed range is usually limited.
Shunt-wound motors and PM motors
used in conjunction with SCR or similarly controlled power sources are better suited for applications requiring relatively constant (with respect to load)
but adjustable, speed over wide ranges.
7) Direction of Rotation? The National
Electrical Manufacturers Association
(NEMA) has established that the standard direction of shaft rotation for all
DC motors, all AC single-phase motors
and all universal motors shall be counterclockwise when facing the end opposite the driveshaft.

Motors or gearmotors with multiple
shafts present special communication
problems. In these cases a point of reference should be the extension that is
depicted as “standard” on the catalog
dimension sheet. For example, in the
case of a motor, the specification might
read: “Rotation clockwise, facing extension at end opposite leads,” or in the
case of a gearmotor: “Driveshaft rotation clockwise, facing end of left-hand
extension.” Use of the sketch space
under Item 20 in the application form
(Fig. 7-18) will help to alleviate any
possibility of error in complex cases.
8) Load Requirements and Conditions?
This question basically asks:
1) what is the power or torque
requirement, and
2) how is it determined.


It is quite possible that the design
engineer has determined the power
requirements analytically or by some

mechanical means, accomplishing the
latter by the string and pulley method
(Section 7.6) or by actually powering
the device with a test motor. If the load
were determined by use of a test motor,
it is probable that tests were run at rated voltage. There is always the possibility that the test motor developed more
power than was actually necessary for
the application and that a motor providing less power, and quite possibly less
costly, would be adequate for the

8a) take on vital significance since the
answers determine the extent to which
heat generated under load will be dissipated during the time the motor is operating at no-load or at rest.
8c) Reversing Service? It might seem at
first that the only reason for this question is to select the winding type. While
this is true, reversing service is also an
important factor in the mechanical life of
gearmotors, and in brush life of DC or
series wound motors.
The reply to this question must be
weighed with other information provided about the load to determine its relative importance. For example, if the
load is inertial and must be reversed, it
could produce excessive shock loads
on the gear train, possibly necessitating
a slip clutch on the output shaft to reduce the shock.

This possibility can best be established
by employing a variable autotransformer and measuring the minimum voltage
required to start and drive the load. By
means of a brake test on the same or an
identical motor, one can then measure
the torque developed at the minimum
voltage and establish the magnitude of
the actual load under starting and running conditions. There is a tendency for
design engineers to specify their power
requirements in terms of horsepower.
It is better, in all cases, to establish
the power requirements in terms of
torque. This is especially true for
gearmotor applications.
8a-b) Continuous Load or Intermittent
Load? Once the magnitude of the
load has been determined, we are
ready to define the duty cycle as
continuous or intermittent. By definition, a motor which continues to
operate after it has reached normal
operating (steady) temperature is
operating under continuous duty
conditions. Conversely, one which
never reaches a steady temperature,
but is permitted to cool between
operations, is operating under inter
mittent duty conditions. Intermittent
duty motors are given a time rating
by the manufacturer. It can be seen,
then, that the subparts of question

Basically, we should be concerned
with the frequency of reversals, and
whether the motor must reverse while
rotating or from rest. In connection with
the latter, there are some applications
where the design engineer may specify
“Motor must reverse in three seconds.”
If this is specified, the inertia of the load
must also be given. (See 8i.) One
would then analyze feasibility of
reversing with different kinds of motors.
A sample motor may need to be built to
determine if the requirement could be
8d) Shock Loads? It is important to
establish if shock loads exist in the
application. Although we all have an
intuitive idea of what shock loading is,
formulating a precise definition (without
resorting to mathematical terms) is
somewhat difficult, and long-term testing by the customer of a drive may be
required to establish the suitability or
fitness of a drive for the application.


Fig. 7-20: Sketch illustrating a typical
overhung load application.

An important area not to be overlooked is whether the load will be
braked or reversed, or both, when
driven by a gearmotor (especially one
with “self-locking” worm gearing). In
the case of inertial loads, such service
can result in severe shock if mechanical
protection devices are not employed.
The method of braking (including point
of application) and reversal should be
described thoroughly.

Fig. 7-19: Typical applications imposing
axial and radial loads.

The important aspect of all common
definitions of “shock” is that they imply
a degree of suddenness and severity.
The combination of these two parameters will have immense consequence in
determining the overall life of a drive
system. When describing a shock load
condition, it is imperative to state as
accurately as possible (in terms of time)
the degree of suddenness and (in terms
of torque) the severity to which the
motor or primarily the gearmotor will be
Running a drive against a stop is the
one most commonly thought of shock
condition. However, since shock
loading is a matter of degree, the
complete load requirements of the
application must be studied. Loads
which vary significantly and can be
classified as shock loads should be
described thoroughly (with a torque vs.
time diagram, if possible). Common
examples of more moderate shock
conditions would be clutched inertia
loads or cam loads. In the case of the
clutch, the amount of inertia and the
time of clutch engagement should be
reported on the application form. For
cam loads, a dimensioned sketch of the
cam on the reverse side of the form and
a description of the load will greatly
assist the drive manufacturer.

The effect of shock loading on the
overall life of a drive system cannot be
overemphasized. Extreme care and
attention should be given to this portion
of the application information form.
8e) Radial or Overhung Loads? These
are loads which are applied in a direction perpendicular to the axis of the
shaft. These may be directly applied as
shown in Fig. 7-19, or reaction type as
shown in Fig. 7-21. Examples of the
first type are loads imposed by belt or
chain tensioning and loads created by
supported weights such as those found
in hoist applications. Examples of the
second type are loads which are developed when the shaft is coupled to the
load through belt or chain drives or
through external spur, helical, bevel or
worm gearing.


A sketch, like the one in Fig. 7-20,
should be used to describe the radial
loads to be expected in an application.
Figure 7-20 shows an application

employing a belt and pulley coupling.
Given the torque at normal operating
speed (Item 8a of Fig. 7-18) and the
pitch radius of the driving pulley, the
driving force at the point of application
can be calculated as follows:
Driving force = torque ÷ pulley pitch
Driving force = tension 1 - tension 2
The overhung load to which the driving shaft will be subjected is determined
by adding the total initial belt tension
applied in a direction perpendicular to
the axis of the shaft.
8f) Axial or Thrust Loads? These are
loads which are applied in a direction
parallel to the axis of the shaft. They
may be directly applied as shown in
Fig. 7-19, or the reaction type as
shown in Fig. 7-21.
Axial fans or directly supported turntables and centrifuges are typical applications developing direct axial loads.
Reaction type thrust loads are typically
found in applications employing helical
or worm gearing to couple the motor or
reducer to the load.
In most cases, directly applied axial
loads are those developed in applications where the motor or reducer shaft
is vertical. In the case of plain motors, it
must be known whether the shaft will
be up or down, since the weight of the
rotor must be taken into consideration.
The thrust developed in gear reaction
loads is the product of the driving force
and the tangent of the external gear
tooth helix angle. It is necessary, therefore, for the designer to provide information about the actual torque loading
and details regarding the external worm
or helical pinion in order for the axial
load to be calculated.

8g) Directly Driven Loads? Properly
aligned directly driven loads are those
which present only “pure” torque loads
to the motor or gearmotor driveshaft
and its bearings. If radial or axial loads
are present, they are carried instead by
bearings in the equipment being driven,
in which case the motor is usually coupled to the load by means of a flexible
coupling to avoid alignment problems
or, in some cases, to reduce shock.
Couplings usually employed for directly driven loads include steel sleeve,
multi-jaw, jaw types with resilient inserts and universal joints. Each has its
own unique characteristics and knowledge of the type of coupling to be employed is of value in determining if the
motor will be properly applied.
8h) Is the Motor Started Under Load?
This section prompts a “yes” or “no”
answer, but in some unusual cases, it
might be answered “sometimes.” There
are many applications where the motor
normally “sees” little or no load at start
but, at certain points in the load cycle,
will experience maximum possible starting load. For example, in an electric
typewriter application, the maximum
load condition normally occurs when
the carriage is being shifted. Should the
operator turn the machine off, or should
the power plug be inadvertently pulled
at this load point, the motor must be
designed to develop sufficient starting
torque to overcome the load when the
power is restored. For applications of
this type, it is useful to obtain information as to load variations expected
throughout the operating cycle.


The answer to the second part of the
question (“If so, what is the starting
torque required?”) should be a real
number expressed in oz-in., lb-in., kg-

cm, n-m, etc. This can usually be determined by the string and pulley method.
(Refer to Section 7.6.)

8i) Is Load of Inertia (Flywheel) Type?
When the reply is “yes,” we should
obtain information about the actual load
inertia or WR2 (sometimes referred to
as WK2). If the information is

If conditions do not permit exact measurements of t1 & t2, the following are generally accepted approximation
Chain: PO = 1.0 WT
V-belt: PO = 1.5 WT
Timing belt: PO = 1.2 WT
Flat belt: PO = 2.5 WT
Gear: PO = 1.2 WT

WT = Tangential force
PO = Overhung load (force)
PS = Separating load (force)
PT = Thrust load (force)
R = Pitch radius (length)
T = Torque (force x distance)
t = Belt tension (force)

Ψ = External gear tooth helix
(zero for spur gear or chain drive)
→ = External gear tooth transverse pressure angle
∝ = angle of force WT along line connecting shaft
centers with respect to a defined datum line on a gearhead
L = Distance from housing datum

Fig. 7-21: Method for calculating overhung and thrust loads on gearmotors: a) for
driving belts and chains (top), and, b) for driving via external spur or helical gearing


unavailable, it may be necessary to
send the device to the motor
manufacturer for testing.
Load inertia information is especially
important if a salient pole synchronous
motor is being considered as the rotary
power source. The reason is that the
“pull-in” to synchronism torque capability of the motor must be great enough to
overcome the WR2 or combined inertia
of the motor and the driven load.
8j) Is Time a Factor in Bringing the
Load Up to Speed? This relates mostly
to inertial loads which invariably use
more power to start and accelerate to
running speed than they do to keep
running at full speed. The torque required to accelerate the load from
stand-still to running speed varies inversely with the time allocated for acceleration. Therefore, it is necessary to
know if there is any minimum time limit.
If so, the limit should be specified here.
9) Life Expectàncy of Motor (Number
of Hours)? Life expectancy is extremely important in the selection of the best
and most economical motor or gearmotor for the application. In addition to
supplying information about total life
expectancy in hours, it is important to
establish the number of starts and the
expected running hours over a given
period of time.
An example of manufacturer standards
for life expectancy under normal operating conditions are:
a) steady load with no shock,
b) continuous duty, eight hours
perday, five days per week,
c) infrequent starts,
d) ambient temperature of 0°C to
e) voltage to be within 10% of
nameplate rating, and
f) frequency to be within 5% of
nameplate rating.

In addition, altitude limits are sometimes specified or implied. The life of
most motors may be greatly affected by
any deviation from normal operating
Temperature is particularly important,
as motor life expectancy is a function of
total temperature. Insulation, lubricant
and seals are all affected by temperature. This is illustrated by the following.
1) As a general rule, ball bearing or
gear lubricant life is halved for every 25°F (approximately 14°C)
increase in temperature. Heat will
eventually degenerate most lubricants and seals, leading to leakage,
increased friction and extra
2) Generally, the motor insulating life is
halved for each 10°C increase in
total temperature.
Therefore, it is apparent that temperature has a direct bearing on the life of a
given motor. When considering life expectancy, we should cross-reference
the following application considerations
that directly affect the motor’s operating
a) bearings,
b) lubricants,
c) duty cycle,
d) radial load,
e) axial load,
f) mounting,
g) enclosure,
h) ambient temperature,
i) ventilation, and
j) electronic controls.
10) How Frequently Will the Motor be
Serviced? Answers to this question in
conjunction with information concerning
life expectancy, duty and ambient temperature are important in selecting the
best bearing and gear lubricant. Similarly, brush selection, in the case of series
wound or DC motors, is dependent to
a degree upon the service anticipated. If


cleaning is seldom or not expected, a
totally enclosed motor may be necessary, depending on the environment.

tion is usually described in a dimension
diagram supplied by the manufacturer.
In the “standard” position, the axis of
the motor lies in a horizontal plane. For
gearmotors in standard mounting position, the axis of the output driveshaft
also lies in a horizontal plane. The
choice of motor mounting may depend
on motor design, operating conditions,
space requirements and life expectancy.
Factors to be considered include:

11) Space and Weight Limitations? If
space is limited, this becomes a very
important consideration. Show the
maximum space envelope (using a
sketch) and indicate how and where the
load should be coupled to the motor or
driveshaft. The sketch should also show
any space restrictions caused by interference with other components.

a) sleeve vs. ball bearings,

In analyzing an application’s space
and/or weight limitations, the associated
cost elements must be recognized. Here
are a few general areas which might be

b) oil vs. grease lubrication,

a) Where a reduction in speed is
needed, an integral speed reducer
motor should be considered. By
combining the motor and speed
reducer in one unit, cumbersome
and complicated speed reduction
transmissions can be avoided. This
alone may resolve the space

In all sleeve bearing motor and/or
gearmotor applications, the mounting
must be specified. If the unit and/or the
output driveshaft is rotated from horizontal to another position, almost without exception a different lubrication
arrangement is required (sometimes at
additional cost).

c) ventilation,
d) care and servicing, and
e) special modifications.

The nature of the differences will depend largely upon the choice of mounting and/or whether the application requires an oil-lubricated or grease-lubricated gearmotor.

b) If space and weight for the motor is
figured too closely, a totally new
redesigned special purpose motor
may be required. This could involve
extensive engineering and special
tooling. One of the most frequent
application mistakes is to ignore the
potential need for more space to
accommodate a larger motor if one
is required at a later time.

Special lubrication arrangements can
include new location of drain, fill, vent
and level indicators, or special oil seals.
Mounting the gearhead above the motor is not recommended and should be
avoided because of the risk of lubricant
leakage down into the motor if a seal
fails or wears out. Lubricant leakage
into the motor can cause motor failure
with additional hazards to personnel
and equipment.

c) If the design does not afford sufficient motor ventilation to keep the
temperature rise within tolerable
limits, a larger and more expensive
motor may be required.
12) Motor Mounting? A sketch should be
used if standard mounting cannot be
adapted. The standard mounting Posi-

13) Temperature Surrounding Motor?
This is the “ambient” temperature and
directly affects a motors life expectancy.


Most locations expose a motor to
the normal operating range (0°C to
40°C or 32°F to 104°F). Temperatures
above or below this range may create
lubrication problems in both motors and
gearmotors or insulation problems.
Temperatures lower than normal may
require special considerations in order
to provide adequate starting torque due
to stiffening of bearing and gear lubricants. Also, a time lag may exist in
reaching operating speed, which could
affect the performance of the driven
Temperatures higher than normal
present lubrication and sealing problems
because of viscosity changes in the lubricant. In addition, the maximum operating temperature for the winding insulation system is established on the basis
of the motor type and insulation class.
14) Is Equipment Designed to Provide
Adequate Ventilation to the Motor?
A motor in a suitable ambient temperature may still overheat if the equipment
confines the motor in such a way that its
generated heat cannot be dissipated.
The ambient temperature in close proximity to the motor should never exceed
the nameplate value (normally 40°C).
A motor external to the equipment in a
suitable ambient temperature is exposed
to circulation of free air and normally
would have adequate ventilation. A
motor housed within the equipment
needs ventilation. Depending upon the
degree of confinement, circulating free
air may be provided from vents in the
equipment housing, or by forced ventilation.
15)What is the Condition of the Air Surrounding the Motor? Dust, grit, humidity and acid fumes can damage motors. Airborne particles may clog


ventilation openings, preventing sufficient heat transfer. Moisture and fumes
may deteriorate motor components.
The answer to this question helps define
the type of enclosure, environmental
treatment, shaft materials and lubricants
Open, ventilated motors are suitable
for clean, dry locations where cooling
air is not restricted. Enclosed products
are suitable for dirty, damp locations.
For outdoor use, wash downs, etc.,
enclosed products must be protected
by a cover while still allowing adequate
air flow.
In open-type motors, sparking of
starting switches in AC motors so
equipped, and of brushes in commutator-type motors can be expected during
normal operation. In addition, opentype enclosures may eject flame if the
insulation fails. Therefore, avoid placing
open-type motors, gearmotors, or controls in the presence of flammable or
combustible materials.
Most totally enclosed products are
not explosion-proof. Explosion-proof
motors, gearmotors and controls should
be used for hazardous locations (flammable/explosive gas, vapor, dust).
When dealing with hazardous locations,
an approved explosion-proof product is
the recommended approach. Exceptions are allowed by the National Electrical Code. NEC and NEMA safety
standards should be studied thoroughly
before exercising this option.
Moisture increases the electrical
shock hazard. Open-type motors
should always be protected from
moisture. Totally enclosed motors will
reduce the hazard if all openings are

16) Shaft End Play Restrictions? Standard “end play” (or axial shaft freedom)
of rotor (or armature) shaft and gearmotor driveshaft is controlled by the
manufacturer during assembly. Some
typical end play specifications are as

supported shafts. Any limited end play
requirement would necessitate special
gauging fixtures for assembly and final
inspection checking. It is not practical in
production to space a sleeve bearing
assembly to zero end play. In subfractional horsepower motors, added frictional losses resulting from a zero end
play tolerance could mean the difference between success and failure. Additionally, within a short period of time
(providing the motor does not overheat
and fail), the washers or bearing faces
will wear away and end play will develop regardless of precautions.

On sleeve bearing supported shafts:
a) Soft spacing washers limit motor
and armature end play to within
0.005 to 0.020 inch.
b) Reducer driveshafts are limited to
within 0.005 to 0.020 inch end play
by means of hardened washers of
varying thickness.
On ball bearing supported shafts:
a) The ball bearings of rotors or
armatures are preloaded by means
of spring-type washers to provide
quiet bearing operation under cold
and normal operating temperatures. This results in essentially no
free end play of the shaft unless a
sufficient axial force is applied.
b) The ball bearings of the secondary
shaft and the driveshaft of many
gearmotors are spaced to a minimum of end play by flat steel
washers of various thicknesses as
On needle bearing supported shafts:
a) In this type of bearing, the drive
shaft acts as the inner race of the
bearing and consequently is similar
in free end play to that of a sleeve
bearing. The sections of the shaft in
the journal area are hardened. End
play is typically limited to within
0.005 to 0.020 inch by means of
spacer washers.
In rare cases, requests are made for
more closely held end play than standard tolerance for sleeve bearing

17) Shaft Dimensions if Other Than
Standard? This detail on the application form. causes little or no problem
unless a designer wants a special feature such as a cross-drilled hole located
“x” inches from the bearing hub or centered to something less than up to .006
TIR. The normal method of dimensioning the location of a cross-drilled hole, a
cross-milled flat or the shoulder of a
reduced diameter on a shaft extension is
from the end of the shaft, and the normal tolerance is ± 0.005 inch (0.196
mm). Ball bearing supported shafts
have no free end play and the normal
tolerance of the extension is ± 0.032
inch (0.8 mm). When checking the
length of a sleeve bearing supported
shaft, the measurement should be made
with the shaft pulled out. Under these
conditions, the tolerance is the same as
18) Electrical Leads? This item offers a
choice of connections from the motor to
the power source. Popular lead materials generally consist of individually
tinned copper strands. Insulation is
polyvinyl chloride or x-linked. polyethylene. If the designer requires something
different, the number of strands and the


type and color of insulation should be
included. Of course, the motor manufacturer’s standard leads are the most
economical choice.

overload relays, thermostats and
inherent overheating protectors. See
Section 7.4. Fuses and relays are
sensitive to motor current only. Ther
mostat devices, usually in direct contact with motor windings, respond to
temperature only. Inherent overheating protectors respond to the total
heating effect whether it is caused by
temperature alone, current alone or
the combined effect of both. Caution
must be exercised if “automatic”
reset protectors are used ≡ they can
reset without warning and be hazardous to personnel.

19) Give Additional Requirements Not
Covered by the Above Data Such as
UL, CSA, Braking, Overload Protection, and Quietness.
Considering the unlimited application
possibilities involving small motors and
gearmotors, it would be impossible to
cover every application consideration in
one questionnaire. This space provides
information for any special requirements
not covered in the form. A continuation
sheet should be attached if needed. The
following comments apply to some of
the specific examples listed.

d) Quietness - This is a complex
problem including both mechanical
and electrical design. See Section
Although the foregoing is by no
means a complete analysis of all the
factors, it should provide a guideline for
motor selection. It should again be
stressed that the more time spent on this
planning phase to provide the motor
manufacturer with accurate, relevant
information about the device to be driven, the easier it will be to match the
right motor to the application.

a) UL, CSA, etc. ≡ Many applications require compliance with one
or more of these organizations’
standards. Their specific require
ments should be made known to
the motor manufacturer at the out
set. At times, there are charges to
the motor manufacturer for “third
party” approvals.
b) Braking ≡ Frequently, the power
transmission must be braked or
stopped by some mechanical or
electrical means. Complete data de
scribing the method of braking required is essential (for example, frequency of braking time required to
stop), and whether or not holding
torque must be present after the motor has been stopped.
c) Overload Protection ≡ This may
be a requirement of the testing or
standards writing organization (as
in No. 19a.) Four basic types of
overload protective devices are normally used with fhp motors: fuses,

Applying fhp
When gearmotors are specified, there
are many factors to consider in addition to
those mentioned previously. This is due to
the gearing and the effects it has on other
parts of the system.

Inertial, Reversing and
Overrunning Loads: Inertial loads
with high reduction ratios often produce
extreme torque multiplication between input and output shafts.


The motor and gearhead must be sized to
sustain the torque developed when starting
or stopping this type of load. Reversing an
inertial load should be avoided unless the
gearing is disconnected from the load, and
the load braked before reversal.
Overrunning loads can be inadvertently
imposed on the gearhead. For example,
power failure or disconnect on an elevating
device driven by a gearmotor can cause
the load to drive the gearmotor in reverse.
If backdriving of a gearmotor is contemplated, the manufacturer should be contacted since many gearheads can be easily
damaged by backdriving.

Service Factors for Gearing: Service factors are correction factors which compensate for nonstandard
load conditions and are applied to torque,
overhung and thrust load ratings of gearing.
These factors compensate for variable and
shock loads. The service factors are not as
well defined for gearmotors below approximately 1/8 hp as they are for larger units,
and judgement should be exercised in their
application. Unfortunately, there is no common agreement among small motor and
gearmotor manufacturers to the magnitude
of various service factors.
Service factors, developed through experience, are useful for estimating the severity of the actual duty, compared with
average duty. The service factors (Fig. 722) as indicated for classes of service defined, are provided as application guidelines. They should be multiplied by the uniform steady, or average torque of the load
resulting in “equivalent required torque:’
Equivalent required torque = service
factor x uniform steady torque.
Equivalent required torque should not exceed rated torque of the gearmotor.

Type of Load
Uniform Steady
Light Shock
Moderate Shock
Heavy Shock

8 Hr.

24 Hr.



Fig. 7-22: Service factors for various
types of loads.

Figure 7-21 provides formulas for calculating overhung and thrust loads on gearmotors. The following application guidelines also apply to the classification of load
Uniform Load ≡A load which does
not vary appreciably during operation or
changes gradually. Blowers or chart drives
would be in this category.
Moderate Shock ≡ A load which
varies significantly during operation or is
applied rapidly. Clutched loads of low
inertia or cam loads would likely be in this
Heavy Shock ≡ A load which varies
greatly in a relatively short time. Inertial
loads braked or reversed through nonlocking gearing would be in this category.
Extreme Loads Not Covered ≡ An
impact load or high speed, high inertial load
driven by self-locking gearing cannot be
covered by service factors and must be
referred to the motor manufacturer.
No matter how well a motor or gearmotor is constructed, improper application
can result in poor performance or complete
failure. The foregoing illustrates the proper
approach in the evaluation of the load to be
driven by a motor or gearmotor. To aid in
the selection procedure, most manufacturers can provide a selection worksheet
which serves as a convenient checklist for
both the customer application engineer and
the manufacturer.


The use of electric motors and
generators is potentially hazardous. The
degree of hazard can be reduced by
proper design, selection, installation and
use, but hazards cannot be completely
eliminated. Hazard reduction is the joint
responsibility of the user, the manufacturer
of the driven or driving equipment and the
motor manufacturer.
Many motors, gearmotors and speed
controls are designed and manufactured to
comply with applicable safety standards,
and in particular with those issued by
ANSI, NEMA, UL and CSA. In addition,
many overseas standards are being followed. In particular, IEC (International
Electrotechnical Commission) standards
are gaining influence.
Furthermore, many products are “third
party approved” with respect to construction. Motors, gearmotors and controls recognized by UL are designated with a code
on their nameplates. The use of codes is
unique to each manufacturer. Each manufacturer must be consulted as to the status
of their “third party approval,” if any.
However, since even well-built apparatus can be installed or operated in a hazardous manner, it is important that safety
considerations be observed by the user.
With respect to the load and environment,
the user must properly select, install and
use the apparatus. For guidance on all
three aspects, see Safety Standards Publication No. ANSI C51.1/NEMA MG-2*.

Before proceeding with the installation,
the user should review the application to
confirm that the proper drive has been

selected. This should be done after thoroughly reading and understanding Section
7.8 and all applicable safety standards. If in
doubt, contact the manufacturer.
Selections or application suggestions made in this Handbook are intended only to assist the reader. In all
cases, the reader is solely responsible
for determining a product’s fitness for
application or use.

It is the responsibility of the equipment
manufacturer or the person installing the
motor to take diligent care in installing it.
The National Electrical Code (NEC),
sound local electrical and safety codes, and
when applicable the Occupational Safety
and Health Act (OSHA) should be followed when installing apparatus to reduce
hazards to persons, other equipment and

Examine the motor for damage from
shipping before connecting. Do not attempt
to turn the output shaft of a gearmotor with
an externally applied torque arm.

Follow the nameplate for voltage, frequency and phase of power supply. See
the accompanying wiring diagram for connections and rotation (and capacitor, if
required). Make sure that the motor, gearmotor or control is securely and adequately
grounded. Failure to ground properly
may cause serious injury to personnel.
(If the wiring diagram shipped with the
drive unit is lost or missing, contact the

*Standards Publication No. ANSI C51.1/NEMA MG-2 Safety Standard for Construction and Guide for Selection, Installation, and Use of Electric Motors and Generators
is available from the National Electrical Manufacturers Association, 2101 L Street, NW,
Washington, D.C. 20037, USA.




For wire sizes and electrical connections, refer to the National Electrical Code
(NEC) article covering motors, motor circuits and controllers, and/or applicable
local safety codes. If extension cords are
used, they should be kept short for minimum voltage drop. Long or inadequately
sized cords can cause motor failure, with
hard starting loads when current draw is at
its highest.

The chance of electric shocks, fires or
explosions can be reduced by giving
proper consideration to the use of
grounding, thermal and overcurrent
protection, type of enclosure and good
maintenance procedures.
The following information supplements
the foregoing safety considerations. This
information is not intended to be all-inclusive, and other applicable sections of this
Handbook as well as local and national
safety codes should be referenced and
understood before operating electric
1) Do not insert objects into motor
ventilation openings.
2) Sparking of starting switches in certain
AC motors, and of brushes in commutator-type DC motors, can be expected
during normal operation. In addition,
open-type enclosures may eject flame
in the event of insulation failure. Therefore, take all necessary precautions to
avoid, protect from or prevent the presence of flammable or combustible materials in the area of open-type motors,
gearmotors and controls.
3) When dealing with hazardous locations
(flammable or explosive gas, vapor,
dust), make certain that an approved,
explosion-proof or dust-ignition-proof
motor is specified.
4) When dealing with any environment that
is unusual such as high humidity, high
altitudes, low humidity, exposure to
weather, etc., make certain that the
proper motor has been specified. Refer
to Section 5.5 for environmental classifications of motors.
5) Moisture will increase the electrical
shock hazard. Special care should be
exercised whenever moisture is present
to avoid electrical shock.
6) Products equipped with thermal protectors are required to be labeled “

Before starting the motor:
1) Check all connections and fuses.
2) Be sure keys, pulleys, etc. are securely
fastened. Proper guards should be
provided to protect personnel from
hazardous rotating parts.
3) Other mechanical considerations include
proper mounting and alignment of products and safe loads on shafts and gearing. Do not depend on gear friction to
hold loads.
When starting the motor:
1) Test-start the motor or gearmotor in an
unloaded state. (Because of possible
reaction torque, the drive should be
securely mounted when started, even
when unloaded.)
2) If the drive unit does not start promptly
and run smoothly, disconnect it at once.
3) If you are unable to correct the problem, contact your purchase source or
the manufacturer, describing the trouble
in detail. Include the serial number, type
and other nameplate data. Do not dismantle the product unless authorized by
the manufacturer; removing screws
voids many warranties.


Thermally Protected.” If severe overloading, jamming or other abnormal
operating conditions occur, such heatsensitive protectors operate to open the
electric power supply circuit. Motors/
gearmotors with automatic thermal protectors must not be used where automatic restarting of the drive unit could
be hazardous, in that clothing or parts
of the human body could be in electrical
or physical contact with a machine that
starts unexpectedly when the thermal
protector cools down. “Manual reset”
protectors or suitable electric supply
disconnect devices/procedures should
be used where such hazards could be
7) Motors/gearmotors which employ capacitors can develop more than nameplate voltage across the capacitor and/
or capacitor winding (depending upon
design). Suitable precautions should be
taken when applying such motors.
8) Abnormal conditions, such as cut-out
switch failure, or partial winding failure
due to overheating, etc., can, on rare
occasions, cause certain types of AC
motors / gearmotors to start in a direction reverse from normal. The chances
are highest when the motor’s rotor
“sees” a relatively light load. One-way
clutches or similar devices are advisable
if such a remote risk is not tolerable in
the intended application.
9) Some additional considerations in
applying speed controls include:
a) Chassis controls should be
properly guarded or enclosed to
prevent possible human contact
with live circuitry.
b) Individual manufacturer’s
specifications should be checked,
but in general, the ambient
temperature should not exceed
40°C (104°F) for encased-type
controls. For chassis-type
controls, maximum permissible

ambient temperature is usually 50°C
c) As in the case of motors /
gearmotors, controls must be
properly grounded to prevent
serious injury to personnel.

Different motors require different types
of maintenance and care. Specific maintenance requirements are outlined in
Section 7.10.
For general safety purposes, however,
the area around an electric motor should
be kept free from dust and dirt or from
obstructions which could interfere with
proper ventilation.
In addition, before servicing motors or
gearmotors employing capacitors, avoid
any contact with the capacitor terminals until it has been discharged. The
capacitor should be discharged in accordance with safety instructions provided
with the motor. If instructions are not available, contact the motor manufacturer for
more information.

With the availability of new and better
insulating materials and the extensive use of
grease-lubricated (“lubricated for life”) ball
bearings, quality electric motors have become more reliable and maintenance-free
than ever before. However, in order to
help obtain the best service from an electric
motor, a few helpful guidelines are given
IMPORTANT: Before servicing or
working on equipment, disconnect the
power source. (This applies especially
to equipment using automatic restart
devices instead of manual restart devices, and when examining or replacing
brushes on brush-type motors/gear motors.)


Regular Inspection
and Maintenance
Small motors usually operate with so
little trouble that there is a tendency to neglect them. Wherever possible, most motors should be inspected twice a year to
detect wear and correct any other conditions which might lead to excessive wear or
premature failure. Special attention should
be given to the following common causes
of motor failure.

Changing Load Conditions:
Sometimes additional friction develops
gradually within the driven machine and
thus imposes an overload on the motor
which will cause overheating. Overload
conditions should be promptly corrected. It
is also important to protect motors with
properly rated fuses. If overloads are likely, then an overload protector should be
specified when selecting the motor. See
Section 7.4, Thermal Protection.

Motor and Load Alignment:
When the motor shaft becomes misaligned
with its load, damage to both the shaft and
the bearings can occur. In some instances,
the driven machine may also be damaged.

Excessive Overhung
Loads: Belt and pulley and other similar
drives which subject the motor shaft to
radial (overhung) loads must not be adjusted too tightly or placed too far out on the
motor shaft. Otherwise, they can cause
excessive bearing wear and/or shaft failure.
Excessive Axial Thrusts: Loads
and couplings must be connected so that
excessive axial pressure is not exerted on
motor bearings that will cause premature

Load Must Not Lock on Gearmotors: A torque-limiting clutch should
be provided if there is a possibility that the
output shaft might be locked or jammed.

Such locking quickly builds up tremendous
forces within the gearhead, stripping gears
or damaging other components. If a flywheel is necessary, consideration should
be given to attaching it to the high speed
motor shaft extension. If a flywheel or high
inertia load is used on a slow-speed gearmotor shaft, it tends to keep the shaft turning after the motor has stopped, causing
the same effects on a gearhead as locking
the driven shaft.

Inadequate Wiring: When installing a new motor or transferring a motor
from one installation to another, it is advisable to check the wiring. Adequate wiring
(depending on the voltage, current, environment and distance from the power
source) should be used to feed electrical
power to the motor. (Consult the National
Electrical Code.) Replacement of old, obsolete wiring will prevent future breakdowns and possible hazards to personnel.
Contamination: Next to overloading or abuse, contamination is probably the most common cause of motor failure. Ordinary dust and dirt can restrict
ventilation and coat motor windings, cutting
down on heat dissipation. This clogging can
lead to continuous overheating and eventual insulation breakdown. Dirt can also
cause wear in such moving parts as
Moreover, dirt which is electrically conductive in nature can cause grounding or
shorting of motor windings. Contaminants
can cause additional problems in motors
having brushes and commutators or internal
centrifugal switches. Therefore, if it is not
possible to keep the motor reasonably
clean, a totally enclosed motor should be
Worn Brushes: Brushes are expected to wear, but they should not wear
excessively. The wear rate of brushes is
dependent on many parameters (armature
speed, amperage conducted, duty cycle,


humidity etc.). For optimum performance,
brush-type motors and gearmotors need
periodic user-maintenance. The maintenance interval is best determined by the
user. Inspect brushes regularly for wear.
Periodically remove carbon dust from the
commutator and inside the motor. This can
be accomplished by occasionally wiping
them with a clean, dry, lint-free cloth. Do
not use lubricants or solvents on the commutator. If necessary use No. 0000 or
finer sandpaper only to dress the commutator. Do not use solvents on a nonmetallic
end shield or other motor parts if the product is so equipped.
Whenever a brush is removed for inspection, care should be taken to put it
back in its original position. Changing brush
alignment or position will result in poor
contact between brush and commutator
surfaces. This can cause excessive sparking with accompanying loss of power and
damage to both the commutator and
brushes. Brushes worn to a length less than
1/4 inch (7 mm) should be replaced with
the same brush type.
Rapid wear of brushes is a symptom of
trouble or misapplication. Rapid wear after
a period of successful commutation may
indicate that the commutator is badly worn.
Resurfacing of the commutator may be
necessary and should be performed by a
qualified service shop or returned to the
service department of the manufacturer.

Lubrication: Under normal operating conditions, the relubrication of sleeve
bearings, ball bearings and gearboxes
should be performed according to the
manufacturer’s recommendations. Under
more severe conditions (higher ambients or
increased exposure to contaminants),
shorter service intervals should be established through frequent user-inspections. A
word of caution: excessive oiling can do
more harm than good if not restricted to a
specific area. Excess oil can contaminate

windings, commutators and internal

Ball Bearing Lubrication:
Ball and roller bearings require only small
amounts of lubricant. Calculations show
that 1/1000 drop of oil will lubricate all the
surfaces of a 10 mm bearing. For ball
bearing lubrication in electric motors,
grease is generally preferred over oil for
long maintenance-free service. This is due
to the availability of improved ball bearing
greases, simplified bearing housings and
elimination of the “human error factor”
which is frequently responsible for too
much, not enough or the wrong kind of
lubricant. Prelubricated bearings and the
elimination of grease fittings help improve
ball bearing life.
Premature bearing failures are caused
by one or more of the following conditions:
1) foreign materials from dirty grease or
ineffective seals,
2) grease deterioration due to excessive
temperature or contamination, and
3) overheated bearings resulting from overlubrication or overload.
Some danger signals are:
1) a sudden increase in the temperature
differential between the motor and
2) running a gearmotor at temperatures
higher than that recommended for the
lubricant. The rule of thumb is that
grease life is halved for each 25°F increase in operating temperature, and
3) an increase in bearing noise, accompanied by a bearing temperature rise, indicating a serious bearing malfunction.

Sleeve Bearing Lubrication:
Lubricants are used with ball or roller
bearings to dissipate heat, prevent rust and
prevent foreign matter from contaminating
the bearings. Sleeve bearing lubricants, on


the other hand, serve a different purpose.
The lubricant must actually provide an oil
film that completely separates the bearing
surface from the rotating shaft member and
ideally, eliminates metal-to-metal contact.
Because of its adhesion properties and
its viscosity (or resistance to flow), oil is
“dragged” along by the rotating shaft of the
motor and forms a wedge-shaped film between the shaft and the bearing. See Fig.
7-23. The oil film forms automatically when
the shaft begins to turn and is maintained
by the motion. The rotational motion sets
up pressure in the oil film wedge which, in
turn, supports the load. This wedgeshaped film of oil is an absolutely essential
feature of effective, hydrodynamic sleeve
bearing lubrication.

Fig. 7-23: Oil film in a hydrodynamic
sleeve bearing.

Without it, no significant load can be
carried without subsequent high friction
loss, heat generation and resultant destruction of the bearing and / or shaft. When an
adequate oil film is maintained, the sleeve
bearing serves as a guide to accomplish
shaft alignment. If the oil film fails, the bearing may function as a temporary safeguard
to prevent damage to the motor shaft and
other rotating members.

Good lubricants are essential to low
maintenance costs. Top grade petroleumbased oils are recommended as they are
substantially noncorrosive to metal surfaces. They are free from sediment, dirt and
other foreign materials, and are stable with
respect to heat and moisture. Their performance-to-cost ratio is very good.
An oil film consists of layers. The internal friction of oil, resulting from the sliding
action of these layers, is measured as viscosity. The oil used should provide enough
viscosity to prevent wear and seizure at
ambient temperature, low speeds and
heavy loads for any given application. Relatively light oils are recommended for use
with fractional horsepower motors since
they offer minimal internal friction, permit
fuller realization of the motor’s efficiency
and minimize the operating temperature of
the bearing.
High ambient and operating temperatures have a destructive effect on sleeve
bearings lubricated with standard temperature range oils because the bearing operates at temperatures beyond the oil’s capability. Such destructive effects include reduction in oil viscosity, an increase in corrosive oxidation products in the lubricant
and a reduction in lubricant quantity. Special oils are available for high temperature
and low temperature motor applications.
The care exercised in selecting the proper
lubricant for the expected extremes in
bearing operating temperatures will have a
decided influence on motor performance
and bearing life.
Although sleeve bearings are less sensitive to a limited amount of abrasive or foreign materials than ball bearings, good
maintenance practices recommend that oil
and bearings be kept clean. In very small
motors, dirty or insufficient oil can add
enough friction to cause the bearings to
seize (especially after cooldown). Frequency of oil changes will depend on local


conditions. A conservative lubrication
and maintenance program should call
for periodic inspection of the oil level
and cleaning and refilling with new oil
every six months.
NOTE: Sleeve bearing motors may
tend to lose their oil film when stored
for extended periods (one year or

Lubrication of
Oil provides the best combination of
lubricating properties for gearmotors and is
nearly always used in 1/10 hp and larger
gearmotors designed for industrial service.
Long service life (over 10,000 hrs.) requires a circulating fluid lubrication system.
All lubricants minimize friction, resulting
in lower heat generation and load support.
The fundamental characteristic of oil is its
free flow and constant presence at the
tooth surfaces of a gearhead during operation, thereby providing a consistent and
continuous lubricating film under load.
The lubricant used in parallel shaft gearmotors (which usually employ spur or helical gearing) is relatively less critical than for
right angle worm-gear types. Usually, a
straight mineral oil suffices if the proper oil
level is maintained. Some fhp gearmotors
use hydraulic-type oils to decrease gearshaft or journal wear.
Right angle gearmotors with worm or
other types of sliding contact gearing require careful attention because the lubricants reach higher operating temperatures
due to lower inherent efficiency. (“Inefficiency” is converted into heat which is
aborbed by the lubricant.) Such lubricants
generally have higher viscosity and contain
protective additives.
Despite its advantages, oil is not
commonly used in smaller gearmotors
because of sealing problems. Smaller

gearmotors characteristically do not have
large gasket surfaces and may not have
sufficient power to withstand the increased
friction of a contact seal on the rotor shaft.
Therefore, grease is used as a compromise
in most small gearmotors under 1/4 hp
(186.5 watts).
When compared with oil, grease provides less consistent lubrication to the gear
teeth under load. Grease does, however,
provide flexibility in mounting and minimizes the risk of leakage. Grease also eliminates periodic visual oil level inspections.
The use of “stiff grease” eliminates the
need for vent hole shipping plugs and their
subsequent removal at the final destination.
However, if a semi-fluid grease is used,
vent hole plugging will be required to prevent leakage during shipment.
Grease requires a shorter service interval, primarily because of reduced lubricant
circulation. Wear of the gear train parts is
invariably higher when grease is used as a
lubricant and the rate of wear increases as
stiffer greases are used. Moderate life (approximately 2,000 hrs.) can be achieved
with grease lubrication in a well-designed
gearhead enclosure.

Relubrication: Oil relubrication
under normal operating conditions primarily
involves maintaining the oil at a recommended and indicated level. Loss of oil by
evaporation or leakage is minimal over long
periods of time under normal conditions
which lengthens the relubricating cycle for
an oil-lubricated gearmotor.
Relubrication periods for greaselubricated gearmotors are shorter and require
complete removal of the old lubricant in the
gear housing, proper cleaning of the residue and replenishing with the recommended quantity and type of grease (manufacturer’s recommendation should always be
followed). With proper maintenance and
loading, life of the grease in the gearmotor
under normal conditions of operation can


be appreciable. Manufacturers take
careful steps to match the lubricant
with the elastomers used in the oil
seals as well as the requirements of the
gearing and bearings in a particular
gearmotor design.
Operating Temperature: Lubricant
life in gearmotors is directly dependent on
temperature. Generally, within the normal
operating ranges, lubricant life doubles for
every 25°F decrease in temperature.
Gearmotors operating in high or low
ambient temperature ranges require special
lubricants or lubricating systems. Gaskets,
motor insulation and lubricant life may be
seriously affected by temperature extremes. When other than normal ambient
temperatures (0° to 40°C or 32° to

104°F) are expected, the gearmotor manufacturer should be consulted.

Mounting Considerations:
Distribution or circulation of gear housing
lubricant is critical to gearhead life. Splash
or special oiling gears are effective methods of oil lubrication. Grease cannot be
circulated in this manner, however. So in
cases where bearings and gears must be
lubricated with grease, felt wicks are often
used to transfer oil from the grease to the
bearings. In other designs, gears are
grease-lubricated and the bearings are externally oillubricated.
Special applications which involve rotating a gearmotor about an axis, or tilting it
periodically, will require modified sealing
and venting arrangements to prevent lubricant leakage. The special mountings, modified castings, additional oil seals or special
lubrication systems will add to the cost.


Motor Controls
Although some applications simply use a
motor to drive a load at a constant or relatively constant speed (up to motor nameplate rating), most applications require
some type of control device to adjust motor speed, sometimes from zero to speeds
above rated. Other situations require velocity, torque and position control. The
type and degree of control capability
needed is determined by the application
and by the type of motor used.
Up to this point in the Handbook, we
have discussed motor theory, types and
construction in a fairly straightforward
manner. When discussing motor controls,
however, it soon becomes obvious that
there is an extremely wide range of control
methods available today, ranging in complexity from the simple series rheostat to
sophisticated electronic controls. The range
of controls can be extended further with
the addition of feedback transducers such
as encoders and tachometers, which allow
position and speed to be controlled quite
In addition, refinements in motor technology such as brushless DC and improvements in stepper motor construction have
increased motion control options even

further. These improvements are being
driven by industry demands for motion
control accuracy and by the need to develop more torque from a smaller motor
frame size. As automation and control systems increase in number and complexity,
new demands for improved performance
will continue to be placed on motor and
control manufacturers.
In the following sections, we will discuss
the many aspects of motion control as they
apply to a variety of control systems and
motor types. The reader should be aware
that choosing a motor control method is
simply another form of problem solving.
The more specifics you know about the
problem, the simpler it will be to select a
control method.
Certain criteria such as the power
source (AC or DC), the degree of control
required, the system controller type, the
process you need to control, and your
budget will all affect your decision. An understanding of these criteria will also allow
you to narrow your focus on a particular
type of motor and control very early in the
process, making the decision easier.


No discussion of motor controls would
be complete without a basic understanding
of the larger world of motion control
systems. In order to select the most
appropriate motor and control method, the
designer must know what role the motor
will play in the total process control
system. If the system is controlling a
number of similar processes, such as a
series of conveyors that transport a
relatively constant load on a continuous
basis, then the motor selection and motion
control method may be quite straightforward. If the motor must drive varying loads
at a constant speed, or at speeds that must
be synchronized with other processes, or if
precise positioning is needed to perform a
process, then motor and control selection
becomes more demanding.
In complex process control systems, the
system control and the motor control must
be considered as well as the interface between the two.

Process Types
Process control systems, as the name
implies, are used to control processes. This
could be a batch process such as mixing
ingredients in a food processing plant or
mixing chemicals used in paint production.
In either case, a specific number of individual steps are performed to get a batch of
raw materials prepared for a process that
is performed on the entire batch.
Another type of process is the continuous
process where raw materials enter one side
of a system and a fabricated or finished
product exits the other side. A web printing
press is an example of a continuous process. The blank paper is fed from a roll
through the printer heads where ink is
applied, then into an ink dryer, and finally
through a variety of finishing machines that

fold, bind and cut the continuous web into
finished printed booklets.
Discrete processing requires a series of
precisely sequenced events to occur in
order to produce a finished product. A
cellular manufacturing operation where a
piece of raw metal stock is placed in a
machine which sequentially bores or drills
holes (on one or more axes), taps the holes
with varying thread sizes, and performs
other similar functions to produce a finished
subassembly is an example of discrete

Control System
Most control systems consist of similar
functional elements that are used to regulate the flow of materials through the system and to control the timing and sequencing of events or processes.

System Controller: The system
controller provides the intelligence for the
process control system. It may be a programmable logic controller (PLC), a microprocessor, an analog computer or a
series of relays. Its primary function is to
act as the system’s timekeeper and traffic
manager so that all of the functions occur at
the right time and in the right order.
Actuators: Electromechanical actuators convert electrical power to some
form of physical action. Motors are actuators. They can accept a control signal and
move a conveyor belt to transport material
to the next process. They can turn a shaft a
set number of degrees to position a product for a specific operation to be performed on it. They can be used for intermittent or continuous processes depending
on the type of motor and the requirements
of the application. Other examples of actuators are brakes, clutches, solenoids, relays, valves and pumps.


Fig. 8-1: Typical open-loop control system.

Actuator Controls: Actuator
controls (such as motor controls) function
as system controllers in very simple systems. In more complex control systems
where the motor is one of many actuators,
the motor control is usually under the command of a separate system controller.
Sensors: A variety of sensors are
used in process control systems to determine the status of each process. They are
used to measure velocity, position, weight,
volume, tension, temperature, pressure,
etc. They are transducers that convert a
physical property to an electrical signal
which can be interpreted by the controller.
The sensor output causes the controller to
trigger some form of actuator to begin, end
or interrupt a process.
Signal Interfaces: Sensors, actuators and controllers all operate on a
variety of signal levels and types. Therefore, interfaces must be employed to translate signals or boost signal levels from one
device to another. For example, the output
of a digital computer must be converted to
an analog signal before it can be used by a
brushless DC motor control. Conversely,
the output of an analog transducer must be
converted to a digital signal before a digital
computer can act on it. The voltage or current levels of sensor outputs are often too
low to be interpreted by a controller, and
therefore need to pass through an amplifier
stage before being processed.

Control System Types
Control system operation is usually divided into two basic types:
1) open-loop (no feedback), and
2) closed-loop (with feedback).
The type of system used depends on the
type of application and the degree of control needed to control the process.

Open-Loop Operation: Openloop control systems do not utilize feedback. In other words, the input to the system is set at a level to achieve the desired
output and the state of the output has no
effect on the input. See Fig. 8-1.
A simple motor-driven conveyor transporting boxes from one work area to another, at a set speed, is an example of an
open-loop system. The speed is set by the
conveyor operator and will vary only
slightly depending on the load. If a person
at the end of the conveyor fails to remove
the boxes in a timely manner, the boxes will
drop off the end of the conveyor. The motor speed will not adjust for variations in
the output unless someone physically reduces the speed or turns the power off.
The boxes dropping off the end of the conveyor (the output) have no effect on the
motor speed (the input).
Closed-Loop Operation: A
closed-loop system measures the output of
the process and feeds a signal back to a
junction point at the input of the system


Fig. 8-2. Typical closed-loop control system.

where it is compared to the input signal.
The input defines the desired output.
Changes in load or component values can
cause the output to differ from the input.
This error signal causes the output of the
system to change in a way that acts to
reduce the error signal to zero.
A conveyor used in an automatic parts
inspection process is an example of a
closed-loop system. Since the parts must
pass through a camera’s field of view at a
steady rate, the velocity of the conveyor
must be held constant. Refer to Fig. 8-2. A
tachometer, located at the drive output,
feeds back a continuous signal to the system input that is proportional to the velocity
of the output shaft. This feedback signal is
compared to a reference input signal. Any
variation in the output signal results in an
error signal which causes the motor control
to alter the speed of the motor until the
error signal is reduced to zero.
The accuracy of such a system will depend on the calibration and stability of the
input reference and the accuracy of the
transducer converting the output quantity
(velocity) to a voltage for feedback purposes. The input reference, feedback
transducer calibration and stability are not
included in the feedback loop, and as a
result, are not subject to the loop’s selfregulation.

Servo Control Systems: Servo systems are closed-loop systems that
follow a velocity, torque or position com-

mand. Servo systems can be divided into
three basic types based on the type of input signals used to control the output.
1) Type 0 results in a constant position
output when a constant input is applied.
2) Type 1 results in a constant velocity
output when a constant input is applied.
3) Type 2 results in a constant acceleration
output when a constant input is applied.
Various types of system controllers can
be used to improve the response of a servo
system by adjusting the error between the
output signal and the input signal in different
1) Proportional (P) controllers adjust the
system gain.
2) Proportional plus Integral (PI) controllers adjust the gain and also increase
the type number of the system by one,
allowing other inputs to be accepted.
3) Proportional plus Derivative (PD)
controllers allow the gain and the transient response of the system to be
4) The Proportional plus Integral plus
Derivative (PID) controller allows the
gain, system type and transient response to be changed in order to improve operation.
For detailed information on servo
control theory, the reader should consult
the many reference sources available on
the subject.


used interchangeably. If you refer to
Chapter 1, you’ll recall that speed is
mathematically represented as the absolute
value of velocity and therefore has no
directional component.
Figure 8-3 shows the four quadrants of
motor operation. Torque (T) is plotted on
the “x” axis while angular velocity (ω) is
plotted on the “y” axis. The direction of
rotation (clockwise or counterclockwise)
determines if a positive or negative torque
or velocity is generated. Operating a motor
within these four quadrants will produce
various speed / torque relationships that
will facilitate varying degrees of motion
control. The designer needs to evaluate the
degree of control that is required by the
application early in the motor and control
selection process to determine which motor is best suited for the application.
Different motors and controls exploit
various aspects of the four quadrants better
than others. A motor which can operate in
all four quadrants offers more control over
speed and torque and direction of rotation.
The down side is that a motor control system, capable of four-quadrant operation, is
usually costlier.

Motors used in servo drive systems
must have certain performane characteristics:
1) linear speed / torque characteristics,
2) smooth torque delivery,
3) rugged construction,
4) high torque-to-inertia ratio,
5) high torque-to-power input, and
6) low electrical time constant.
The performance requirements of the
system will determine which of these features are necessary. However, linear speed
/ torque characteristics are generally considered critical requirements for servo

Motor controls can be designed to
regulate speed, torque, velocity and
position. In some cases, acceleration and
deceleration time constants can also be
regulated. When motor velocity vs. torque
is plotted on ±x and ±y axes, it reveals the
characteristic speed / torque curve. In this
discussion, velocity and speed are often

Fig. 8-3: Four quadrants of motor operation.


When selecting a motor control method,
it is often advisable to discuss the control
aspects with the motor or control manufacturer. Sometimes, solutions can be provided early in the design phase which will save
considerable amounts of design time and
money, For instance, servo motors are
designed for high performance applications, which makes them more costly.
However, not all four-quadrant applications require servo motor performance.
Therefore, a system designer can often
save money if the requirements of the application can be met by a less costly motor
control system.

Single Quadrant
A typical speed / torque curve for a
permanent magnet (PM) motor or brushless DC motor is shown in Fig. 8-4a. The
direction of shaft rotation is clockwise. By
convention, when a motor shaft turns in a
clockwise direction, it delivers some degree of positive torque at a given positive
velocity. These characteristics are plotted
in the first quadrant of the graph. A motor
operating in the first quadrant is doing

work. It is generating a force to displace a
mass at a certain speed.

Two Quadrant
Figure 8-4b shows the characteristics
for the same brushless DC motor running in
a counterclockwise direction. The velocity
and torque are negative since the direction
of rotation is reversed. All motors are
capable of first quadrant operation.
Reversible motors can operate in the first
and third quadrants. This simply means that
they can provide positive torque at a
positive velocity and negative torque at a
negative velocity.

Controlling Motors
with Linear
Speed / Torque
Motor design engineers have learned
that controlling motor speed is easier when
the motor exhibits linear speed / torque
characteristics. A close look at the relationship between velocity and torque and
how certain motor designs can exploit their

Fig. 8-4: Typical PM or brushless DC motor speed / torque curves: a) forward direction, positive velocity, positive torque (left), and b) reverse direction, negative velocity,
negative torque (right).


linear characteristics will help the reader to
understand why these motors provide
more versatile control capability.
In Chapter 1, Section 1.2, we learned
that force (F) on a current-carrying conductor immersed in a magnetic field is a
product of the magnetic flux density (B),
the conductor’s current (I) and the length
of the conductor (l):
F=BlI [1]
A somewhat similar effect occurs when
a conductor of length (l) is moved with
velocity (ν) through a magnetic field (B). A
voltage (V) appears between the ends of
the conductor according to the relationship:
E =∫ (ν x B) dl [2]
This formula reduces to E = Blν. In a
motor, the effect of current on the force
generated and the effect of velocity on
voltage occur together. Motion is produced by applied current and a generated
voltage is produced by the resulting motion. The generated voltage (E) always acts
to oppose and limit the normal applied
current flow. It is referred to as counter
emf or back emf.
In rotating machines, the conductors
take the form of coiled turns. The torque
developed on each turn of such a coil is
often alternately expressed as the product
of the current and the rate of change of the
flux linking the turn. Therefore:
T = i --- [3]
where λ is magnetic flux linking
the winding and θ corresponds to the angular displacement.
Similarly, the voltage generated in each
turn of the coil may be expressed as the
rate of change of flux linkage with respect
to time.
E = ---- [4]
Since λ is a function of rotary position θ

Fig. 8-5: Equivalent circuit for a single
winding of a PM type or a brushless type
DC motor.

the equation may be written:
dλ d θ
E = ---- x ---- [4]
where ---- = angular velocity.
Figure 8-5 shows the equivalent electric
circuit for one phase of a PM brush-type
DC motor. The same circuit also applies to
a brushless DC motor. It is represented by
a voltage source (V) connected to a series
combination of RW (winding resistance),
LW (winding inductance), with shunt resistance (RL) and a voltage source (Eg) representing the counter emf. The resistance
(RL) is usually of a high enough value that
its effect on motor operation is insignificant
and can therefore be omitted from the
circuit model.
Since the normal commutation function
connects each phase or combination of
phases in sequence to the voltage source
(V), the circuit model for the overall motor
is represented by the same basic circuit,
except for the fact that the circuit values
may represent more than one winding “on”
at a time. The circuit model shows that the
voltage generator (Eg) acts in opposition to
the normally applied source voltage (V).
Consequently, the current flowing in the
phase will result from (V-Eg) acting across
the impedance made up of RW and LW.
The equation for the motor equivalent
circuit is written:
V= LW ---- + RW i + Eg [5]
For the steady state analysis and since
the inductance of the typical motor is usually small enough that it can be ignored, the


Fig. 8-6: Speed / torque curve of either a PM brush-type or a brushless DC motor
using a simplified model.

above equation can be reduced to:
V = RW I + Eg [6]
V = RW I+ Keω [7]
where Ke is a function of turns and magnetic flux. Ke is called the voltage constant. It
is a proportionality constant that relates the
generated voltage to shaft speed (ω).
If the motor current (I) is constant, a
proportional torque is produced:
T=Kt I [8]
where Kt is a function of turns and magnetic flux. Kt is called the torque constant and
is a proportionality constant that relates
current to developed torque.
Solving the torque equation for current
and substituting the resulting expression for
I in the voltage equation yields:
V = -------+ Keω [9]


Fig. 8-7: Typical speed / torque characteristic of either a PM brush-type or a
brushless DC motor.

Solving for ω results in a linear equation
relating velocity (ω) to the developed
torque (T):
ω = ---- - -------[10]
- ———
is the slope
Kt K e
— is the axis intercept.
The intercept corresponds to the operating point at which T = 0 (no load).
ωNL = ---- [11]
Torque at stall may be solved in similar
fashion by setting ω= 0.
TS = ------[12]
Figure 8-6 shows a plot of the speed/
torque relationship. Both noload and stall
torque are influenced equally by changes in
applied voltage (V). Increasing V shifts the
speed / torque characteristic outward away
from the axis in a parallel fashion. A given
motor will therefore display parallel speed /
torque characteristics corresponding to the
different applied voltages as shown in
Fig. 8-7.
Stall torque may be influenced independently by adjusting the equivalent circuit
series resistance (RW). An increase in


Fig. 8-8: Effect on speed / torque curve
of varying RW.

resistance has the effect of increasing the
slope of the speed vs. torque characteristic
while no-load speed remains unaffected.
Figure 8-8 illustrates the effect of changing
RW. The motor design variables that affect
Kt and Ke tend to have interrelated effects
on the speed / torque characteristics. In
developing the model for the speed/torque
characteristic we assumed that the winding
inductance (LW) was negligible.
A further examination of the speed/
torque equation reveals that velocity (ω)
decreases as the torque load (T) is increased with voltage (V) held constant.
This is the expected result, and is typical of
a permanent magnet DC motor. Similarly,
velocity (ω) will increase with increasingly
applied voltage if the torque is held constant. This relationship is significant in the
control of motor speed. An increase in
torque load will decrease the motor speed,
but the speed can be corrected by a small
increase in the applied voltage.
Speed control of PM brush-type and
brushless DC motors is accomplished by
adjusting the voltage applied to the motor.
Figure 8-9 illustrates how a constant speed
is maintained by varying the voltage. If the
load is held constant, the speed (ωc) can
be maintained by applying a constant voltage (Vc). But if the load increases, as illustrated by the dashed line (L2) and the voltage remains constant, the speed will decrease to ω2. In order to maintain the constant speed (ωc), the voltage must be

Fig. 8-9: Controlling DC motor speed by
varying applied voltage.

increased to V2. Likewise, if the load decreases (L1), the speed will increase unless
the voltage is reduced to V1. With a
smooth stepless range of voltage adjustment, the motor may be operated at any
point (T,ω) within the rated maximum
torque and rotor speed.

Fig. 8-10: Typical speed / torque characteristics for a 1/4 hp DC motor.

Rating Point: Figure 8-10 shows a
speed / torque characteristic curve for a
typical 1/4 hp DC motor. The rating point,
in this example, corresponds to a voltage
of 130 VDC, a torque of 100 oz-in. and a
speed of 2500 RPM. We learned earlier
that we can maintain a constant speed by
increasing or decreasing the voltage proportionally to changes in load. In this example the voltage limit is set at V max. This is
the maximum voltage that can be applied to


the motor for safe operation. It also establishes a limit on the amount of torque which
can be delivered at higher speeds, which
we will illustrate next.

Regulated Speed: Many motor
applications require a regulated speed over
a varying load range. A conveyor application where a constant speed must be maintained regardless of the number of items on
the conveyor is an example.
Theoretically, a DC motor could maintain a constant speed for any load if it had
an unlimited current and voltage source. In
reality however, every motor and control
has a current and voltage limit. In many
electronic controls, the current limit is adjustable, allowing for variable torque in
addition to variable speed.
If the 1/4 hp motor in Fig. 8-10 was
attached to a control device, a series of
regulated speed characteristic curves could
be developed like those shown in Fig. 811. The current curve has also been added
to show the effects of current limiting on
regulated speed.

ometer which allows this point to be
adjusted. Adjusting the current limit increases or decreases the available torque.
The regulated speed curves show that
for a rated speed (ωr) of 2500 RPM, this
system is capable of delivering above-rated
torque at a constant speed up to a point
near the current limit value. Just prior to the
current limit value, the speed will start to
drop off sharply until it reaches current limit
at which time the motor will stall. The degree of drop off or slope of the regulated
speed curve is determined by the design of
the motor and control.
If the motor is operated at a speed lower than the rated 2500 RPM (ω1), it will
again deliver a maximum torque up to the
current limit point. At lower speeds however, it will not require as much voltage. At
higher than rated speeds (ω2), the motor
speed will be affected by the voltage limit.
It will deliver a constant speed until the
voltage limit is reached. The speed will then
decrease at a rate determined by the slope
of the V max curve until it reaches current
limit, at which time the motor stalls. The
regulated speed / torque curves indicate
how much the speed will vary over a given
torque range.

Four Quadrant

Fig. 8-11: Regulated speed curves for a
typical 1/4 hp DC motor and speed

The dashed vertical line represents the
current limit point for this motor and control. Some controls provide a trim potenti-

Some applications require a greater
degree of motor control. For instance, the
motor may be required to reverse while
running, thus generating a negative torque
while running at a positive velocity, or vice
versa. To accomplish this, a motor and its
control must be able to operate in the second or fourth quadrants where load torque
is in the direction of rotation. Motors with
linear speed / torque characteristic provide
the best four quadrant operation. Servo
application which follow a velocity, torque
or position command require four quadrant
operation to achieve optimum system


Fig. 8-12: A typical DC motor in four quadrant operation.

response. That is why a linear speed /
torque relationship is a strict servo motor

Reversing Motor Direction:
Now that we have examined the control
theory of motors with linear speed / torque
characteristics, we can demonstrate their
control capabilities by showing a typical
four quadrant application.
Figure 8-12 shows a linear speed /
torque characteristic curve typical of a PM
brush-type as well as a brushless DC
motor. Since it is applying a positive torque
at a positive velocity, the characteristics are
plotted in the first quadrant. Point 1 on the
characteristic curve represents the
operating point for a given load value.
Assume for this example that the motor
runs constantly and is being controlled by a

system controller. At certain points in the
process the motor must reverse direction
when it receives the command from the
controller. For simplicity of discussion, all
losses due to windings, hysteresis and
other physical properties are considered
negligible in this example.
At the instant the motor receives the
reverse command, the current direction will
switch to a negative value and the motor
will begin to operate in the second quadrant. In other words it will instantly begin to
generate a negative torque while maintaining a positive velocity represented by point
2 on the graph.
At point 2, the current is reversed and
the applied voltage is reversed. The motor
is still putting out a positive velocity so the
back emf, which is a function of velocity
and which normally limits the current, now


becomes an additive component for the
time it takes the velocity to decay to zero
(point 3). This can be seen if we analyze
the equivalent circuit formula:
V = RWI + Eg
Under first quadrant conditions, V and
RWI are both positive while the Eg
component is negative. Therefore, Eg
opposes the applied voltage. When the
reverse command was given, the polarity
of the applied voltage and current were
both switched. The negative current
immediately begins generating a negative
torque. However, the rotor and shaft are
still turning with a positive velocity. During
the period of time from point 2 to point 3
as the positive velocity is decaying, the Eg
component of the equation is still negative.
Therefore, instead of opposing the applied
voltage and limiting the current, Eg
instantaneously aids in developing
additional torque. Although this time is
quite short, the motor control (if any) and
the load must be able to tolerate the
instantaneous increase in torque at point 2.
Once the velocity decays to zero at
point 3, the motor stalls. Because Eg is a
function of velocity which is now zero,
there is no back emf until the current
generates a force in the opposite direction.
When the negative current exerts a force in
the opposite direction, the resulting
counterclockwise movement causes a back
emf to develop and the motor velocity
increases in the negative direction to a
value limited by the load. This is represented by point 4.
Since quadrants three and four are mirror images of quadrants one and two,
when the reverse command is given again a
similar series of events occur in quadrants
four and one (represented by points 5 and
6 on the graph) until the motor again returns to full load speed.

Regenerative Drives: When a

motor performs work, it dissipates power
in the form of heat and other losses. There
are times when the motor must maintain a
constant velocity or torque while being
aided by other physical forces. For example, when a conveyor on an incline moves
a box in an upward direction, it is performing work and normal losses occur. But
when the same conveyor is reversed and
the box is lowered, the motor is aided by
the force of gravity and the mass of the
box. The inertia of the load tends to overhaul the motor and puts power back into
the power supply.
Most motor control systems do not
offer regenerative capability. A control system must be specifically designed to absorb or store the additional power for a
time until it can be dissipated. The example
given earlier where a switch is thrown to
reverse a DC motor is another example of
where power must be absorbed momentarily by the control power supply. During
the few seconds between the time the current is reversed and the motor stalls, power
is being put back into the system because
there is no back emf to limit the current.

Motor controls can be divided into two
basic categories:
1) passive device speed controls, and
2) solid state controls.
Passive device controls consist of fixed
or variable resistors, or variable transformers that are used to adjust the magnetic
field strength, voltage levels or other motor
characteristics (depending on the motor
type), in order to control motor speed.
Solid state controls utilize more complex
circuits consisting of active devices like
diodes, thyristors, transistors, integrated
circuits and in some cases, microprocessors to control motor voltage, power


supply frequency, or to provide electronic
commutation and thereby control motor
Electronically commutated motors use
logic circuits which develop rotating magnetic fields by rapidly switching coil currents on and off. The on/off timing of the
logic circuits is usually controlled by built-in
sensors or specialized motor construction
features which monitor rotor position.
Brushless DC, switched reluctance and
stepper motors use electronic commutation. They cannot be operated by simply
connecting them to a power source; the
control is required for proper operation.
Electronically commutated motors with
the appropriate controls can generally control position, direction of rotation and
torque in addition to speed. Usually, they
operate in closed-loop mode except for
stepper motors which operate in openloop mode because of their unique construction. These electronically commutated
motors were discussed in Chapters 3 and
4. We will examine the control aspects of
these motors later in this section.

The most economical motor speed
controls use passive devices such as
variable resistors and transformers to
control motor electromagnetic characteristics. These controls are described below
for both DC and AC motors.

Controlling DC
Motor Speed
The speed and torque of a DC motor
can be described by the following

Va - Ia Ra
RPM = k ————— [13]
T = KφIa [14]
RPM = revolutions/minute
Va = armature voltage
Ia = armature current
Ra = armature resistance
φ = field flux
T = motor load or torque
k, K = constants
Equation [13] indicates that speed can
be varied by changing any of the variables,
Va, Ra or φ. Consequently, there are three
methods by which the speed of a DC motor can be controlled:
1) Field Weakening The field flux (φ)
in some motors can be altered by
means of a series rheostat.
2) Armature Resistance Control 
Voltage across the armature can be
changed by introducing variable resistance in series with the armature resistance (Ra). Improved speed regulation
can be obtained by incorporating two
variable resistances, one in series and
one in parallel with the armature.
3) Armature Voltage Control Voltage
across the armature (Va) can be varied
through the use of a controlled voltage
source to a motor with separately excited field and armature circuits.

Shunt-Wound DC
Motor Passive Speed
Let’s apply the three basic methods of
speed control to the various types of DC
motors beginning with the shunt-wound

Field Weakening Control: In
order to weaken the field of a shunt-wound
DC motor, a rheostat can be connected in


Fig. 8-13: Simple series field resistance
circuit and shunt-wound DC motor speed
/ torque characteristic.

Fig. 8-14: Simple series armature resistance circuit and shunt-wound DC motor
speed / torque characteristic.

series with the field winding while the armature voltage is kept at the “rated” or line
voltage (V1 = Va). As shown in Fig. 8-13,
the introduction of a field rheostat will permit adjustment of field current from point X
(no additional resistance and full field current) to point Y (maximum resistance and
minimum field current). An increase in field
resistance will decrease the available field
current and consequently, the field flux (φ).
The effect of reducing the field flux while
maintaining the armature voltage is an increase in motor speed. Therefore, field
control or “field weakening” will normally
produce speeds above the base (rated)
speed. It should be noted, however, that
the field can only be weakened within limits. Weakening the shunt-wound DC motor
field beyond a certain point can result in
excessively high and unstable speeds. It
can also result in overheating the armature
as can be seen from equation [2] in that a
reduction of field flux (φ) will produce a
corresponding increase in armature current
(Ia) in order to maintain a given load (T).
Furthermore, with an excessively weak
field and a high armature current, the shuntwound DC motor will be increasingly susceptible to armature reaction, excessive
brush arcing and loss of breakdown
torque. To prevent this, the maximum permissible limit for this speed control method
is generally 150% of the motor’s rated
basic speed. Furthermore, the maximum
load of the motor must be reduced when

operating above the basic speed so that its
horsepower rating is not exceeded.

Armature Resistance Control: Essentially opposite to the field
weakening method, armature resistance
control calls for a variable resistance connected in series with the armature, while
the field winding is excited at rated or line
voltage. See Fig. 8-14. By reference to
equation [13], if the voltage across the
armature (Va) is reduced (by increasing
resistance), motor speed will decrease.
Therefore, armature resistance control will
always reduce speed below the rated base
speed of the motor.
As indicated in equation [14], an increase in load will result in an increase in
armature current which, in turn, causes an
increase in voltage across the series connected resistor. For this reason, if the motor is started with no load at some setting
below the base speed and a load is subsequently applied, there will be a sharp drop
in motor speed and a corresponding I2R
power loss across the resistor. Therefore,
the series resistor must have enough capacity to match the load current.
Using a resistor in series with either the
armature or field is also very inefficient and
is not considered practical for most applications. This method however, is relatively
inexpensive and will effectively control DC
motor speed both above and below the
base speed in some applications.


Shunted Armature Connection: In a variation of the armature resistance method, both series and shunt resistors may be used “in tandem” to improve
speed regulation characteristics of a DC
shunt-wound motor by making the operating speed somewhat less susceptible to
changes in load torque. This factor may
become especially important in cases
where the precise nature of the load torque
is not well known, yet it is desirable to preset the operating speed.
In the shunted armature connection
method, a variable resistor connected in
parallel (shunt) with the armature acts to
increase the current through the series resistance and thus reduce the difference
between the no-load and the full-load current. The series resistance may be used to
control armature voltage in the same way
as with armature resistance control. See
Fig. 8-15. Shunt resistors also assist dynamic braking and are, therefore, used in
cases where a shunt motor is applied to a
load which must be braked.

load changes, speed regulation is equivalent to the inherent regulation of the motor
as shown in the speed / torque curves in
Fig. 8-16.

Fig. 8-16: Example of variable armature
voltage supply.

The feedback type is a silicon controlled
rectifier control and will be discussed with
solid state controls in Section 8.5.

Permanent Magnet
(PM) Motor Passive
Speed Controls
The motor equations [13] and [14] at
the beginning of this section can be applied
to a permanent magnet (PM) motor.
Notice, however, that a PM motor has a
fixed field strength, and therefore, the field
flux (f) cannot be varied. Hence, there are
only two methods to control the speed of a
PM motor.

Armature Resistance Control: This is the same method described

Fig. 8-15: Shunted armature speed control method.

Armature Voltage Control:
There are two types of armature voltage
1) nonfeedback type, and
2) feedback type.
The nonfeedback control consists of a
field power supply and a manually adjustable armature power supply. As motor

for shunt-wound motors. A variable resistance placed in series with the armature
can be varied to increase or decrease the
voltage across the armature and cause the
motor speed to change. See Fig. 8-17.

Armature Voltage Control:
By increasing or decreasing the voltage
supply to the armature of a PM motor, the
motor speed can be adjusted. Voltage adjustment can be achieved through the use
of a variable voltage transformer.


Fig. 8-17: Armature resistance control circuit for a PM DC motor and associated
speed / torque characteristics.

Series Wound
(Universal) Motor
Passive Speed
A series wound motor is suitable for
AC or DC operation and is capable of
supplying high starting torques, high speeds
and high outputs. The speed of a series
motor can be changed by varying the voltage across the motor. This can be achieved
by either using a variable resistor, a variable voltage transformer (autotransformer)
or an electronic control.
Series Resistance Control: A
variable resistor or rheostat in series with
the motor will decrease the speed of the
motor at any load as the resistance is
increased. In theory, the motor speed can
be adjusted to a standstill. However, due
to starting torque limitations, armature
cogging and reduced ventilation, the
minimum speed is usually limited to some
higher value.
A series resistor introduces a voltage
drop in the circuit directly proportional to
the current flowing. The voltage across the
resistor, therefore, will increase as the motor is loaded (since the motor current will
increase with load). It follows that the voltage across the motor will decrease with an
increase in load and the speed will drop
more rapidly with load whenever a series

resistor is used. The higher the resistance
value, the greater the drop in speed as the
load is increased. Also, a series resistor
will have its greatest effect on the starting
torque of the motor since at starting, the
maximum current is flowing and will limit
the motor voltage to its lowest value. The
minimum full-load speed at which a series
motor will operate on AC with a series
resistor is usually limited by the starting
torque available to start the load with that
value of resistance.
Typically on AC, the speed range of a
series motor using a variable series resistor
will be from 1.5:1 to 3:1, depending upon
the motor. On DC, the speed range will be
increased because of the improved
regulation and corresponding increase in
starting torque. Typical characteristic
curves for a series motor are shown in
Figs. 8-18a and b.

Shunt Resistance Control:
A series motor can also be controlled by
shunting an adjustable resistor across the
armature. The speed range is usually limited by this method because of the increased
current passing through the field coils and
the corresponding heating effect. A wide
speed range may only be employed if the
application has a very intermittent duty
Using the same motor as above, typical
characteristic curves are shown in Figs. 818c and d. Although the speed range is
limited, this method of control improves the


Fig. 8-18: Series wound motor passive device speed control methods: a) AC series
rheostat control, b) DC series rheostat control, c) AC shunt rheostat control, d) DC
shunt rheostat control, e) variable AC voltage control, and f) variable DC voltage control.

speed regulation of the motor and maintains good starting torque characteristics. It
is an excellent method for matching motor
A combination of series and shunt resistors is sometimes used to obtain characteristics between the two types of controls.

Variable Transformer Control: By using a variable transformer to
vary the voltage across a series motor,
speed ranges of 4:1 to 7:1 are typical depending upon the motor. If a full-wave
bridge is used to convert the output of the
transformer to DC, the speed range will be
increased because of improved regulation

and starting torque. Figures 8-18e and f
show typical characteristic curves for the
motor used in Figs. 8-18a, b, c and d.

AC Motor Passive
Speed Controls
One of the principal characteristics of
the AC induction motor is its ability to
maintain constant or essentially constant
speed under normal voltage and load variations. Therefore, this type of motor does
not lend itself to a simple method of speed
control over a wide range.


Some types of loads, however, make
practical some degree of speed adjustment
if the proper motor and control means are
chosen. First, it should be understood that
there are variations of conventional induction motors which are designed for the
express purpose of improved speed control. These motor types may employ
wound rotors with variable resistance,
brush shifting means and other special features. This discussion, however, will be
confined to induction motors having the
conventional squirrel cage nonsynchronous,
reluctance synchronous and hysteresis synchronous rotors.
The speed of an AC motor is related to
the power supply frequency (Hz) by the
RPM = —— [15]
RPM = revolutions/minute
(nominal synchronous speed)
f = frequency (Hz)
P = number of poles
The above speed represents the synchronous speed of the revolving magnetic
field of the stator in a nonsynchronous motor or the actual rotor speed of a synchronous motor.
While a synchronous AC motor rotates
at the exact speed defined by the above
formula, the nonsynchronous motor never
operates at synchronous speed. The difference between the synchronous speed and
the actual speed is known as rotor “slip”:


adjusting supply frequency,
changing the number of stator poles,
adjusting power input, and
controlling rotor slip.

The change in frequency method requires the use of solid state driven power
supplies and falls in the category of solid
state controls, which will be discussed in
Section 8.5.

Change in the Number
of Stator Poles
The pole-changing method (Fig. 8-19)
is also suitable for both synchronous and
nonsynchronous motors, but has the limitation of offering only a few speeds (usually
no more than four), which are widely separated from each other. By nature, the polechanging method requires that a portion of
the winding be idle during the operation of
one or more speeds. This results in motor
inefficiency and a considerable reduction in
the output rating for any given frame size.
Switching methods for pole-changing are
also expensive and complicated, making

Sync. Speed - Actual Speed
Slip = ———————————— [15]
Sync. Speed
The magnitude of slip depends upon the
rotor design, power input and motor load.
As in the case of the DC motor, the speed
of an induction motor can be made to vary
by changing any of the variables in the fundamental speed equation, such as:

Fig. 8-19: Simplified pole-changing


obtaining the maximum speed change
for a given change in motor power

the method useful in relatively few

Changing Rotor Slip
The changing of rotor slip is simpler,
less costly and the most widely used technique for varying the speed of an AC induction motor.
There are three types of nonsynchronous motors to which this method is best
suited: shaded pole, permanent split capacitor and polyphase. The latter is not widely
used in fractional horsepower motor sizes.
NOTE: Due to the sensitivity of the
centrifugal or relay starting switches,
the rotor slip method should not be applied to split-phase start and capacitor
start motors unless the speed will never go low enough to engage the starting switch. If the motor is running at
reduced speed with the starting switch
closed, the auxiliary winding or switch
contacts would soon burn out.
To obtain the optimum speed control
effectiveness in applications employing the
change in rotor slip method, the following
guidelines should be followed:

There are several ways to change the
power input to an induction motor, and
thereby increase or decrease the amount of
slip. Listed below are those which are most
frequently used.

Series Resistance Method:
A variable resistor can be used to vary
voltage across the winding of an induction
motor See Figs. 8-20a and b. Series resistance can be used with either shaded pole
or PSC motors.

Variable Voltage Transformer Method: This method may be used
in place of a series resistor to reduce voltage across the winding. It has the advantage of maintaining substantially the same
voltage under the starting condition when
the current is higher than during the running

1) Since the principle is based on changing the power input, it is important
to match the motor closely with the
load. This will ensure that with a change
of power input, a noticeable change in
speed will result.
2) The load should have a substantial
component of inertia. If the load is not
of the fan or blower type, it may be
necessary to add a fly-wheel to provide
this necessary inertia. NOTE: A noninertial load cannot be satisfactorily
controlled by the change in rotor
slip method.
3) It is advisable to use a rotor specifically designed and constructed for
high slip (high degree of slope of the
speed / torque curve). This will aid in

Fig. 8-20: a) Simplified series resistance
circuit (top), and b) change of motor
speed by series resistance method


speeds. See Figs. 8-22a and b.

Shunt Resistance Method:
Also confined to the PSC motor, this
method has been found to provide stable
speed in four-pole, 60 Hz motors up to 1/
100 hp (7.5W) over a range from 1500
RPM down to 900 RPM with a constant
torque output. See Figs. 8-23a and b.
With this method, it is necessary to use a
high slip type rotor.

Tapped Winding Method:

Fig. 8-21: Variable voltage transformer

mode. There is also much less power lost
as heat than with a resistor. See Fig. 8-21.
By reducing the voltage across the main
winding of a PSC motor, full voltage is
maintained across the capacitor winding,
providing more stable operation at lower

Fig. 8-22: a) Variable voltage transformer
method in a PSC motor (top), and b)
varying PSC motor speed by the variable
transformer method (bottom).

This method is most widely used in shaded
pole fan motors. The change in input is
obtained by changing motor impedance
through the use of various portions of the
total winding. See Fig. 8-24. The number
of speeds is determined by the number of
taps introduced into the winding. In addi-

Fig. 8-23: a) Shunt resistance method
(top), and b) change of PSC motor
speed by shunt resistance method (bottom).


Fig. 8-24: Tapped winding circuit.

Fig. 8-25: Winding function change

Winding Function Change
Method: Applicable only to the PSC

package that is smaller and just as economical over the life of the application as
some of the earlier, less sophisticated
This section will cover solid state control of both DC and AC motors. We will
begin with the simpler speed controls such
as SCR and PWM and end with electronic
commutation controls.

motor, the winding change method can be
used in applications requiring no more than
two speeds. See Fig. 8-25. The functions
of the main and the capacitor (starting)
windings can be switched to provide “high”
and “low” speeds. High speed is obtained
when the winding with fewer turns is functioning as the main, while lower speed is
achieved with the winding with more turns
functioning as the main. This is an extremely efficient technique, but it does require
that the motor winding be exactly tailored
to the load in order to provide the desired
two speeds.


Active vs. Passive
Control of DC
Motor Speed
In Section 8.4, you’ll recall that the
speed of a DC motor can be varied by
changing any of the variables in the basic
speed formula:
Va - Ia Ra
RPM = k —————

`Advances in solid state electronics such
as VLSI technology as well as improved
manufacturing techniques like surface
mount component technology have led to
many improvements in motor controls. The
continuing drive for miniaturization has led
to smaller controls which offer better performance and greater reliability than their
predecessors. Many of these changes have
also driven down the cost of controls.
Control system designers are discovering that an electronic control, when
matched with the right motor, can offer a

Passive devices such as resistors increase the motor circuit resistance, causing
increased power dissipation in the form of
heat. This additional heat produces no useful work and decreases the overall efficiency of the system. With the development of
semiconductors, it became possible to vary
motor speed through voltage switching
rather than by adding resistance to the
drive circuit.
Instead of varying the level of resistance, switching amplifiers vary the time
during which full line voltage is applied to
the armature. The net effect is an average
voltage which is roughly equivalent to a


voltage level obtained by the variable resistance-type control.
To see how these two techniques work,
think of two simple circuits, each with a
light bulb, a power source and a current
control device. In Fig. 8-26a, a variable
resistance controller is used. In Fig. 8-26b,
a switch is connected in series with the light
bulb and power source. In the variable
resistance system, the resistor can be regulated to control the current and produce a
light intensity from 0 to maximum rated.

Fig. 8-26: a) Simplified variable resistance control circuit (top), and b) switching circuit technique (bottom).

In the switching system there are only
two possible states: “on” or “off.” To vary
the light intensity, the switch may be turned
on and off many times per second. Each
combination of on/off states represents one
cycle. Since semiconductor switching can
take place at very high frequencies, the eye
perceives an average intensity somewhere
between off and maximum. The longer the
bulb is left in the “on” state during each
cycle, the brighter the light will seem to
In a similar fashion, semiconductors
vary motor speed by switching voltage to
the motor windings on and off very rapidly.
The longer the voltage is “on”, the higher
the average voltage will be and concurrently, the higher the resultant motor speed.

Pure DC vs.
Rectified AC
The quality of the direct current and
voltage used to drive a motor has a significant effect on its efficiency. Before we discuss the various solid state controls used to
control DC motor speed, it is important to
review some basic DC theory and to see
how DC motors are affected by various
grades of DC.

AC Rectification: Rectification is
essentially the conversion of alternating
current (AC) to unidirectional current
(DC). It is the most economical means of
generating DC, since it utilizes commercially available AC sources. However, the
degree to which the alternating current is
converted will determine the overall efficiency of the motor and control system.
A simple diode can be used for halfwave rectification. Full-wave rectification
can be obtained by using two diodes in a
center-tapped transformer circuit. A fourdiode bridge circuit will also provide fullwave output. These circuits are shown in
Fig. 8-27.

Fig. 8-27: Typical half-wave (top) and
fullwave (middle and bottom) rectification
circuits employing diodes.

Later we will see how SCRs are used
to create full and half-wave rectification in
DC motor controls. We can see from the
wave shapes (current diagrams) that rectification provides unidirectional current, but


not uniform or pure DC. It is the measure
of departure from pure direct current that
can have a significant effect on motor

Form Factor: Form factor is a
measure of departure from pure DC. It is
defined as the root-mean-square (rms)
value of the current divided by the average
value of the current. Pure DC has a form
factor of 1.0 or unity. For half-wave rectified current, the form factor is 1.57. For
full-wave rectified current, the form factor
is 1.11 when measured with a resistive
The form factor is an important consideration with motors designed to operate on
direct current. When operated from rectified power vs. pure DC, the increase in
motor heating for a constant output is approximately proportional to the square of
the form factor. For example, a motor operating from half-wave rectified DC current
will have approximately 2½ times the heat
rise of the same motor operating on unity
form factor DC.
To accommodate the increased heating
effect of high form factor current, continuous duty applications generally require a
larger (and more costly) motor to drive a
given load. Stated another way, a designer
may save money by using a low cost, high
form factor speed control, only to sacrifice
much of the savings by using a larger motor
to keep the motor operating temperatures
within design limits.
High form factor also means that a high
peak current is required to maintain an
average current output for a given power
requirement, thus contributing to rapid
brush and commutator wear.
Filtering: Filtering methods act to
“smooth out” the rectified current or voltage waveform by means of series inductance and/or parallel capacitance. The effects of filtering can be seen in the waveforms in Fig. 8-28.

Fig. 8-28: Filtered vs. unfiltered full-wave

The filter capacitors in Bodine controls
improve the armature current form factor
to near unity (1.00), and also result in higher average voltage available for a relatively
wider range of speed control. The advantages of full-wave rectification with filtering
can be seen in the chart in Fig. 8-29.
Typical Feedback
Controller Speed

Form Factor


Unfiltered. . . . .1.6 - 2.0 . . . . . . . . 65%
Filtered . . . . . . 1.1 - 1.5 . . . . . . .120%
Unfiltered . . . . 1.1 - 1.6. . . . . . . . 80%
Filtered . . . . . . 1.0 - 1.1 . . . . . . . 130%
Fig. 8-29: Effects of various types of rectification and filtering on form factor.

SCR Phase Control of
DC Shunt and PM
Motor Speeds
While the speed of a shunt-wound motor can be changed by varying either the
field or armature voltage, a PM motor’s
speed can be varied only by changing the
supply voltage to its armature. Some
controls utilize the field weakening method


for shunt-wound motor speed control. This
is not the preferred method however, since
changing the field voltage directly affects
the output torque capability of the motor
and should only be used where relatively
light loads are encountered. Changing the
motor armature voltage, on the other hand,
allows full torque to be developed.
Most motor controllers for the fractional
horsepower DC shunt-wound and PM
motors use silicon controlled rectifiers
(SCRs) as the control element for varying
the power applied to the motor. The SCRs
control the armature voltage and thus, the
motor’s speed.
An SCR is a three-terminal device
made from four layers of alternating P and
N-type semiconductor materials. See Fig.
8-30. It functions as a diode (only conducts current in the forward direction), but
will do so only when a trigger voltage is
applied to its gate.
Once an SCR is fired, the gate signal
can be removed without stopping conduction. Conduction ceases when the positive
voltage is removed from the anode. The
typical gate signal required to activate an
SCR is about two volts and 10 milliamps
for three microseconds. Although these
values are representative trigger requirements, an SCR gate can tolerate much
higher power inputs without damage.
The rectifying capabilities of SCRs

make them popular in speed controls. They
can be directly connected to the AC
source to form a half-wave rectifier without
AC-to-DC conversion circuitry. When an
SCR is used to rectify alternating current,
the point during the positive half cycle of
the input current at which the rectifier is
turned on can be adjusted by the timing of
the application of the trigger signal to the
gate. At the end of the positive half cycle,
the SCR will turn off as the applied polarity
of the voltage reverses. By controlling the
phase relationship of the trigger to the zero
axis crossing of the positive half cycle of
alternating current, the amount of power
transmitted through the SCR can be varied.
This is called phase control. One or more
SCRs can be used to provide phase-controlled half-wave, full-wave or multiplephase control.
The combination of a counter emf sensing element, a triggering unit whose phase
is controlled by the counter emf sensor,
and one or more SCRs constitutes a basic
feedback speed controller.

Half-Wave SCR Controls: In
a half-wave SCR motor control, the gate
signaling characteristics of the SCR are
used for speed selection and as feedback
for compensation of load changes.

Fig. 8-31: Feedback control circuit using
the counter emf of the motor as the feedback control voltage.

Fig. 8-30: Function diagram and standard schematic symbol for a silicon controlled rectifier (SCR).

The circuit illustrated in Fig. 8-31 uses
the counter emf of the motor as a feedback
control voltage (motor speed is proportional to counter emf). Gate firing occurs


Fig. 8-32: Voltage waveforms of a half-wave SCR control. If motor speed decreases,
the SCR will automatically fire sooner in the cycle. The shaded areas are proportional
to the power delivery.

when the divided fraction of the supply
voltage (developed at the center arm of the
potentiometer) exceeds the counter emf
developed by the motor. At this moment
and for the remaining portion of the half
cycle, the input voltage is applied to the
motor. If the motor should slow down due
to an increase in load, the counter emf will
be lower and the SCR will automatically
fire sooner in the cycle (thus allowing the
SCR to be on for a larger portion of the
half cycle). The voltage waveforms associated with this control operation are shown
in Fig. 8-32. With this circuit, the SCR can
be controlled only through the 0 to 90
degree range.
Half-wave rectified SCR controls, while
inexpensive, do not operate a motor at its
full potential. For example, a motor operating from a half-wave rectified DC current
will have approximately 2½ times the temperature rise of the same motor operating
on pure DC. Since motor life is inversely
related to temperature, the motor will have
a much shorter life. This temperature rise is
directly related to the form factor discussed

Full-Wave SCR Controls:
Full-wave SCR controls optimize a motor’s performance. They can be constructed using two SCRs with a center-tapped
transformer or as a full-wave bridge where
two of the diodes are replaced by SCRs.
See Figs. 8-33a and b.
By using full-wave rectification in conjunction with filtering to smooth the rectified current or voltage waveform, the form

Fig. 8-33: a) Full-wave SCR control using
a center-tapped transformer (top), and
b) using a bridge configuration (bottom).

factor is improved significantly. Refer to
Fig. 8-29 for the effects of filtering on form
Like the half-wave control, the timing of
the control signal of the full-wave SCR
determines the “firing angle” (the electrical
angle from the zero crossing point when the
SCR fires). See Fig. 8-34. When the SCR
is switched on, current flows to the motor
winding. The position of the firing angle
determines the average voltage and in turn,


Fig. 8-34: The effect of the firing angle
on the average voltage of a full-wave
SCR control.

the output speed. If the SCR fires early in
the cycle, current flows to the windings for
a longer time and the average voltage is

IR Compensation: Speed can
be maintained at a nearly constant level
regardless of changes in motor load with
the addition of IR compensation. While the
voltage developed by a tachometer is
sometimes used as an output speed signal,
in most controllers it is the counter emf
generated by the motor that is compared
with a reference voltage to regulate speed.
To compensate for varying loads, the
applied armature voltage and armature
current (proportional to load) are sensed.
The difference (V-IR) is proportional to
motor speed. This voltage is compared to
the reference voltage established by the
external speed setting potentiometer. The
difference or error is used to automatically
increase or decrease the armature voltage
and thus, the motor shaft speed. If the controller senses a counter emf that is lower
than the reference voltage, it will increase
power to the motor. This will increase the
speed and the generated counter emf. This

action will continue until the difference between the counter emf and the reference
voltage equals zero. If the counter emf exceeds the reference voltage, the controller
will decrease the power to the motor.
Figure 8-35 illustrates an SCR speed
control consisting of a counter emf sensing
element, an emf phase-controlled triggering
unit and an SCR. Inherent motor characteristics combined with a reflected load
make it impractical to achieve regulation
closer than about 1% using counter emf
and armature current as the feedback signals. However, a tachometer generator can
be incorporated as the feedback element
to achieve speed regulation approaching
0.1%. In Chapter 9 we will discuss feedback devices such as tachometer generators and encoders in greater detail.

Fig. 8-35: Interrelationship of elements in
a basic SCR feedback speed control.

Other Compensation
In addition to providing feedback circuitry which adjusts output power to maintain constant speed as load varies, the following features can also be included in a
well-designed SCR control.


Line Voltage Compensation:

AC line voltage typically varies by as much
as ± 10%. Since motor speed is proportional to voltage, motor speed will fluctuate
as the line voltage varies. Hence, it is important to incorporate line voltage compensation circuitry features in the motor
control to maintain speed settings.

Temperature Compensation: A
motor’s armature winding resistance (Ra)
is not always constant during its operation.
It rises and falls with the ambient and operating temperature and can cause control
instability. Selection of circuitry components with low temperature coefficients can
help reduce speed changes caused by temperature variations. However, some temperature compensation devices must also
be built into the control circuit to sense the
winding temperature and make up for the
resistance variations due to temperature

Torque Limiting (Current
Limiting): In some drive applications, a
limit must be placed on maximum torque
output. For example, a winding machine
may require that wire tension be limited to
a maximum to avoid breakage.
Since motor torque can be expressed
by the equation T = kIa, torque is directly
proportional to armature current. Therefore, limiting the current to the armature
also limits the torque. A controller with a
torque limiting circuit can draw current up
to a preset value, after which the motor’s
speed will “drop off.” The nature of the
drop-off is dependent on control design,
initial speed, inertia, rate of torque increase, etc.
In addition to maintaining a limiting
torque, torque control is also useful for soft
starting (controlled acceleration) of loads
that are essentially inertial in nature.

Surge Suppression: An abnormal voltage “spike” can damage the sensitive components of a controller. A transient

protector or surge suppressor should be
used to divert the voltage surges. Thyrectors and varistors are two devices commonly used for this purpose. Figure 8-36
shows a varistor used in a bridge circuit to
protect a controller’s circuitry.

Fig. 8-36: Surge suppression using a

RFI Suppression: Any instantaneous change in voltage across an energy
storage network will result in the emission
of RFI (radio frequency interference). The
RFI that is created is simultaneously propagated through the air and conducted
through the elements of the system. In the
case of electronic motor controls, rapidly
changing voltage across a capacitor
through the use of an SCR or arcing at the
motor brushes may result in RFI which
may cause disturbances in nearby electrical
RFI can be prevented from reaching
places through the use of filters for the conducted portion and shielding for the portion
propagated through the air.
The shielding of electrical equipment to
prevent the propagation of RFI through air
is difficult. This is because the strength of
the RFI signal at any given distance from
the source depends not only on the
orientation of the RFI source with respect
to the receiver, but also on the amount of
amplification of RFI due to the antenna
action of objects to which it is physically
connected. For this reason, shielding


should be individdually designed for each
Prevention of conducted RFI from
reaching and introducing noise to the supply line can be accomplished with a filter
placed between the line and the control as
shown in Fig. 8-37.

“firing angle” resulting in the correct
average voltage for a given desired speed
When the transistor is switched on, current flows in the winding. Just like the SCR
control, the firing angle in a PWM circuit
(the electrical angle between the start of the
cycle and the angle at which the transistor
begins to conduct) determines the average
voltage and in turn the output speed. A
wider pulse width will result in a higher
average voltage. See Fig. 8-38.

Fig. 8-37: Simple RFI filter.

A simple filter design consists of an inductance choke put in series with the input
and a bypass capacitor put across the line.
The impedance of the choke increases with
increased frequency. Its impedance is negligible at 60 Hz but presents a high impedance at the frequency of the RFI range,
which causes some portion of the conducted RFI to drop across it. The impedance
of the capacitor decreases with increased
frequency. It is virtually an open circuit at
60 Hz but almost a short circuit at the RFI
frequency, and so some portion of the RFI
is shunted across it.

Fig. 8-38: Effect of pulse width on average voltage in a PWM circuit.

Figure 8-39 shows that a power
amplifier is used to amplify the control
voltage to provide the actual drive current,
while a feedback circuit tracks the
armature voltage level.

Pulse Width
Modulation Control of
DC Motor Speed
Pulse width modulation (PWM) circuits
use transistor switches instead of SCRs as
voltage control devices. The circuits are
similar in their basic function. In a pulse
width modulation control, a DC-to-pulsewidth converter converts a control signal
voltage to an appropriate pulse width or

Fig. 8-39: Typical PWM control circuit.

PWM controls operate from pure DC
and require an external power supply with
a high degree of rectification and filtering.
As a result they can be costlier than SCR


However, unlike SCRs which can turn current on but not off, transistors are not dependent on the negative cycle of the AC
source for turning off the winding current.
Because the PWM drive operates from a
pure DC source, the relationship between
pulse width and motor voltage is linear
(Fig. 8-40) and has little lag. This gives
PWM controls the quick response necessary for many servo applications.
The pulse repetition rate (cycle duration) ranges from 1 100 kHz, depending
on the characteristics of the motor and
application. The transistor’s ability to generate a wide range of pulse widths gives
PWM controllers a very wide speed range
and precise control of peak motor current.

Fig. 8-40: Linear characteristics of a
PWM control.

Commutation of
DC Motors
Some motors are controlled through
electronic commutation. Brushless DC and
DC stepper motors are examples of
electronically commutated motors. Both of
these motors were described in Chapter 3.
Electronically commutated motors cannot
be operated by connecting them to a
power supply. The control is required for

commutation (motor action) as well as for
speed, position and torque control. In this
section, we will examine the types of
controls used with brushless DC motors
and stepper motors and the effects they
produce on the motors.

Brushless DC Motor
Brushless DC motor controls perform a
variety of functions. One primary function
of the control is commutation. Commutation takes place by sequentially switching
the current in one or more stator phase
windings to generate a revolving magnetic
field. The magnets in the rotor cause motor
action by chasing the revolving magnetic
field generated in the stator windings.
The on/off switching of phase current is
a function of rotor position. Rotor position
is determined by sensors located in the
motor itself. The rotor position information
is fed to the commutation logic circuits in
the control which determines the correct
firing sequence of the transistors that supply current to the windings. Since the current is switched just before the magnets in
the rotor align with the magnetic field generated in the stator, and since the current
switching is governed by the rotor position,
the rotor never catches up with the field.
Brushless DC motors run at higher speeds
than PM DC motors because their speed is
not limited by the frictional components of
mechanical commutation, but by the voltage limit of the control circuit and motor

Trapezoidal vs. Sinusoidal
Characteristics: Brushless DC motors can exhibit either trapezoidal or sinusoidal torque characteristics. It is the arrangement and type of windings as well as
the physical characteristics of the stator
and rotor that determine whether a motor


Fig. 8-41: Overlapping torque waveforms of a sinusoidal DC motor being driven as a

will produce trapezoidal or sinusoidal
The back emf of a DC motor always
follows the waveform which a motor produces when it is externally driven. In other
words, as a result of the motor’s construction, the waveform which it produces when
it is run like a generator determines the
characteristics of the back emf. The commutation cycle and ultimately the torque
output are dependent on the back emf. The
shape of the waveform, therefore, is important. There is considerable difference of
opinion among motor manufacturers as to
which wave shape is better.
Figure 8-41 shows the overlapping
torque waveforms of an externally driven,
three-phase DC motor with sinusoidal

characteristics. From the curve we can see
that torque is a function of rotor position. If
you follow a single waveform you’ll see
that minimum torque occurs when the
waveform crosses the axis. It then
progresses to a maximum torque value
before returning to a state of stable equilibrium. The back emf waveform follows this
same path. Peak torque at constant current
occurs when the back emf peaks. Therefore, in a brushless DC motor, by sensing
the rotor position and timing the commutation circuits so that the phase coils are
turned on near the top of the back emf
waveform, we will generate a torque ripple
output similar to the waveform shown in
Fig. 8-42.

Fig. 8-42: Torque ripple of a sinusoidal brushless DC motor.


Fig. 8-43: Output waveforms of a DC motor with trapezoidal characteristics.

The torque output has a considerable
amount of ripple. This could be reduced by
increasing the number of motor phases and
thus commutating on shorter cycles. This
approach adds considerable cost to the
control since more transistors and logic
circuits are needed for commutation.
Another way to reduce the amount of
ripple is to construct the motor to produce
a trapezoidal characteristic waveform. Figure 8-43 shows overlapping torque waveforms of an externally-driven trapezoidal
DC motor. Notice that the tops of the
waveforms are flat by design. Therefore, if
the commutation takes place at or near the
top of the waveform, there is less ripple
than with the sinusoidal design. This is represented by the bold line at the top of the
waveforms in Fig. 8-43.
In general, motors with sinusoidal outputs are easier to construct and therefore,
less costly. However, they generate considerably more torque ripple. High accuracy sinusoidal controls in combination with
high resolution position sensors can produce very smooth torque outputs from a
sinusoidal motor. However, the additional
control circuitry and sensors add to the
cost of the system.
Brushless DC motors with trapezoidal
characteristics have flat torque curves and
lend themselves to digital and pulse width
modulation control techniques. The controls for trapezoidal characteristic motors
are more cost-effective to produce than
those for sinusoidal motors having the same

number of phases. Figure 8-44 shows the
relationship between the various waveforms of a three-phase brushless DC motor
with trapezoidal characteristics.

Fig. 8-44: Waveform relationships in a
three-phase brushless DC motor with two
phases energized and one off at all


Brushless DC motors can be used as
servo motors depending on the application.
They are capable of four quadrant operation and develop considerably more torque
per frame size than their PM DC counterparts. Most brushless DC controls provide
variable current limiting. Acceleration and
deceleration response times are usually

Stepper Motor
Stepper motors can carry out extremely
varied patterns of precise movements. Position is determined by the number of steps
taken in either direction of rotation. Velocity is determined by the step rate. To produce the same sequence by other means
might involve more expensive apparatus
(resolvers, tachometer generators, etc.)
and considerably more system maintenance. Perhaps the most distinct advantage
of stepper motors is that they can perform
a variety of complex operations with a
noncumulative unloaded step error of 3%
to 5% maximum of one step.
The basic function of any stepper control, no matter how simple or complex, is
to provide the means of directing a stepper
motor to complete a specific sequence of
steps. Stepping is accomplished through
the sequential energization of the motor’s
phases. The heart of any stepper system,
the driver, is the device which actually conducts current from the power supply to the
motor windings. This is accomlished via
power transistors (represented by switches
in Fig. 8-45). There are three principle
types of stepper drivers:
1) Series R (also known as L/R),
2) chopper, and
3) bilevel.
Each can be configured in unipolar and
bipolar modes which will be explained later
in this section.
To prevent motor overheating, each

Fig. 8-45: Simplified representation of a
stepper motor drive scheme.

power driver circuit uses a different method to limit current beyond the specified
maximum for the motor. The differences in
system performance are reflected in the
time required for each driver type to bring
the stepper motor up to full current, and
the shape of their phase current vs. time

Series R (L/R) Driver: The simplest, least expensive stepper driver is the
Series R (or L/R) driver. In this scheme,
resistors are connected in series with the
motor windings. See Fig. 8-46. These resistors limit the maximum winding current
to a safe operating level by adding to the
divisor in the formula:
Imax = ——————————
R series + R Winding
The electrical time constant for current
rise is:
RSeries + RWinding
To get adequate high speed performance, winding current must rise and decay quickly. This is accomplished by using
high resistance series resistors to minimize
the time constant, and correspondingly,
high power supply voltages to attain adequate levels of current. Since a significant
amount of energy is dissipated as heat in
the resistors, Series R drivers are limited to
applications which can tolerate additional
heat and relatively low system efficiency.
Advantages of Series R are low initial cost,
system size and simplicity.


Fig. 8-46: Simplified representation of Series R drive scheme and associated waveforms.

Chopper Driver: With chopper
drivers, external resistors are not used to
limit the maximum flow of current. Limited
only by the relatively small winding resistance, current would tend to rise to an unsafe level. To prevent this, the chopper
driver will turn off the voltage across the
windings when current reaches a preset
maximum. See Fig. 8-47. The driver then
monitors current decay, until it reaches a
minimum level at which it reapplies the
voltage to the windings.

Instead of a pure exponential curve,
chopper drivers produce a sawtooth
shaped current waveform like the one
shown in Fig. 8-47. Since chopper drivers
do not dissipate energy through series resistors, it is practical to increase voltage for
much higher horsepower output. The onand-off chopping action maintains current
at safe operating levels. The high supply
voltages used by these drives allow chopper drivers to reach maximum currents
much faster than Series R drivers.

Fig. 8-47: Simplified representation of chopper drive scheme and associated waveforms.


Fig. 8-48: Simplified representation of bilevel drive scheme and associated waveforms.

The one operational characteristic inherent in chopper circuits that may cause
problems in some applications is the tendency for oscillating current to produce
system resonances at certain frequencies or
motor speeds. Vertical dips on the speed /
torque curve represent narrow speed ranges in which the torque dips unexpectedly.
These resonance effects can be diminished
and sometimes eliminated by using electronic compensation circuitry.

Bilevel Driver: Rather than chopping current at a prescribed maximum,
bilevel drivers switch between two separate input voltage levels. See Fig. 8-48. To
bring the motor windings rapidly up to

maximum current, a relatively high voltage
(typically above 24 V) is initially applied.
Once the desired operating level has been
reached, the driver quickly switches to a
much lower maintenance voltage (typically
under 10 V).
This dual voltage approach provides the
rapid acceleration which is not possible
with a Series R design, while minimizing
some of the resonance effects found in uncompensated chopper drivers. The principle disadvantages encountered with bilevel
drivers are the added expense for switches
or transistors and the dual power supply
needed to deliver the two voltages used by
the scheme.


Fig. 8-49 Simplified representations of unipolar (top), bipolar series (left) and bipolar
parallel (right) drive schemes.

same changes in direction of magnetic flux
in the motor stator.

Unipolar vs. Bipolar
Each of three drive circuits used in stepper motor applications can be configured
into three basic modes:
1) unipolar,
2) bipolar series, and
3) bipolar parallel.
Each mode has advantages and disadvantages in terms of cost and performance.
See Fig. 8-49. In the unipolar mode, two
of the four windings are energized at any
given instant, and current flows in only one
direction through each winding. The
sequence in which the windings are
energized determines the direction of shaft
rotation. In bipolar operation, all windings
are on simultaneously. Rotation is produced by changing the direction of the
phase current in the windings. No matter
which method is used, the rotor “sees” the

Unipolar Circuits: Unipolar
drive circuits are generally simpler, more
reliable and less expensive. They require
only four drive transistors and a single
power supply. Though they deliver somewhat lower torque for a given power input
at low speeds, they usually produce higher
torques at higher speeds.
Bipolar Circuits: With bipolar
circuits, as many as eight power transistors
or four power transistors and a dual power
supply are needed. This adds cost and
vulnerability to failure. But when high
torque and very low speeds are application
requirements and there are constraints on
motor size, a bipolar driver may be the
most desirable alternative. Since all four
phases are energized at any given instant,
the bipolar circuit generates a stronger
magnetic field, delivering more torque to
do the work.


Under static or low speed conditions,
bipolar drivers can increase torque output
by 20% to 40%. When connected in parallel, effective phase resistance and inductance are reduced by half. This allows current per phase to be increased to 140% of
the “two-phase on” unipolar rating. When
connected in series, the effective number of
winding turns is increased, so the series
bipolar circuit makes more efficient use of
the windings. Voltage across the windings
can be increased, while keeping current
low (70% of the “two-phase on” unipolar
rating). In some cases this permits less expensive power supplies and drive components to be used.

Stepper Motor
Stepper motors operate in either of two
speed ranges:
1) error-free-start-stop (EFSS), and
2) slew.
This combination of two operating ranges is unique to the “stepping” design. For
each increment in the phase energization
sequence, the stepper motor takes a precise known angular step. As mentioned
earlier, the rotor follows the established

magnetic field through a series of detent
positions one at a time, with a noncumulative unloaded step error of no more than
3% to 5%, provided the speed and acceleration capabilities of the motor are not
If an application requires that the motor
get from position “a” to position “b” as
quickly as possible, a stepper motor must
be carefully accelerated or “ramped” from
its low to high speed range or it will lose
synchronism with the magnetic field. Just as
an internal combustion engine will stall if
accelerated too quickly, “racing” a stepper
will cause it to act unpredictably. In a
typical application the motor may be
commanded to “ramp” between low and
high speed ranges many times, and each
time the shape (slope) of the ramp will be
an important factor in maintaining step

Operating Speeds: The term
EFSS (error-free-start-stop) is used to
describe the stepper’s low speed operating
region. In EFSS, the motor phases are
switched relatively slowly, usually no faster
than 1500 steps per second (even slower
with larger motors). The maximum EFSS
rate is dependent on load torque and load
inertia. See Fig. 8-50. In the EFSS region,

Fig. 8-50: EFSS and slew curves for a 34 frame single-stack stepper. Dashed lines
are EFSS curves for zero, one and four times the stepper motor’s rotor inertia.


the motor can be started and stopped instantaneously without losing steps. If one
or two phases are left on, the rotor will
stop at the exact detent position corresponding to those phases.

Low-Frequency Resonance:

Slew Speed: The high speed area
of operation in a stepper is called the slew
region. Here the windings can be sequenced quickly (up to 20,000 steps per
second with the smallest stepper motors).
If the sequence is suddenly stopped while
the motor is operating in the slew region,
inertia will cause the rotor to go beyond the
desired holding position by at least four
steps and possibly more.
In order to reach the slew region, the
motor must first be started in EFSS and
carefully accelerated to the desired slew
speed. Then after rotating a particular number of steps at the higher step rate, the
motor must be “ramped down” or decelerated to a suitable EFSS speed before it
can be stopped at the desired position. In
this way, “ramping” allows us to dramatically reduce traverse time.
By starting in EFSS and then ramping
up to slew, we can run for most of the
traverse at the higher slew speed, and still
come to a complete stop at the desired
point without losing (or gaining) steps. Of
course, the shape of the required velocity
profile is dependent on the ability of the
motor to accelerate the load.

Operational Limitations
There are certain inherent regions within
which a stepper motor will not provide
stable operation. At both the natural frequency of the motor, and the mid-frequency resonance region, stepper motors may
oscillate noisily, lose steps or even stall.
Electronic and mechanical means can be
used to compensate for these effects, and
they do present an added dimension to be
considered in the application process.

The low-frequency resonance region of a
stepper motor is usually a narrow band
centered between 80 and 200 steps per
second (sps). In this region, the motor load
must contain some friction, either inherent
or added by the user, to assure stable operation. Although it is possible to calculate
with some certainty the amount of friction
required, system performance should always be verified by actual testing.

Mid-Frequency Resonance:
Mid-frequency resonance is the term used
to refer to a region within the mid to upper
stepping rates in which there is a steep
drop-off in available torque. In this area,
motor performance is extremely erratic and
stalling can occur. Once this region is
passed, normal operation resumes. The
actual location and width of the mid-frequency resonance region is dependent on
the type of control, the power supply voltage and the motor load conditions. However, speed / torque curves provided by
the manufacturer usually indicate probable
unstable areas. Although continuous operation in resonance areas is not possible
without some type of damping, steppers
can operate at these speeds momentarily
during acceleration and deceleration.
Since resonance is a function of motor
design, load characteristics and control
circuitry, it can often be avoided, compensated for or even eliminated by a variety of
Ramping If operation beyond the
mid-frequency resonance region meets
application requirements, it may be possible to ramp through it by properly matching
motor to load. Since steppers are normally
used in processes which require frequent
acceleration and deceleration, the effects of
resonance can generally be overcome.
Electronic Antiresonance Techniques
Various electronic methods are available
to minimize resonance effects. A common


and relatively inexpensive technique is to
“half-step” the motor by energizing the
windings alternately one and two at a time.
The motor takes two half steps to advance
a full step angle. This produces smoother
shaft rotation with reduced resonance
When extended mid-frequency operation is unavoidable, more sophisticated
antiresonance circuitry is needed to electronically dampen the instabilities that cause
resonance. Contact the motor manufacturer for more information.
Mechanical DampersSeveral mechanical methods may be used to successfully overcome the effects of resonance.
Viscous inertia, ferro-fluidic and eddy current dampers all operate on the principle
that a sacrifice in the rate of acceleration
produced by adding inertia produces increased momentum to cancel out oscillation in the resonance region.
Viscous inertia dampers are coupled to
the stepper motor shaft opposite the load.
A damping rotor rotates in a fixed housing
filled with a viscous fluid. Once the motor
is brought up to speed, the inertia sets up
an added momentum which damps the
oscillations in the resonant area. Ferrofluidic dampers create inertia in a nonmagnetic
housing filled with magnetic particles. Energy is absorbed by the interaction of inertia,
mass and housing. Eddy current devices
substitute a cup made from conductive
material (usually aluminum) for inertia and
fluid. As the shaft rotates, eddy currents
are built up in the aluminum cup. The
damper then acts like a friction drag on
shaft rotation and resists deviation from
operating velocity.

Oscillation (Ringing): Another
control system characteristic which can be
a factor in positioning application is the
tendency for stepper rotors to oscillate or
“ring” when the pulse train is stopped. See
Fig. 8-51. The ringing effect usually lasts

Fig. 8-51: Oscillation or “ringing effect” in
an undamped stepper with no load.

no longer than a few hundred milliseconds.
If this poses a problem, there are several
ways to damp stepper motor systems.
Motor plugging circuits make it possible
to electronically damp oscillation by “backstepping” the stepper motor so that the
rotor is at zero velocity when it reaches the
desired final position. With delayed last
step damping, the EFSS rate is selected so
that the rotor overshoots the next to last
position and reaches the final detent with
zero velocity. It can then be held with little
or no oscillation. Either method effectively
reduces motor oscillation. See Fig. 8-52.

Fig. 8-52: Damped stepper response
with no load attached.

If electronic damping is not applicable
because system parameters vary (friction
load or inertia), viscous inertia or frictiontype dampers attached directly to the motor or load are excellent substitutes for
electronic damping circuits.

Inertia: Inertia plays an important
role in stepper applications. To obtain desired operation, the load inertia must be
within the capability of the motor control
system to accelerate and decelerate. Too
much load inertia can cause the motor to
lose steps or stall during acceleration. If


there is insufficient load inertia, the width of
the resonance region may be too large.
To determine whether or not inertia will
pose a problem in an application, first consult the motor control performance characteristics. If the intended operation is within
acceptable design guidelines, inertia should
be manageable. If the desired stepping rate
is within the midfrequency region and the
load system cannot be altered to allow a
different stepping rate, more inertia may be
added, or electronic means may be employed to arrive at a balanced combination
of motor, load and control.

Advances in AC motor control have
been slower to evolve than those for DC
motors. As a result, AC motors have been
slow to shake their image as constant
speed drives. Nevertheless, progress is
being made in many areas. Adjustable frequency AC drives are becoming more
prevalent even for motors in the fractional
horsepower range. These drives offer programmability of functions such as preset
speeds, resonance compensation, and acceleration and deceleration rate control.
Some sophisticated controls combine voltage and frequency control within the same
unit. Other controls with specialized memory chips allow for keyboard-programmable, motor air gap flux adjustments.
It is beyond the scope of this Handbook
to cover all of the latest innovations in electronic AC motor controls. However, familiarization with some of the basic solid state
control methods is necessary.

Change in Frequency
As mentioned earlier, one way to
control AC motor speed is by changing the
power supply frequency. This is based on
the speed formula for AC motors. The
speed of an AC motor is related to the
power supply frequency (Hz) by the

120 f

RPM = revolutions/minute
(nominal synchronous speed)
f = frequency (Hz)
P = number of poles
Change in frequency has the advantage
of providing stepless speed changes over a
relatively wide range, and may be used
with either synchronous or nonsynchronous
induction motors. The synchronous motor
has the obvious advantage of following the
speed adjustment called for by the control.
The nonsynchronous motor, even though it
develops more torque per frame size, will
slip in speed from the control setting depending upon motor load. The major disadvantage encountered with this method is
the relatively high cost of the frequency
changing power supply.
With an increasing number of manufacturers making three-phase adjustable frequency drives, the three-phase motor is
gaining popularity in adjustable speed applications. This is particularly true where
ruggedness, reliability and low maintenance
are requirements.

Polyphase Power Supplies:
Small motors wound for operation with
two-phase power supplies seem to be best
suited for adjustable frequency applications. These motors will provide performance similar to three-phase designs, but
the two-phase adjustable frequency power


Fig. 8-55: Typical PWM voltage waveform.

Fig. 8-53: Ideal speed / torque curves of
a polyphase motor operated from an
adjustable frequency drive (V3 > V2 > V1
and Hz3 > Hz2 > Hz1).

supply is more practical. Small two-phase
motors can be optimized to operate over a
range of 10 to 120 Hz by proper voltage
adjustment. The voltage must be increased
as the frequency is increased in order to
compensate for the change in motor reactance. See Fig. 8-53.
One of two basic techniques are used to
obtain adjustable frequency power:
1) Six Step MethodThis method is
named for the shape of the waveform it
generates. See Fig. 8-54. Line voltage
is rectified to an adjustable DC level.
This voltage is then fed into an inverter
which produces an alternating square
wave voltage. At low motor speeds, the
six step inverter can produce pulsations
of torque and speed, called cogging.

Six step inverters also produce harmonics in the output waveform which cause
motor heating without contributing to
motor torque.
2) Pulse Width Modulation (PWM)
With PWM, line voltage is rectified to a
constant potential DC voltage. This DC
voltage level is fed into a PWM inverter
which generates a series of short pulses
at varying widths to yield the voltage,
frequency and harmonic relationship
desired. See Fig. 8-55. The average
voltage is determined by the width of
the pulse (wide for high average voltage
and determined by the rate at which
polarity is reversed (which is much
smaller than the pulse rate, so there are
many pulses per cycle).
One disadvantage is that PWM
inverters produce high frequency minor
currents at the pulse repetition frequencies. The rapid high voltage pulses can
also produce insulation stresses, and
noise and vibration problems in motors.
There are several variations of these
two techniques. Since they produce non-

Fig. 8-54: Typical six step voltage waveform.


sinusoidal waveforms, they cause additional motor heating which may require that the
motor be derated from the output that is
obtainable from a pure sine wave.
Small polyphase motors are often rated
for dual frequency (50/60 Hz) use at a
single voltage level. These motors will run
hotter on 50 Hz than 60 Hz because the
input will be higher on 50 Hz and their ability to self-regulate will be reduced due to
the reduction in speed to approximately 5/
6 of the 60 Hz speed.

Single-Phase Power Supplies: On single-phase power supplies,
split-phase start or capacitor start motors
are least suitable for dual frequency operation. It is difficult to find a starting relay
suitable for dual frequency operation.
When a centrifugal cut-out switch is used
instead of a relay it is difficult to obtain the
correct operating speed. In addition, if a
60 Hz split-phase motor is designed to
operate close to its temperature and magnetic limits, then operation on 50 Hz will
not be satisfactory since the current and
watts will increase excessively and the motor will overheat. This could even occur at
no load.
The permanent split capacitor (singlephase power supply) motor presents a
problem in adjustable frequency operation
over a range of frequencies. This is primarily because the capacitor value should be
decreased with an increase in frequency
and vice versa. However, when specifically
designed for the purpose, the permanent
split capacitor motor is the best choice for
operation in the narrow frequency range of
50 to 60 Hz.
When the frequency is changed from 60
to 50 Hz, the current in the main winding
will increase and the current in the capacitor winding will decrease so that the total
current may actually remain approximately
the same regardless of the frequency. Generally speaking, any PSC motor can be

wound so that it will accommodate the
same input power at 50 or 60 Hz.
However, a dual frequency, constant
voltage design sacrifices power output
compared with single-phase versions.
Therefore, for a given frame size, optimized
dual frequency motors will have lower hp
ratings than single frequency motors.

Vector Control of
Induction Motors
AC motors have long been used as
constant speed drives while their DC counterparts have been employed in numerous
variable speed and positioning applications.
This phenomenon is due to the DC motor’s
inherent adaptability to variable speed
techniques and its linear speed/torque
This adaptability is a function of DC
motor construction and the ability to control torque and motor field flux independently. We learned earlier that by weakening the magnetic field of a DC motor, the
field current is also weakened and consequently, the back emf is reduced. If the
armature voltage is held constant while
weakening the field flux, motor speed increases. DC motors become very unstable
at high speeds due to brush arcing and
armature reaction. Therefore, high speed
DC motors require special construction to
overcome these inefficiencies.
AC motors which have no brushes and
more rugged construction have been unsuitable for variable speed applications
because their torque and field flux are interrelated. Any change in either one will
cause a corresponding reaction in the other. Vector control (or field-oriented control) allows independent control of an induction motor’s field flux and rotor current
to achieve linear torque characteristics like
those of DC motors. To do that, the motor
control must regulate the instantaneous
magnitude and phase of the stator currents


or voltages in order to develop a linear
relationship between torque and slip frequency. This involves numerous calculations and algorithms. Although vector control techniques have been known for some
time, they have only become cost-effective
with recent advances in microprocessors
and integrated circuit technology.
The instantaneous angular position of
the field flux vector rotating at synchronous
speed must be known for accurate vector
control. This can be measured (direct vector control) or it can be estimated from the
computed slip which is based on the rotor
time constant, Tr (indirect method). The
rotor time constant is a function of rotor
resistance and inductance and can vary
significantly from its nominal value depending on operating conditions. It is critical that
Tr be tuned correctly. If it isn’t, the calculated slip will be in error and consequently
so will the field flux vector. If the estimated
Tr is not matched to actual Tr, field orientation will be lost and the actual torque will
differ from the expected torque. A popular
method for calculating Tr is by using the
inverse Gamma form model equivalent
circuit, but that is beyond the scope of this
Handbook. It suffices to say that vector
controllers require extensive processing
power in order to achieve effective results.
Machine tool spindle drives have benefited from the use of vector controlled induction motors. They can be operated at
higher speeds than thyristor-controlled DC
motors for increased application performance and they require less maintenance,
both of which often justify the cost of the

Switched Reluctance
Motor Control
The switched reluctance motor was
described in Chapter 4. It possesses
qualities of both AC and DC motors. The
switched reluctance motor has been

receiving more attention in recent years as
a variable speed drive for the same reasons
that vector control of induction motors has
grown in popularity: faster processors and
decreasing cost of building and implementing controls.
But unlike induction motors which are a
staple in the industry, switched reluctance
motors are not widely used nor understood
by designers. Therefore, there is considerable controversy over the methods of controlling switched reluctance motors, especially in servo systems or four quadrant
Since they possess AC motor qualities,
they require signal processing in order to
compensate for inherent nonlinear properties. Control algorithms are needed to
smooth irregularities from the motor as well
as from the rotor position feedback devices that are required. A considerable degree
of wave shaping is also required on the
input side of these motors.
Rotor position is a critical factor in
controlling a switched reluctance motor.
Transducers for measuring position and
current add considerable cost to the
system. Although there are several
methods for estimating the rotor position,
they are cumbersome and can often create
undesired effects.

Some motor controls are provided in
separate enclosures for simple applications
where the motor speed is controlled
manually or where the motor control is
used as a stand-alone device. Other times,
a motor control is simply one element of a
more complex motion control system and
is mounted in a large central equipment
enclosure with other process control
equipment. In the latter applications, the


manufacturer may provide the control
without an enclosure.
Motor control enclosures, like motors
themselves, are rated and tested against
safety criteria established by various third
party standards organizations such as the
National Electrical Manufacturers Association (NEMA) and Underwriters Laboratories (UL). Designing to these standards is
voluntary and compliance to standards is at
the manufacturer’s discretion. When a control enclosure meets various third party
standards, end-users are assured of certain
safety and operating characteristics.
Standard UL-508 covers safety design
requirements for industrial control equipment enclosures. UL-50 covers cabinets,
cut-out boxes and junction boxes. NEMA
has also established standards for industrial
control equipment enclosures to meet a
wide range of applications.
A brief overview of NEMA enclosure
types is given below. If additional information or specific details are required about
motor enclosure standards, the reader
should contact the various standards organizations and industry associations listed in
Appendix 1.

NEMA Type 1: This type of enclosure is suitable for indoor general applications under normal atmospheric conditions.
Type 1 enclosures protect users from
touching the equipment and protect the
control from falling dirt.

NEMA Type 2: This is a general
purpose indoor enclosure with drip shield
protection to protect the control from falling liquid or dirt. It is not intended to protect against dust or internal condensation.
NEMA Type 3: These enclosures
are for outdoor use and provide some
protection from windblown dust, rain and
moisture. They also protect the control
from external ice formation. They will not
protect against internal condensation or

NEMA Type 3R: The same as
Type 3, this enclosure only protects against
falling rain, sleet and external ice formation.
NEMA Type 3S: Also the same as
Type 3, this enclosure meets additional
provisions for operating external controls
when ice-laden.
NEMA Type 4: These enclosures
are for indoor or outdoor use and protect
against windblown dust and rain, splashing
water and forcefully directed water from a
hose. They do not protect against internal
condensation or icing.
NEMA Type 4X: The same as
Type 4, this enclosure provides added
protection against corrosion.
NEMA Type 6: These enclosures
are for indoor or outdoor use and can
withstand temporary submersion in water
at a limited depth.
NEMA Type 6P: The same as
Type 6, this enclosure also has the ability
to withstand submersion for prolonged
NEMA Type 11: These enclosures
are intended for indoor or outdoor use and
protect against corrosive liquids and gases.
They can be submerged in oil for added
protection against fumes and gases.
NEMA Type 12: These enclosures
are for indoor use and provide a degree of
protection against dust, falling dirt and
dripping noncorrosive liquids.
NEMA Type 13: These enclosures
are for indoor use and provide a degree of
protection against dust, spraying water, oil
and noncorrosive coolant.
NEMA Type 7 (Class I,
Groups A, B, C and D indoor):
These enclosures are intended for hazardous areas as defined by the National Elec-


trical Code. They meet explosion, hydrostatic and temperature tests.

NEMA Type 9 (Class II,
Groups E, F and G indoor):
These enclosures are intended for use in
Class II hazardous areas as defined by the
National Electrical Code. They also protect against the ingress of dust.
In addition to local standards, an international classification system has been established by the International Electrotechnical Commission (IEC) to rate the sealing
effectiveness of electrical equipment enclosures. IEC-529 utilizes an alpha-numerical
system. See Fig. 8-56. The letters “IP”
stand for “Ingress Protection” and are followed by two numerical digits which indicate degrees of protection against solid
objects and moisture.
The first digit indicates the degree of
protection that the enclosure offers against
solid object entry:
0 - No special protection.
1 - Protection from solid objects larger
than 50 mm.
2 - Protection from solid objects not
greater than 80 mm in length and 12

mm in diameter.
3 - Protection from entry by objects
greater than 2.5 mm in diameter.
4 - Protection from objects greater than
1.0 mm in diameter.
5 - Protection from dust.
6 - Dust-tight.
The second digit indicates the degree
of protection that the enclosure offers
against moisture:
0 - No special protection.
1 - Protection from dripping water.
2 - Protection from vertically dripping
3 - Protection from sprayed water.
4 - Protection from splashed water.
5 - Protection from water jets.
6 - Protection from heavy seas.
7 - Immersion protection.
8 - Continuous submersion protection.
IEC-529 does not cover mechanical
damage, explosions or harsh environmental
conditions such as high humidity or corrosive fumes.


Fig. 8-56: IEC-529 enclosure classifications.


Feedback Devices
In Chapter 8, we explored the world of
open and closed-loop motion control systems. We concentrated on motors and their
associated controls and how they can be
operated in either of the two modes.
Closed-loop motion control systems depend on feedback transducers for the
speed and position error signals which regulate the system’s functions. Open-loop
systems merely require an input signal to
initiate some type of action.
Sensors and feedback transducers provide information which the motor or system
controller uses to stop, start, speed up,
slow down or reverse a motor’s direction
of rotation. Sensors usually monitor the
object or material being processed. They
include photoelectric sensors, viscosity and
flow sensors, temperature sensors and
thermocouples, ultrasonic sensors, limit
switches, force and torque transducers,
and strain gauges.
In closed-loop systems, these devices
measure a specific characteristic of the
process and send a corresponding signal
back to controller to initiate some form of
actuator control. For instance, when an

object trips a limit switch, the controller
may send a stop signal to a motor, shutting
down the process. A flow sensor monitoring fluid pressure can provide the necessary feedback to cause a motor to open or
close a valve.
Feedback transducers generally monitor
the characteristic of the drive train for
changes due to load variations or driveshaft
position. These are the types of controls
which we will examine in this Chapter.
They include tachometer generators, encoders, resolvers, synchros and magnetic
sensors. Accuracy of resolution, dynamic
response, noise characteristic, temperature
stability, environmental conditions and cost
all play a role in deciding which feedback
device should be employed in a specific
When considering price vs. performance, it must be stressed that the accuracy of the error signal cannot exceed the
capability of the feedback device. It is important to weigh price / performance decisions carefully, especially if an application
has tight control tolerances.


Fig. 9-1: Typical motor speed control feedback loop employing a tachometer.

A tachometer generator is an electromechanical feedback transducer that generates an analog voltage output directly
proportional to the angular velocity at
which it is driven. Tachometers may be
used in simple applications to provide
speed readout signals which are monitored
on a meter and calibrated in RPMs. They
are also used to deliver feedback signals in
speed control systems or in velocity damping systems in position control. High performance tachometers are usually specially
designed for servo applications. In less
demanding situations, however, certain
types of DC motors can act as tachometers by being driven mechanically to generate the desired feedback signal. Figure 9-1
shows a typical feedback control system
loop employing a tachometer generator.
Tachometers can be separate devices
or integral parts of a motor design. Tachometer-motor combinations may consist
of a motor winding and tachometer winding
on the same armature or they may utilize
separate windings connected on a common
shaft. The common armature type has the
disadvantage of magnetically coupling the
tachometer and motor windings, making it
unsuitable for some high performance servo applications. Brushless tachometers are
also available.

Tachometer design follows the same
basic rules of DC motor design, except
that certain critical requirements such as
output voltage linearity, low voltage ripple
and temperature stability must be maintained for feedback signal accuracy.

Encoders are position and motion sensing devices that produce a digital signal
which can be easily interpreted by a system
controller or microprocessor. There are
two distinct types:
1) rotary encoders, which sense the
movement or position of drive train
components rotating about an axis, and
2) linear encoders, which sense position or
velocity of an arm moving parallel to an

Rotary Encoders
Most drive trains produce some form of
rotary motion. All motors, except for linear
motors (Chapter 4), are rotary drives. In
order to accurately control certain processes, the exact angular position of a rotating drive train must be known. Encoders
are feedback transducers that sense angular displacement.
Most rotary encoders are available with
optical or magnetic-type detecting elements. A contact-type has found limited
use in some applications and laser-type


Fig. 9-2: Typical optical rotary encoder.

encoders are used in many robotic applications. Rotary encoders fall into two different categories:
1) incremental, and
2) absolute,
depending on their construction and the
type of output signal they generate.
Optical Rotary Encoders: Optical
encoders use the same basic components
regardless of whether they are absolute or
incremental in nature. A light source, usually an LED (light emitting diode), is used to
pass light through slots in a rotating code
wheel. The light transmission is interrupted
by the pattern on the code wheel. The light
is detected by a photoelectric diode
mounted opposite the light source. A signal
processor accepts the signals from the
photoelectric diode and may convert them
into binary or another code such as grayscale code. Figure 9-2 shows a typical
optical encoder configuration.
The physical characteristics of the code
disk and the resulting output signal separate
the incremental encoder from the absolute
type. The incremental encoder passes a

beam of light through a series of small slits
on a stationary mask and an identical pattern on a rotating disk. The photo diode
detects a pulsed light source due to the
alternate opening and closing of the slits
resulting from the rotating disk. The light
pulses can be counted to obtain the angular
position, but to obtain direction information
a second stage is required.
Incremental encoders can provide two
channels of output pulses, displaced by 90
electrical degrees, known as an “A quad
B” output system. See Fig. 9-3. The direction of rotation is determined from the occurrence of the edges of the A and B pulse
trains. An A transition (0 to 1) occurs before a B transition during one direction of
rotation, and vice versa for the other direction. As the shaft rotates through the null
point, a reference pulse is generated.
Incremental encoders provide no indication of shaft position upon power-up.
They must be rotated through the null point
or provide a marker pulse in a third channel in order to obtain a reference position.
This re-initialization or resetting of the system must be performed after a power interruption. Strong electrical interference
can also cause miscounting. They are considered volatile position indicators and are
best suited for short cycle and rate applications.
Absolute optical encoders use similar
components except for the coded pattern
on the rotating disk. See Fig. 9-4. The
absolute encoder disk pattern provides an
individual code for each position. Because

Fig. 9-3: “A quad B” output from an incremental rotary encoder.


Fig. 9-4: Optical rotary encoder coded disks: a) incremental type (left), and b) absolute type (right).

no two codes are alike, the exact position
is always known at start-up even if the
system position was moved during a power
outage. They are preferred in robotic applications where zeroing several axes can
cause considerable production delay or
where personal injury might occur if an
erroneous position was detected.
Absolute encoders may provide very
high resolution and accuracy. They are
more expensive than incremental optical
encoders and may produce outputs in three
standard codes: binary, binary coded digit
(BCD), or gray code. They are available in
single-turn and multiturn configurations.
Single-turn absolute encoders produce
a unique “word” output for each position
over 360°. If the shaft rotates more than
one full turn, however, the position information will repeat and the actual position
cannot be determined unless the shaft turns
are counted. Multiturn absolute encoders,
on the other hand, are equipped with gear
trains which keep track of the number of
shaft turns. They produce a unique “word”
output corresponding to the shaft location
and the number of turns.

domains recorded at selected pitches. The
degree of magnetic pitch defines the angular position. A magnetization system, similar to that used in conventional magnetic
recording equipment, is used to saturate
the permanent magnet material on the rotating disk. The angular position of the disk
is synchronized to the charging pulses so
that an entire array is written during one
revolution. The disk is programmable, allowing for customization and changes.

Magnetic Rotary Encoders:
A typical magnetic rotary encoder is illustrated in Fig. 9-5. It consists of a rotating
magnetic disk or drum with magnetic

Fig. 9-5: Typical magnetic rotary encoder


A magneto-resistive sensor, which
changes its resistive value under the influence of the rotating magnetic field, detects
the magnetization on the rotating drum and
produces a corresponding output signal.
Magnetic rotary encoders provide good
stability under varying temperature ranges,
have low power requirements and offer
good resolution in a small package size.

Linear Encoders
In some motion control applications,
linear encoders are preferred over a combination of rotary encoders and lead screw
arrangements. A linear encoder consists of
a scanner and a glass or steel tape scale
(depending on the length of the unit) which
is fixed to a support. The fixed scale functions much like the coded disk in a rotary
encoder. The scanner contains a light
source, photocells and an additional graduated scale or reticle. See Fig. 9-6.

Fig. 9-6: Typical optical linear encoder

Light is projected through the openings
on the reticle and the fixed scale and is
detected by photosensors. The fixed scale
modulates the light as the scanner moves
and produces sinusoidal photosensor outputs in quadrature (phase shifted by 90°).
These outputs are compared to a reference
voltage and combined to produce two

symmetrical square wave outputs in
quadrature. The square waves are then
counted to indicate speed and direction.
Linear encoders may be used very effectively in precision applications such as
inspection tables, microlithography tools
and printed wiring board drilling machines.
They are subject to error because of the
signal processing that takes place. Care
must be exercised in mounting the scale.
There are several rules of thumb for error
correction in linear encoders and the manufacturer should be consulted to determine
the degree of error which an application
may inflict and how to minimize such errors
before a linear encoder is selected and

Resolvers and
Resolvers and synchros are analog output position transducers. Both resolvers
and synchros look like small AC motors
and function like rotating transformers. The
output voltages of a resolver and synchro
are uniquely related to the input shaft angle.
They provide absolute positioning over the
full 360° shaft rotation.
A resolver usually has a single-winding
rotor and two stator windings positioned at
right angles to one another. The rotor is
excited by an AC reference voltage which
in turn is coupled to the stator windings.
See Fig. 9-7a. The relationship between
the output voltage of a resolver and a reference input voltage (V SINωt) is derived
from the following:
V(S1 to S3) = V SINωt SINθ
V(S4 to S2) = V SINωt COSθ
A resolver-to-digital converter is required to convert the analog resolver output to two digital signals that are 90° out of
phase. These digital outputs are required
by the system controller and are a direct
representation of the input shaft angle (θ).


Fig. 9-7: Comparison of transducers: a) resolver (left), and b) synchro (right).

The synchro is represented in Fig. 9-7b.
It consists of three stator windings in a
Wye configuration. The relationship between input reference voltage V SINωt
and the synchro output voltages is:
V(S1 to S2) = V SINωt SINθ
V(S3 to S2) = V SINωt SIN(θ + 120°)
V(S2 to S1) = V SINωt SIN(θ + 240°)
A Scott T transformer is needed to convert the three 120° out-of-phase analog
signals into two 90° signals so that a resolver-to-digital converter can be used to
generate digital signals.
Since synchros and resolvers are transformers, they have inherent signal isolation
and minimize electrical interference.
Another advantage of synchros and
resolvers is that there is no signal processing performed at the drive train as with
encoders. The resolver or synchro can be
positioned where the angle needs to be
measured while the r-to-d converter can
be located in the cabinet with the controller
or processor. This makes synchros and
resolvers highly suitable for harsh manufacturing environments where electrical interference and temperature fluctuations could
degrade an encoder signal.

Magnetic Pick-ups
A typical magnetic sensor is shown in
Fig. 9-8. The sensing element consists of a
wire coiled around a permanent magnet.
Magnetic sensors detect the motion of
moving ferrous objects that come within
their magnetic field. When positioned near
a moving gear, they will sense each tooth
as it cuts through the magnetic field. The
change of flux through the coil resulting
from the passing ferrous object generates a
voltage at the coil terminals.
Magnetic pick-ups are capable of relatively high resolution and can sense very
small objects. When used to sense rotating
shaft speed, the output of the sensor must
be converted to RPMs by an analog-todigital converter. They are particularly suited for high temperature applications since
they contain no solid state components and
they are highly shock-resistant.

Fig. 9-8: Typical magnetic pick-up.


They do not perform well at
extremely slow speeds because they
depend on the rate of change of flux
from one ferrous object to the next. At
slow speeds, the output voltage drops
below tolerable levels.
A variation of the magnetic pick-up is
the eddy current sensor which detects ferrous and nonferrous objects. Eddy current
sensors are not as simple as magnetic pickups because they require an oscillator and
other circuitry in order to provide an output
voltage. They do function well at extremely
low speeds because they are not dependent on the rate of flux change like the
magnetic pick-up.


Clutches and
Braking Techniques
In many applications, it is desirable or
necessary to accelerate the driven load
smoothly from rest or to engage two independent drive trains in order to transfer
power from one to the other. It often becomes necessary to bring a driven load
down from its operating speed to zero
speed (standstill) more rapidly than the
normal coast time experienced when the
motor is merely disconnected from its
power source. Smooth acceleration, or the
transfer of power from one drive train to
another, is accomplished with clutches.
Deceleration is accomplished by braking
Clutches and brakes are quite similar in
functionality and method of operation. The
basic difference is that in clutch applications, both drive trains are free to rotate. A
brake, on the other hand, is a clutch with
one member held stationary. In fact, the
functionality is so similar that, for some
applications, clutches and brakes can be
combined into a single unit called a clutchbrake.
In less precise applications, electromechanical brake assemblies can be costly. In

these situations, dynamic braking can often
be used to provide a cost-effective method
of quickly reducing the speed of the driven
load. We’ll first look at electromechanical
clutches and brakes and their actuation
methods. Then we’ll discuss the various
dynamic braking methods for both DC and
AC motors. Many of the clutches and
brakes that will be discussed have limited
or no use in fractional horsepower motor
applications but are included so that the
reader will have a better understanding of
the scope of clutch and brake techniques.

Electromechanical clutches are categorized by both the techniques used to engage or stop the load as well as by their
method of activation.


The techniques include:
1) friction,
2) electromagnetic, and
3) mechanical lock-up.

Friction Techniques
This type of clutch or brake uses the
friction developed between the two mating
surfaces to engage the two drive trains or
stop the load. One surface is made of metal and the other consists of a high friction
composition material.

Disc Type: This type of clutch or
brake consists of a friction plate and a disc.
Figure 10-1a shows a simple plate style in
which one plate is pressed against the other. The friction created by their contact
causes one of two things to happen:
1) in the case of a clutch, both plates will
turn or,
2) if one plate is held stationary as in a
brake, the other plate will stop when
contact is made.
Quite often a caliper arrangement is
used for braking. See Fig. 10-1b. The
pinching action of the caliper against the
rotor makes this a very effective braking
technique. Caliper disc brakes require high
activation pressure and dissipate heat much

Fig. 10-2: Typical drum type clutchbrake.

better than plate style discs. They are also

Drum Type: Drum type clutches
and brakes have cylindrical shaped surfaces mounted on a common axis. See Fig.
10-2. The friction shoes either expand outward to contact the machined surface of
the rotating drum or they can contract inward to engage a rotating shaft. As before,
if both shafts rotate, the contact results in a
clutch action. If the drum is stationary, the
shoes provide braking action.
The contraction type is especially suited
for high cyclic operation because centrifugal force causes rapid withdrawal of the
shoes when released. Drum clutches and
brakes transmit high torque.
Cone Type: Cone type clutches
and brakes are a cross between disc and
drum types. They provide the benefits of
light engagement forces and high torque
transfer but are difficult to disengage. Consequently, they are rarely used.


Fig. 10-1: Typical disc type clutch or
brake mechanisms: a) plate type (left),
and b) caliper type (right).

Clutches and brakes employing
electromagnetism are classified as
nonfriction type. They are used in
applications requiring variable slip. They
utilize the principles of electromagnetic


Fig. 10-3: Eddy current type clutchbrake.
Fig. 10-4: Hysteresis type clutch-brake.

attraction to cause engagement or to reduce load speed by adjustable slip.
Eddy Current Type: Eddy current
type clutches are used in adjustable speed
applications but cannot be operated at zero
slip. As brakes, they have no holding
power and are used primarily for drag
loads. They have a tendency to run hotter
than the other electromagnetic types and
sometimes require additional cooling
Eddy current type clutches and brakes
consist of a stationary field coil, an input
drum and a coupling pole assembly which
functions as an output rotor. Refer to
Fig. 10-3.
A coil sets up a magnetic field, linking
the input drum with the output rotor. Eddy
currents induced in the input drum create a
new magnetic field which interacts with the
magnetic field in the output rotor. A resulting coupling torque is created which is proportional to the coil current.

Hysteresis Type: This type of
clutch provides constant torque which can
provide varying degrees of slip as long as
the heat dissipating capacity of the clutch is
not exceeded. Torque is transmitted by
hysteresis effect. Torque is independent of
speed, except at high speeds. It is also a
linear function of the control current except
at low currents and near magnetic saturation. As a result, precise control can be
achieved with hysteresis type clutches and
A coil on the input rotor generates a
magnetic field in the rotor and the drag
cup. Refer to Fig. 10-4. Torque is transmitted through the drag cup because the
hysteresis effect in the drag cup causes the
drag cup flux to change at a slower rate
than the rotor flux. Hysteresis type clutches
and brakes are used quite often in fractional horsepower motor applications.


Mechanical Lock-up
Mechanical lock-up techniques apply to
clutches only and use direct mechanical
connections between the input and output
components to transmit torque. Operation
of mechanical lock-up devices usually requires speed, a speed differential between
input and output components, or a specific
rotational direction. Many use centrifugal
force, wrapping action or wedging action
to lock the two members together, and are
sometimes considered to be self-activating.

Square Jaw Type: A square jaw
clutch is shown in Fig. 10-6. The square
teeth of one member mate with the cutouts on the other member to provide a
positive lock-up which cannot slip. It is
limited to low speeds (under 10 RPM)
because of its nonslip characteristics.
Fig. 10-5: Typical magnetic particle type

Magnetic Particle Type: The
input disc of a magnetic particle clutchbrake is located within the output housing.
See Fig. 10-5. The space between the disc
and the housing is filled with magnetic particles. An electromagnet surrounds both the
input and output housings. Energizing the
electromagnet causes the metallic particles
to form a rigid bond between the two
housings and transmit torque from one to
the other.
The amount of particle bonding is controlled by the current flow and is directly
proportional to the torque. The torque slip
can be adjusted by varying the current flow
in the coil of the electromagnet. These
types of clutches and brakes are useful in
variable speed tensioning and positioning

Fig. 10-6: Square jaw clutch.

Spiral Jaw Type: Because of its
sloped surface design (Fig. 10-7), the spiral jaw clutch offers smoother running engagement than the square jaw type. It can
be engaged at speeds up to 150 RPM.
However, it has a tendency to freewheel,
and can only run in one direction. Reversing the direction of rotation will cause

Fig. 10-7: Spiral jaw clutch.


Fig. 10-10: Wrap spring clutch.

Toothed Type: Toothed clutches
combine the benefits of electrical, pneumatic or hydraulic actuation with positive
mechanical lock-up. They can be engaged
at speeds up to 300 RPM. See Fig. 10-8.

Wrap Spring Type: This type of
clutch uses a coiled spring to attach one
shaft to the other. Rotation in one direction
tightens the spring around the output shaft
and transmits torque. Rotation in the other
direction uncoils the spring and releases the
output shaft. Refer to Fig. 10-10.
Roller Ramp Type: Rollers sliding on the ramped surfaces of a hub provide
the means of transmitting unidirectional
torque in these types of clutches. See Fig.
10-11. When actuated, the clutch causes
the roll cage to position the rolls at the top
of the ramp and engage the hub and sleeve.
When the clutch is disengaged, the roll cage
forces the rolls down the ramp away from
the sleeve.

Fig. 10-8: Toothed type clutch.

Sprag Type: A sprag type clutch
has an inner and outer race with sprags in
between. See Fig. 10-9. Because of their
shape and size, they wedge themselves
between the races when rotation occurs in
the proper direction. The wedging action
locks the two races together and transmits
torque from one shaft to the other. They
are unidirectional.
Fig. 10-11: Roller ramp clutch.

Actuation Methods
There are four basic methods used to
actuate clutches and brakes:
1) electromagnetic,
2) mechanical,
3) pneumatic, and
4) hydraulic.
Fig. 10-9: Sprag type clutch.


Electromagnetic is the primary method
of actuation in fractional horsepower applications because it offers the most control
and flexibility. The other methods of actuation will be discussed for completeness, but
they are usually reserved for specific application or higher horsepower motors.
Before choosing an actuation method,
the applications engineer should ask
several question:
1) How much torque is needed?
2) What is the best available engagement
3) Does the application require electronic
or remote control?
4) How much response time is needed?
5) Are there any special environmental
requirements that must be satisfied?
6) What is the duty cycle?
7) What are the temperature requirements
of the clutch or brake?
8) What is the maximum operating speed
of the system?
9) What space or weight requirements
must be satisfied?
10) What are the service life and maintenance requirements?

applications where it would be difficult,
impractical or too expensive to run the
piping or tubing required for the other
types of actuation.
A typical electrically actuated clutch or
brake is shown in Fig. 10-12. One half
consists of an armature attached to the
drive motor or input shaft. The other half is
an electromagnet embedded in an iron shell
and covered with a friction pad. When
voltage is applied to the coil of the electromagnet, it attracts the armature and engages the clutch. If both components turn freely, the unit functions as a clutch. If one is
held stationary, braking action takes place.
Electromagnetic clutches and brakes
can have rotating or stationary coils. Rotating coil types (Fig. 10-12a) use slip rings
and brushes which can cause sparking,
making them unsuitable for explosive atmospheres. The stationary field type with a
fixed coil (Fig. 10-12b) eliminates this
The simplest type of electrical actuator
consists of a plug-in module which converts AC line voltage to DC and uses on/
off switching circuits. More sophisticated
controls include solid state modules with
integral time delayed outputs. Some are
equipped with torque adjustment controls
for soft starts and stops.

Based on this information, the best
choice of clutch or brake type and actuation method can be determined.

Pneumatic Actuation: Air actuation methods are common in industrial
applications involving larger horsepower
motors. Compressed air supplies are
readily available in most industrial settings.
Pneumatic actuation requires piping or tubing as well as pressure regulators, filters,
lubricators, control valves, exhaust valves
and mufflers to control various aspects of
the pneumatic system. This support equipment and the associated costs and maintenance they require are the main disadvantages of pneumatic actuation systems.

Electromagnetic Actuation:
Extremely fast cycling rates are achievable
through electromagnetic actuation. Its
torque range is limited, compared to hydraulic and pneumatic actuated clutches
and brakes.
Fractional horsepower motor applications often involve some form of automatic
operation involving electrical commands.
That is why electrical actuation is more
common in these applications. Electrical
actuation also works well in remote


Hydraulic Actuation: Hydraulic
actuation provides fast response and
smooth engagement when control valves
are used to control hydraulic pressure.
Hydraulic pistons can deliver high torque
requirements needed to operate heavyduty clutches and brakes. Like pneumatic
actuation systems, the piping and associated control mechanisms are the main dis
Mechanical Actuation: This is
the simplest and least expensive form of
actuation. Mechanical actuation depends
on human strength to depress a pedal or
move a lever, so force is limited to about
75 lbs. This limits torque transmission and
cycling rates. Mechanical actuation is

usually reserved for vehicles and industrial
equipment like cranes and hoists.
Centrifugal clutches which engage when
a motor reaches a predetermined speed
are also examples of mechanical actuation.
Centrifugal clutches cannot be controlled
externally, however.

Motors should not coast more than a
few shaft rotations after being deenergized. If the application requires
precise braking, electromechanical brakes
and clutches like those previously
discussed should be used. In less critical
applications, dynamic braking techniques
can be employed.

Fig. 10-12: Electromagnetic clutch: a) rotating coil (left), and b) stationary coil (right).


Dynamic braking is achieved by altering
the connections to the motor with or without the aid of an auxiliary power source,
depending on the motor type (DC or AC).
In either case, the motor acts like a generator and the kinetic energy of the motor and
the driven load is used to exert a retarding
force to slow the forward rotation of the

DC Motors
Various techniques are used to accomplish dynamic braking in fractional horsepower DC motors and gearmotors. Each
will be explained in detail.

Shunt-Wound Field DC Motors: Perhaps the easiest motor to dynamically brake is the shunt-wound field
motor. A shunt-wound motor is a DC
brush-type machine with field and armature
connected in parallel across a DC power
supply. See Fig. 10-13. The interaction of
the magnetic field set up by the field winding and the current flowing in the armature
conductors produces torque or normal
motor action.

Fig. 10-13: Dynamic braking circuit for
four-wire shunt-wound motor or two-wire
PM motor.

A counter electromotive force (cemf) is
generated in the conductors of any armature rotating in a magnetic field. While the
unit operates as a motor, the cemf opposes
the line voltage and limits the current in the
armature winding to a value just sufficient
to supply adequate output shaft power
requirements. Braking is simply accomplished by disconnecting the armature from
the power source and placing either a short
or current limiting resistor across the armature terminals while the field coils remain
At the instant this is done, the rotation
will continue because of the inertia of the
armature and its driven load. The armature
rotating in a magnetic field will continue to
have voltage (cemf) generated in it that will
be proportional to its speed and the
strength of the magnetic field. The armature
circuit which is now closed by a short or
current limiting resistor will have a current
flowing in it opposite to that originally produced by the power source.
The reversal of current will produce a
torque opposite to the original motor action
and the motor will begin to reverse itself.
However, during the reversing process, the
speed in the forward direction will be rapidly reduced and so will the voltage generated in the armature. At the point of reversal or zero speed, the generated voltage is
zero. The motor stops at this point since no
current can flow and no torque is generated to continue the reversing process. The
motor has been dynamically braked.
The rate of braking is controlled by the
value of the shunting resistor. A small resistance will allow a large amount of current
flow and, since the reversing or braking
torque is proportional to the current, the
motor and load will stop in a minimum
amount of time. Some resistance is usually
recommended to limit the severity of the
braking action, especially with gearmotors.


NOTE: The field winding should be
disconnected from the power source
after the motor stops unless the field is
meant to be connected continuously
across the line at standstill.

Permanent Magnet Field DC
Motors: Dynamic braking of PM motors is accomplished in the same way as
the shunt motor with some additional advantages. The shunt motor cannot be dynamically braked to a stop in the event of a
power failure because a field voltage must
be present to generate the braking action.
With a permanent magnet (PM) motor,
a power failure will not affect the motor’s
braking capability because its magnetic
field (a permanent magnet rather than a
coil) is not affected by a power outage. A
normally closed relay or similar device
across the armature will automatically function in case of a power failure, shorting the
armature’s terminals and initiating the braking action. This inherent characteristic is
important, for example, on reel drives to
prevent unwanted spillage of tape.
Figures 10-13 and 10-14 apply to PM
field motors (except that the shunt field in
Fig. 10-13 should be replaced by a permanent magnet field). Figure 10-14 illustrates the use of electronic components to
achieve dynamic braking in a unidirectional

Fig. 10-15: Dynamic braking circuit for a
four-lead series wound motor operated
from a DC source. This may not function
properly if the motor is operated from an
AC source.

PM motor application. The diode biases
the transistor off in the run mode. When the
armature no longer draws current from the
line (brake mode), the transistor will conduct because the polarity of the armature
cemf is opposite to the line voltage.

Series Wound Motors: Universal (AC/DC) or series wound motors may
be dynamically braked in several different
ways. One method that applies to a fourlead series wound motor is quite similar to
that described for the shunt-wound motor.
See Fig. 10-15. The only difference is the
addition of resistance in series with the
much lower resistance of the field circuit to
prevent excessive heating during frequently
repeated or extended braking cycles. This
method is not generally successful when the
motor is powered by AC as the motor
tends to continue running without braking
because of repulsion motor behavior.
A three-lead, reversible series wound
motor can be very conveniently braked by
simply connecting the armature across the
opposite set of field coils. See Fig. 10-16.

Fig. 10-14: Dynamic braking circuit for a
unidirectional two-wire PM motor.


Fig. 10-16: Simplified dynamic braking
circuit for a three-wire series motor.

It should be noted that the series wound
motor scheme shown in Fig. 10-16 is “selfexcited” since it brakes the motor without
the need for any external source of power.
However, because of the self-excited feature, braking by these methods is less consistent or reliable than the schemes presented for the shunt or PM motors in Figs.
10-13 and 10-14.

Compound Wound Motors:
A compound wound motor, having characteristics of both a shunt and a series wound
motor, can be braked by:
1) a shunt or series braking circuit,
2) a self-excited series wound braking
circuit, or
3) a combination of both.
However, because of the slower speed
of the compound wound motor, the shuntwound braking circuit is preferred.

Plugging as a Means
of Braking
Reversing a motor by reversing the
power to the armature while the field remains connected is called “plugging.” This

technique can be used to brake a motor if
the power to the motor is removed at the
point when the armature passes through
zero speed in its attempt to reverse itself.
Plugging is more severe than the braking
methods described earlier because the
voltage across the armature (in the case of
a shunt motor) and across the entire motor
(in the case of a series motor) is approximately twice its normal value at the instant
of reversal. The generated voltage in the
armature is added to the line voltage from
full speed down to zero. Under normal
running conditions, the generated voltage
(cemf) opposes the line voltage.
Plugging is not always recommended as
a means of braking. In wound field motors,
for example, the braking torque generated
is no longer proportional to the high armature current which is drawn. Excessive
armature heating and brush arcing occur
without the advantage of significant increases in torque.
In the case of PM motors, the coercive
force of the magnets may be exceeded,
causing a resultant decrease in magnet
strength. If plugging is contemplated, the
motor manufacturer should be consulted to
establish motor limitations.

Other Considerations
Relays, switches and electronic devices
shown in Figs. 10-13 through 10-16 are
meant to suggest only some of the possible
ways of braking the motors discussed.
Before using relays, switches and contactors in DC circuits, check that the devices have a DC rating of sufficient capacity. It is also important during the braking
action that these devices be equipped with
“break before make” contacts. Overlapping of the breaking and making functions
can cause problems.
Some applications require that the holding torque be continued after the motor has
stopped rotating. Of the braking circuits


described, the only one capable of
providing a reasonable holding torque for a
wound field motor is the circuit in Fig. 1013. Permanent magnet motors have
inherent holding. The strength of both
depends upon the slot effect of the
The nature of the load is often a vital
factor in dynamic braking applications.
Caution must be exercised in applying
motors which are to be dynamically braked
or plugged. In such applications, high
currents and dynamic mechanical forces
are generated during the braking period.
For safety reasons, the thermal and
structural capabilities of the drive system
should not be exceeded. Dynamic braking
of high inertia type loads require additional
consideration because of the mechanical
and thermal strains which can be induced in
both the motor and other associated torque
transmitting components.
While temperature rise is important in
the normal operation of a motor, it is even
more important in the dynamic braking of
the motor. Since the braking torques
generated with some schemes are higher
than normal running torque, the energy
which the motor must dissipate rises
Brush life can be expected to decrease
when the frequency or duration of dynamic
braking is substantial. Special brushes are
usually required.

AC Induction Motors
We will restrict this discussion to those
AC motors which utilize a nonenergized
rotor typically found in small motors. In
most cases, this means some form of a
squirrel cage rotor except for the capacitor
hysteresis type which uses a permanent
magnet type rotor. In most cases, no
distinction will be drawn between a
synchronous and a nonsynchronous motor
since any braking method discussed usually

will be applicable for either version in a
particular winding type.
In general, AC motors are dynamically
braked by removing the AC power from
the motor and substituting DC. When this
is done, the motor is very similar to the DC
shunt motor described earlier. The stator,
with DC applied, is similar to the field
winding of a shunt motor and the squirrel
cage rotor is similar to a shorted armature
in the braking mode. In essence, the motor
now acts like a DC generator with a shortcircuited armature.
The electrical output of the generator
has high circulating currents in the shorted
rotor bars. The mechanical input of the
generator is the kinetic energy of the rotor
and the connected load. This rotational
energy is dissipated in the form of heat (in
the rotor) when the motor is quickly
brought to a stop. The source of DC for
braking purposes can vary from batteries
and highly filtered supplies, to full wave and
half-wave sources. DC may also be supplied by a charged capacitor. The choice is
dependent on economics and the degree of
braking required. Pure DC is best but
more expensive to provide than rectified or
nonpure DC.
Whether one or all of a motor’s windings are used to brake, it is also a question
of economics and power supply availability. “Plugging” may also be used to brake
AC motors. Again, plugging consists of
reconnecting the motor (while running) so
that it wants to reverse itself. However, at
zero speed (before the motor can rotate in
the opposite direction), the power is removed. This method is limited to those
motors which are capable of reversing
while running.
A third method of braking small AC
motors, called “capacitor shorting,” is
limited to permanent split capacitor (PSC)
motors of the highslip nonsynchronous and
hysteresis synchronous types. The
procedure is to short the capacitor, placing


Fig. 10-17: Dynamic braking circuit for
shaded pole motor.

both the main and the capacitor windings
directly across the AC line. This method
eliminates the rotating field associated with
these motors and its torque producing capabilities. The two windings (main and capacitor) must be identical for the capacitor
shorting method to be effective.

Shaded Pole Motors: A shaded
pole motor is normally unidirectional with
only one stator winding connected to the
AC line. The only way to dynamically
brake this motor type is to apply some
form of DC in place of AC. See Fig.1017. Because of low motor impedance on
DC, voltage must be removed immediately
after braking (unless the DC is low enough
that it won’t overheat the winding). An
acceptable continuously applied power
level for braking can be obtained from the
motor manufacturer.
Split-Phase Motors: Motors
with split-phase windings employ centrifugally operated switches or starting relays

Fig. 10-18: Dynamic braking circuit for a
split-phase motor.

which serve to “cut out” or disconnect the
starting windings from the electrical supply
when the motor has come up to 75% of
running speed. To prevent burnout, starting
windings are intended to be connected to
the line for no more than a few seconds.
Since it is not normally recommended
that these motors be reversed while running, the only feasible way to dynamically
brake a split-phase motor is to apply some
form of DC in place of AC as in Fig. 1018. Again, because of low motor impedance, the DC voltage should be less than
the AC. The braking voltage should be
removed immediately after braking, since
the drop in speed will cause the centrifugal
switch or the start winding relay to reconnect the starting winding.
An electrolytic starting capacitor, in series with the starting winding, is recommended for this type of operation, since it
would overcome the starting winding heating problem by blocking the DC power
(the capacitor would also tend to provide
additional starting torque on AC).

Fig. 10-19: Dynamic braking circuit for
permanent split capacitor motor using
main winding only for braking.

Permanent Split Capacitor
Motors (including hysteresis
synchronous): Several different
braking methods can be considered for
permanent split capacitor (PSC) motors.
DC can, of course, be applied. Figure 1019 shows that the capacitor will prevent
the auxiliary winding from being used for
braking because the capacitor blocks the
flow of DC. In order to use the second
winding, a three-pole or three-contact
switch must be used to provide either a


Fig. 10-22: Dynamic braking circuit for
permanent split capacitor motor using
capacitor shorting method.

Fig. 10-20: Dynamic braking circuit for
permanent split capacitor motor using
windings in parallel for braking.

Fig. 10-21: Dynamic braking circuit for
permanent split capacitor motor using
windings in series for braking.

parallel or a series winding arrangement as
shown in Figs. 10-20 and 10-21.
The “plugging” method can also be used
on permanent split capacitor (PSC) motors
which can be reversed while running. This
is usually restricted to nonsynchronous
designs using a high slip rotor and to hysteresis synchronous motors. Plugging can
be accomplished by reversing either wind-

ing. However, the main winding is preferred to avoid high voltage problems associated with the capacitor.
On small motors (approximately 1/75
hp or smaller), the “capacitor shorting”
method can be used when the main and
capacitor windings are identical. As with
plugging, “capacitor shorting” is not applicable to low slip nonsynchronous motors
or reluctance synchronous motors. As the
size of the motor increases, this braking
method becomes less effective and there
may be a tendency to “creep” or to continue to rotate slowly at some very low
speed. The capacitor shorting method is
illustrated in Fig. 10-22.

Three-Phase Motors (Polyphase): A three-phase motor may be
dynamically braked by applying DC or by
plugging. For a Wye or a Delta-connected
motor, the braking circuit is shown in Fig.
10-23. In order to plug a three-phase motor for braking purposes, two input leads

Fig. 10-23 Dynamic braking circuit for three-phase motor.


be charged during normal running and then
used to supply the DC voltage necessary
to stop the motor in the braking mode.

Other Considerations
Fig. 10-24: Dynamic braking by capacitor
discharge method.

must be reversed. At the point of zero
speed, the motor is disconnected from the
AC line.

The DC Supply
All AC motor types can be braked by
applying DC to the windings. It was stated
earlier that pure DC is more effective than
rectified AC. It should be noted that some
motors (PSC type) may continue to rotate
at very low speed if braked by a half-wave
supply. The effectiveness of any combination cannot always be predicted, so some
trial and error tests should be conducted to
establish the best circuit for each application. In some cases, DC may be merely
supplied by the discharging of a capacitor.
Figure 10-24 shows how a capacitor may

Fig. 10-25: Parallel shaft gearmotor
(helical and spur gearing) with inertial
load on output shaft.

Holding torque must be considered with
AC motors. AC motors are not very effective at holding the load after bringing the
speed down to zero. The best holding
characteristics are provided by reluctance
synchronous motors. Because of their construction characteristics, reluctance type
rotors will tend to lock into preferred positions. Of course, if any of the motors discussed are energized to maintain holding
power, the electrical input must be low
enough to prevent winding and lubricant

Gearmotors must be given special consideration, particularly if they are to be
used to dynamically brake inertial loads.
Because of the high kinetic forces generated, gearing and other machine elements
may be damaged if not selected and applied properly.
It is important to remember that the
gearhead of a gearmotor is positioned between the inertial load and the motor’s rotor. Because an inertial load “wants to
keep on rolling” and backdrive a gearhead
after the normal forward driving power is
removed, both the inertia of the motor’s
rotor and an external inertial load can subject the gearhead components to dynamic
stresses that exceed their design capabilities. Therefore, the dynamic braking of
gearmotors driving inertial loads must be
carefully analyzed.
When considering the dynamic braking
of external inertial loads, it is useful to calculate the effect of the load as seen at the
output shaft of the gearhead. Figures 1025 and 10-26 show inertial loads (flywheels) directly connected to gearmotor


Fig. 10-26: Right angle worm gearmotor
(two stages of worm gearing with inertial
load on output shaft).

output shafts. It is also possible for considerable inertia to be “seen” by the gearhead
in applications employing pulleys and belts
(or sprockets and chains) in the drive system. If the output shaft is not directly coupled to the driven load (with speed altering
elements separated from the gearhead), it
will be necessary to calculate the equivalent
inertia at the gearhead driveshaft using
Equation 1 in Fig. 10-27.
Equation 1 shows that speed reductions
beyond the driveshaft reduce the inertia
seen by the gearhead output shaft. Speed
increases have the opposite effect according to the square of the speed ratio. Equation 1 is useful for analyzing the effects of
speed changes due to gears, belts or chain
drives coupled to the gearmotor output
shaft. Equation 2 illustrates the calculation
of inertia for simple discs like the flywheels
shown in Figs. 10-25 and 10-26.

Estimating Torque During
Dynamic Braking: If a gearmotor is
required to dynamically brake an inertial
load from full (normal) speed in a specified
period of time, one must consider whether
the gearhead would be capable of absorbing the stored kinetic energy of the inertial

load during the braking period (as opposed
to its normal function of transmitting the
torque necessary to drive the load).
Equations 3a and 3b in Fig. 10-27 provide useful approximations for analyzing the
effects of inertial loads when considering
dynamic braking of gearmotors. Equation
3a is a general equation, while Equation 3b
approximates the external braking torque
required to bring the system to rest in the
period (ds) after the electrical power is
disconnected from the motor (but dynamic
braking not yet applied). Equation 3a is
derived by assuming that the kinetic energy
of a mechanical system, driven by a gearmotor, uniformly decelerates and is converted into work done (dissipated energy).
In addition, Equation 3b ignores the inertia
of gearhead components and does not
consider additional dynamic loading imposed due to gearing backlash, or system
misalignments and inefficiencies.
Note that Equation 3b shows that two
inertial components are of major
WKr2 x R2 (internal inertia)
WK2lds (external intertia)
When the internal inertia component is
significantly larger than the external inertia
component, it is feasible to dynamically
brake the load through the motor winding.
However, if the external inertia component
is larger than the internal component, the
load should be externally braked or
If the internal inertial component in
Equation 3b is disregarded, it is apparent
that for a given output speed, the inertia of
the external load (WK2lds) and the braking
interval (ds) have great torque multiplying
possibilities that can be fed into the gearhead. A gearmotor which performs acceptably at its rating when driving a load
forward can easily fail due to excessive
loading imposed during dynamic braking


Equation 1: [WK2](lds) = [WK2](ls) Nl
[WK2](lds) = inertia of the external load as seen by the driveshaft at its speed.
[WK2](l) = inertia of the load at its driven speed.
= speed of the driveshaft (revolutions / minute)
= speed of the load (revolutions/minute)
Equation 2: [WK ]c = weight (lbs) x
[WK2]c = inertia of solid cylinder or disc rotating about its own axis
NOTE: Many handbooks provide formulas for calculating the
inertia of other geometric shapes.
Equation 3:
(Ids) (Nds)2
[(WK2r x R2) + WK2 lds] (Nds)2
a) Tds =
or: *b) Tds =
573 (∆ds)
221,185 (∆ds)
Tds = indicated torque required to bring the gearmotor driveshaft to rest
during braking (lb-in)
Ids = inertia of te entire mechanical system as seen by the gearmotor
WK2r = internal inertia contributed by the motor’s rotating member (rotor or
armature) (lb-in2)
R = ratio of the gearmotor’s gearhead
WK2 lds = inertia of the external load seen by the gearmotor driveshaft
Nds = gearmotor driveshaft speed (RPM)
∆ds = driveshaft revolutions during the braking period
221.185 = A constant associated with inch system with inch system units.
Note: If SI units are used (newtons and meters instead of pounds of
force and inches), the constant becomes 5,615.
*When R = 1, Equation 3b applies to nongeard motor.
Fig. 10-27: Formulas for calculating the effects of inertial loads on gearmotors.

(i.e., the backdriving torque caused by an
inertial load can exceed the forward driving
torque and be beyond the capability of the
Consider what happens when a gearmotor is forced to dynamically brake a
relatively high external inertial load. An
external inertial load on a gearmotor tends
to “backdrive” the motor through the gear-

head. Because the electrical braking torque
applied to the rotor is resisting rotation, an
almost instantaneous torsional binding effect occurs in the gearhead. Under this
condition, the motor winding is unable to
absorb all of the stored kinetic energy of
the rotating load and the remainder must be
absorbed by the torsion and deflection of
the various gearhead members, including


the axial movement and bending of the
motor’s rotor (the gearhead, in effect, becomes a mechanical spring).
The amount of energy absorption of
each of the gearhead members involved is
a function of their respective stiffness.
Therefore, the stored kinetic energy of the
load must be dissipated or absorbed by the
gear teeth, intermediate gearshafts (if more
than one stage), preload washers, the
driveshaft and gear housing. Moreover,
some of the kinetic energy is dissipated as
heat, due to friction from such sources as
the rotor and gearshaft bearings sliding in
their bores.

Considerations with Spur
and Helical Gearing: This type of
gearing is common to parallel shaft or inline (concentric shaft) gearheads. In fhp
gearmotors, such gearheads typically have
recess action type gearing which provides
advantages when driven forward, but offers relatively greater frictional resistance
than standard gearing when driven backwards. The resistance to backdriving manifests itself as a locking effect. It follows that
the amount of resistance to backdriving
increases with the number of stages of
gearing. Gearheads with many helical and
spur stages offer considerable resistance to

Special Considerations with
Worm Gearing: Dynamic braking of
gearmotors with worm gearing presents
additional considerations that do not exist
with spur or helical gearing. A primary
condition peculiar to worm gearing is the
possible self-locking effect. (“Self-locking”
is a term that describes an inherent characteristic of certain worm gears that prevent
them from being backdriven. The slow
speed shaft cannot be driven by an applied
force.) In worm gearmotors, self-locking is
a characteristic of higher gear ratios (typically greater than 15:1).

It is possible that during dynamic braking, gearing that is normally non-self-locking will lock. This can occur when the lead
angle of the worm gear in the lower ratios
is such that during dynamic braking, the
friction in the gearing increases to the point
where self-locking occurs. At the moment
of locking, the contacting gear teeth and
other gear train parts must dissipate the
energy of the load. For applications where
the braking forces exceed the shear
strength of the gear teeth, failure will occur.
Braking forces slightly under the shear
strength of the gears and other parts will
not show up as immediate failure, but can
severely shorten gearmotor life through

General Guidelines for High
Inertia Gearmotor Applications: Dynamic braking of high inertial
loads on gearmotors requires that the energy be absorbed or stored in the various
gear train parts, which act like springs. A
significant decrease in the stresses imposed
on these parts can be effected by utilizing a
torsionally resilient coupling (the effect is
that of a torsional spring) or clutching that
disconnects or limits the transmitted torque.
Protection of the gear train members can
also be accomplished by stronger gearhead
parts, or by reduction of external inertia or
load speed.
A good general rule to follow in
applying dynamic braking to gearmotors is
to use the minimum power for braking
necessary to obtain the desired results. If it
is required that the maximum allowable
coast is to be held to 90 degrees at the
driven shaft, it would be unwise to apply
dynamic braking that limits the coast to a
much lesser amount.
For the same reason that temperature
rise is an important consideration under
normal operating conditions, it is even
more critical when dynamic braking is applied. If dynamic braking is required at


frequent intervals, operating temperature of
the gearmotor and its lubricants would be
higher than that of a nondynamic braking
application with the same load. It is better
to limit the braking so it will not exceed the
allowable temperature rise of the winding
or gear lubricant.
Adhering to these guidelines results in a
cooler running, more service-free gearmotor, and places lower stresses on the gears
and other mechanical components affected
by dynamic braking.

In the majority of motor and gearmotor
applications, the dynamic braking capability of the motor is normally not the determining factor in the motor selection. Voltage, frequency, speed, torque, etc. are
usually more important considerations in
establishing whether the motor should be
an AC or DC motor, or one of the particular types of AC or DC construction.
Under these circumstances, obviously,
one accepts the braking capability that the
particular motor offers. This generally presents no problem since all winding types
and most of the dynamic braking methods
described substantially reduce the stopping
time and satisfy the majority of the less
critical braking applications.
For example, a 1700 to 1800 RPM
NEMA 42 frame motor (approximately
4.5" diameter) typically would coast from
40 to 120 revolutions when the power is
removed without the aid of dynamic
braking. With dynamic braking, the rotor
would come to a stop within one to six
revolutions with no load attached (except
for gearing). Any load would, of course,
reduce the stopping time if it were frictional

in nature and would increase it if it were
highly inertial.
The previously mentioned range of
stopping times without dynamic braking
may seem excessive, but it is based on a
number of different motor types, each of
approximately the same horsepower level.
This criteria results in differences in rotor
lengths and construction which, along with
the differences in windings, provides an
even wider stopping range when dynamically braked.
As might be expected, a smaller motor
would stop more rapidly than a larger motor. A 1700 to 1800 RPM 32 frame motor
(approximately 3.5" diameter) would typically stop in about half the time taken by
the larger 42 frame motor, or 20 to 60
revolutions without dynamic braking and
0.5 to 3 revolutions with dynamic braking
(at no load but with gearing included).
Since the time to stop a rotating part is
directly proportional to its inertia, the
smallest possible motor should be used to
drive the load where fast braking is desired. Motors with centrifugal switches and
high density rotors should be avoided
(since their relatively higher inertia in small
motors may be significant).
Although we have limited our discussion
to stopping time using a speed of 1700 to
1800 RPM, it should not be forgotten that
the kinetic energy of a rotor is proportional
to the inertia times the speed squared.
Therefore, the speed of the rotor should be
kept to a minimum for best braking results.
(High speed series wound motors are particularly difficult to brake rapidly and consistently.)
When using induction-type motors, the
additional braking torque generated by
using a high resistance rotor over a low
resistance rotor, or a reluctance synchronous over a nonsynchronous type, should
be considered when fast braking is desirable. Also, the reluctance synchronous
motor will provide some holding torque, a


criteria which might not be satisfied by any
other motor type. The holding torque difference between a nonsynchronous induction motor and a synchronous reluctance
induction motor may be as much as 10:1
with continuous DC applied.
There appears to be a definite advantage to using an AC induction motor over a
Fig. 10-29: Shunt motor speed / torque

The reason appears to be the result of
differences in rotor inertia.
As mentioned earlier, when using a
split-phase motor, it is advisable to use a

Fig. 10-28: PSC motor speed / torque

DC shunt-wound motor which is traceable
to the braking torque generated by each.
Comparison of Fig. 10-28 with Fig. 10-29
will illustrate the basic differences between
a PSC motor and a shunt-wound motor
(both of the same hp rating).
In Fig. 10-28, the first (right-hand)
quadrant represents the normal running
characteristic curve. The second (lefthand) quadrant shows the normal braking
characteristic. Since the AC motor has a
high braking torque close to zero speed, it
tends to be “snubbed down” to a stop
quite nicely, whereas the DC motor (Fig.
10-29) will tend to lose its braking force as
the speed is reduced and tends to coast
more. The area under the curve divided by
the operating speed represents the average
braking torque.
In some cases, the split-phase motor
(Fig. 10-30) may not brake as quickly as
the PSC motor (compare Fig. 10-28 with
Fig. 10-30) even though its generated
braking torque is as high or higher than
thatof a permanent split capacitor motor.

Fig. 10-30: Split-phase motor
speed / torque curves.

starting capacitor in series with the starting
switch and winding to limit the DC braking
current and prevent overheating of the
starting winding and destructive arcing at
the starting switch contacts.
Although the series wound motor can
be furnished in a smaller package than other motor types with the same horsepower,
it does not brake as consistently as other
motor types because of its higher operating
speed (high kinetic energy) and limited
braking power available by the normal
regenerative method.
The capacitor discharge method, described earlier, is only effective on small
subfractional induction motors driving low
inertial loads, since a reasonably sized ca-


Fig. 10-31: Braking by half-wave with
capacitor discharge.

Fig. 10-32: Half-wave braking of a PSC

pacitor has only a limited amount of stored
energy to dynamically brake or counteract
the kinetic energy of the motor and its load.
Frequently, a capacitor is used in conjunction with a diode to provide a “combination” half-wave and capacitor discharge
braking circuit to eliminate the shortcomings of each. Used by itself, the capacitor
has limited energy to release while the halfwave brake by itself may cause a PSC
motor to rotate slowly after its speed has
been brought down from its original level.

Figure 10-31 is a typical capacitor/halfwave braking circuit that may be used in
place of full wave or pure DC to provide
dynamic braking almost equivalent to the
latter. The slow rotational speed experienced with a PSC motor after the initial
braking period with half-wave DC can
often be eliminated by bypassing the motor
capacitor in the braking mode as shown in
Fig. 10-32.


Appendix 1
List of Associations and Standards Organizations
Most motor and control manufacturers design their products to conform to a variety of
safety standards. For convenience, a partial list of these standards organizations and associations is given below. Specific standards are referenced throughout the Handbook. If
the reader wishes to obtain more detailed information about a specific standard, the appropriate agency or association should be contacted directly.
American National Standards Institute (ANSI)
1430 Broadway
New York, NY 10018
American Society of Testing and Materials (ASTM)
1916 Race Street
Philadelphia, PA 19103
Canadian Standards Association (CSA)
179 Rexdale Boulevard
Rexdale, Ontario, Canada M9W 1R3
Electronic Industries Association (EIA)
2001 Pennsylvania Avenue, NW
Washington, DC 20006-1813
Institute of Electrical and Electronic Engineers (IEEE)
345 East 47th Street
New York, NY 10017
International Organization for Standardization (ISO)
1 Rue de Varembe
1211 Geneva 20, Switzerland
Mechanical Power Transmission Association
1717 Howard Street
Evanston, IL 60201
National Electrical Manufacturers Association (NEMA)
2101 L Street, NW
Washington, DC 20037
National Fire Protection Association (NFPA)
Batterymarch Park
Quincy, MA 02269
Underwriters Laboratories Inc. (UL)
333 Pfingsten Road
Northbrook, IL 60062


Appendix 2
Troubleshooting fhp Motors
IMPORTANT: Before servicing or working on equipment, always disconnect
the power source. This applies to all equipment, but special attention should be given
to thermally protected equipment using automatic restart devices, control equipment which
is under the control of external logic circuits, or brush-type motors and gearmotors when
the brushes are being examined or replaced. All of these situations present a higher potential for shock hazard or for injuries which might occur due to unanticipated mechanical
Before attempting to service any motor, read the manufacturer’s warranty information. In many cases, service by unauthorized persons will void the warranty.
If an external examination cannot determine the cause of the problem, always
consult the manufacturer before examining internal parts.
The motor environment should be cleaned regularly to prevent dirt and dust from interfering with ventilation or clogging moving parts. Refer to Chapter 7 for information on the
care and servicing of motors and gearmotors. Refer to Chapter 5 for motor environmental
protection information.
Before servicing motors or gearmotors employing capacitors, always discharge the
capacitor by placing a conductor across its terminals before touching the terminals with
any part of your body. Failure to discharge the capacitor could result in electrical shock.
In many cases, easy-to-detect symptoms will indicate exactly what is wrong with a
fractional horsepower motor. However, since general types of motor trouble have similar
symptoms, it is necessary to check each possible cause separately. The accompanying
table (on page A-3) lists some of the more common ailments of small motors and the likely causes.
Most common motor troubles can be checked by a series of inspections or basic measurements. The order in which these tests are performed are a matter of preference, but it
is advisable to perform the easiest first.
In diagnosing troubles, a combination of symptoms will often point to a specific source
of trouble. For example, if a motor will not start and yet heating occurs, there is a good
likelihood that a short or ground exists in one of the windings.
In the case of brushless motors, a symptom exhibited in the motor may actually be
caused by a problem in the control. External motion control circuitry often associated with
motor control systems can also be a source of problems. It is always wise to check the
connections between system components first before attempting to isolate internal motor
or control problems.
Centrifugal starting switches are occasionally the source of fhp motor problems. These
switches have a finite life and can wear in many ways depending on their design and use.
Open switches will prevent a motor from starting. When stuck in the closed position, the
motor will operate at slightly reduced speed and the start winding will overheat quickly.
Other problems can be caused by oxidized or out of alignment contact points on the
switch. It is important to remember that any adjustment of the switch or contacts should
be made by the manufacturer or an authorized service representative.
Because of the wear effects of brushes and commutators, commutated motors require
more maintenance than nonbrush types. The wear rate of brushes is dependent upon
many parameters (armature speed, current, duty cycle, humidity, etc.). For optimum performance, brush-type motors and gearmotors need periodic user-maintenance. Refer to
Chapter 5 for information on maintaining brushes.





High no-load speed.

List of Probable Causes:
1. Open circuit in connection to line (blown fuses,
overload protector tripped of faulty).
2. Open circuit in motor winding.
3. Defective starting switch.
4. Defective capacitor.
5. Starting (or auxiliary) winding open.
6. Starting switch not opening.
7. Overloaded motor (mechanical failure in load).
8. Winding shorted or grounded.
9. One or more windings open.
10. High mica between commutator bars or
rough commutator.

11. Dirty or out of round commutator.
12. Worn or stricking brushes and / or annealed
brush springs.
13. Open circuit or short circuit in armature winding.
14. Oil-soaked brushes.
15. Open shunt field or demagnetized magnets (PM).
16. Tight of seized bearings.
17. Interference between stationary and rotating member.
18. Failure of ventilation (blocked or obsturcted ventilation openings).
19. Shorted or grounded armature winding.
20. Wrong connection of motor.




Jerky operation-severe vibration.

* Note: Caution must be exercised since motor winding may be grounded or sudden start-up of motor may cause injury.



Excessive brush wear.



17, 22, 23, 24, 25

7, 16, 18, 21, 22, 28

7, 8, 16, 17, 20


1, 2, 7, 8, 16, 17, 20, 21

8, 16, 17, 20, 21

8, 16, 17, 20, 21



17, 22, 23, 24, 25

7, 16, 18, 21, 22, 28

4, 7, 8, 16, 17, 20

4, 5, 9, 20

1, 2, 4, 5, 7, 8, 9, 16, 17, 20,


4, 8, 16, 17, 20, 21

8, 16, 17, 20, 21



Sluggish-sparks severely at the brushes.

Reduction in power-motor gets too hot.

17, 22, 23, 24, 25

17, 22, 23, 24, 25

Excessive noise (mechanical).

7, 16, 18, 21, 22, 28

6, 7, 8, 16, 17, 20

6, 7, 8, 16, 17, 20
7, 16, 18, 21, 22, 28

3, 4, 5, 20

1, 2, 3, 4, 5, 7, 8, 9, 16, 17,
20, 21

Starts, but heats rapidly.

3, 5, 20

1, 2, 3, 5, 7, 8, 9, 16, 17,
20, 21


Split Capacitor


Capacitor Start

Runs too hot after extended operation.

Will not always start, even with no load, but will run in either direction
when started manually*.

Will not start.


Motor Type


7, 8, 16, 17, 20

10, 11, 12, 13, 19

10, 11, 13, 19, 23, 26, 27

13, 16, 17, 19, 20, 21


10, 11, 12, 13, 14, 19

17, 22, 23, 24, 25

7, 16, 18, 21, 22, 28

7, 8, 16, 17, 19, 20


1, 2, 7, 8, 12, 13, 16, 17, 19, 20,

Series, PM, Shunt Compund)

21. Improper or low line voltage (not within ±10% of
nameplate rating).
22. Worn bearings.
23. Unbalanced rotor or armature (vibration).
24. Poor alignment between motor and load,
loose motor mounting.
25. Amplified motor noises due to mounting conditions.
26. Incorrect spring tension.
27. Lack of moisture.
28. High ambient temperature.



8, 9, 16, 17, 20, 21



17, 22, 23, 24, 25

7, 16, 18, 21, 22, 28


1, 2, 7, 8, 9, 16, 17, 20, 21

(2 or 3-Phase)

Appendix 3
Helpful Shortcuts
Resistor Value Codes
The color code adopted by the Electronic Industries Association is used to standardize
the markings on resistors so that their resistance value can be determined. The color band
system is the the most common marking method.
The first color represents the first significant digit of the resistor value. The second color represents the second significant digit. The third color corresponds to a power of ten
multiplier. Quite simply, it represents how many zeros to add after the significant digits. A
fourth color is used to indicate the tolerance of the resistor. The body-end dot and bodyend band systems are also used.
Resistor Color Codes






No Color





Guaranteed Minimum Value

Left-Hand Rule for Electromagnetism
This rule is helpful in remembering the principle of electromagnetism used in electric
motors and for determining the direction of current flow in relation to magnetic field and
conductor motion. Bend your left hand in the shape shown in Figure A3-1. The thumb
points in the direction of force on a conductor. The first finger points in the direction of the
magnetic field, north to south. The second finger points in the direction of current flow.

Fig. A3-1: Left-hand rule for electromagnetism

Fig. A3-2: Right-hand rule for

Right-Hand Rule for Induction
This rule helps you to remember the relationship between current flow and magnetic
fields in a generator. See Fig. A3-2. The thumb points in the direction of conductor motion. The first finger points in the direction of the magnetic field and the second points in
the direction of the emf or generated current flow.
Determine surface speed in feet per minute (sfm) for a given spindle speed
and work (or tool) diameter by multiplying ˘ of the speed by the work or tool diameter.
For example: For a lathe running 400 rpm and a 3" work diameter, what is the sfm? Answer: ˘ of 400 rpm = 100. 100 x 3" = 300 sfm.
When you want to select a speed to give a desired surface speed, divide desired sfm
by work or tool diameter and multiply by 4. For example: You want 300 sfm for a 3"
work diameter. To find machine speed: 300 ÷ 3" = 100. 100 x 4 = 400 rpm.
The correct wrench size for nuts and bolts doesn’t have to be found by trial
and error. To pick up just the tool you need, you merely have to remember that the right
wrench is almost always 1˚ times the nominal thread size of the bolt or nut. For example:
A 1˚” bolt takes a ∫” wrench (˚” x 1˚” = ∫”).
Equivalent hardness. In the most commonly used portions of scales, Brinell
hardness numbers equal approximately 1/10th of the equivalent hardness indicated on the
Rockwell “C” Hardness Scale. Multiply Rockwell “C” numbers by 10 to get the approximate Brinell equivalent. For example: 50 RC x 10 = about 500 Br. 350 Br ÷ 10 = about
35 RC.
Find spur-gear DP (diametral pitch) by measuring the number of times two
pitches will fit in 3" on a scale, and halve it to produce the pitch. Measure halfway between the teeth at about the middle of the tooth depth. Disregard small departures to arrive at an even answer. (Standard pitches in the size normally encountered are whole
numbers from about 4 diametral pitch through 40 DP.)
Determine exact pitch diameter of any standard spur gear without so
much as touching the gear in question. Simply count the teeth and divide by the diametral


Appendix 4
Motor Application Formulae
T = torque or twisting moment (force x moment arm length)
π = 3.1416
N = revolutions per minute
hp = horsepower (33,000 ft-lbs. per minute); applies to power output
J = moment of inertia
E = input voltage
I = current in amperes
P = power input in watts
T(lb-in.) x N
hp = ————————
hp = T (oz-in.) x N x 9.917 x 10-7 = approximately T (oz-in) x N x 10-6
hp x 746
P = EI x power factor = ————————
motor efficiency

Power to Drive Pumps:
gallons per minute x total head (including friction)
hp = ————————————————————————
3,960 x efficiency of pump
approximate friction head (feet) =
pipe length (feet) x [velocity of flow (fps)]˝ x 0.02
5.367 x diameter (inches)
Efficiency = approximately 0.50 to 0.85

Time to Change speed of Rotating Mass:
J x change in rpm
Time (seconds) = ————————————
308 x torque (ft-lb.)
weight (lbs.) x [radius(feet)]˝
J (disc) =—————————————
weight (lbs.) x [outer radius(feet)]˝ + (inner radius in feet)˝]
J (rim) = ———————————————————————

Power to Drive Fans:
cubic feet air per minute x water gauge pressure (inches)
hp = —————————————————————
6,350 x efficiency


Ohm’s Law:
amperes = ———

Power in DC Circuits:
Watts = volts x amperes
volts x amperes
Horsepower = ————————
volts x amperes
Kilowatts = ————————
volts x amperes x hours
Kilowatts hours ———————————

Power in AC Circuits:
volts x amperes
Apparent power: kilovolt-amperes (KVA) = ————————
Power factor = ————————
volts x amperes x power factor
Single-phase kilowatts (Kw) = ————————————————
volts x amperes x power factor x 1.4142
Two-phase Kw = ————————————————————
volts x amperes x power factor x 1.7321
Three-phase Kw= ——————————————————

Geometric Formulae:
π = 3.1416
D = diameter
Area of circle = ——
Area of sphere = πD˝
π D≈
Volume of sphere = ——
Area of triangle = ˚ altitude x base


Appendix 5

Alcohol (100%)
Carbon Tetrachloride
Castor Oil
Glue Liquid
Hydrochloric Acid
Lard Oil
Linseed Oil
Machine Oil
Sodium Silicate
Sulphuric Acid
Tung Oil




Point oF




per oF

% Pure











Appendix 6
Temperature Conversions




The numbers in italics in the center column refer to the temperature, either in Celsius or
Fahrenheit, which is to be converted to the other scale. If converting Fahrenheit to Celsius, the equivalent temperature will be found in the left column. If converting Celsius to
Fahrenheit, the equivalent temperature will be found in the column on the right.

-100 to 30




31 to 71



72 to 212



87.8 22.2 72
89.6 22.8 73
91.4 23.3 74
93.2 23.9 75
95.0 24.4 76
96.8 25.0 77
98.6 25.6 78
100.4 26.1 79
102.2 26.7 80
104.0 27.2 81
105.8 27.8 82
107.6 28.3 83
109.4 28.9 84
111.2 29.4 85
113.0 30.0 86
114.8 30.6 87
116.6 31.1 88
118.4 31.7 89
120.0 32.2 90
122.0 32.8 91
123.8 33.3 92
123.8 33.9 93
127.4 34.4 94
129.2 35.0 95
131.0 35.6 96
132.8 36.1 97
134.6 36.7 98
136.4 37.2 99
138.2 37.8 100
140.0 43 110
141.8 49 120
143.6 54 130
145.4 60 140
147.2 66 150
149.0 71 160
150.8 77 170
152.6 82 180
154.4 88 190
156.2 93 200
158.0 99 210
159.8 100 212



213 to 620



621 to 1000







Appendix 7
SI (Metric) Conversion Table
SI Unit

Imperial/Metric to SI

SI to Imperial/Metric


meter (m)

1 inch = 2.54 x 10 -2m
1 foot = 0.305 m
1 yard = .914 m

1 m = 39.37 inches
= 3.281 feet
= 1.094 yards


kilogram (kg.)

1 ounce (mass) = 28.35 x 10 -3kg.
1 pound (mass) = 0.454 kg.
1 slug = 14.59 kg.

1kg = 35.27 ounces
= 2.205 pounds
= 168.521 x 10 -3 slug


square meter
(m2 )

1 sq. in. = 6.45 x 10 -4 m2
1 sq. ft. = 0.93 x 10 -1 m2
1 sq. yd. = 0.836 m 2

1 m2 = 1550 square inch
= 10.76 square feet
= 1.196 square yard


cubic meter
(m3 )

1 cu. in. = 16.3 x 10 -6 m3
1 cu. ft. = 0.028 m 3

1 m3 = 6.102 x 10 4 cubic inch
= 35.3 cubic feet


second (s)

same as Imperial/Metric

same as Imperial/Metric

Electric Current

ampere (A)

same as Imperial/Metric

same as Imperial/Metric

Plane Angle

radian (rad.)

1 angular deg. = 1.745 x 10
1 revolution = 6.283 radians

1 radian = 57.296 angular


hertz (Hz)

1 cycle/sec. = 1 Hz

1 Hz = 1 cycle / second

Force (f)

newton (N)

1 oz. (f) = 0.278 N
1 lb. (f) = 4.448 N
1 kilopond = 9.807 N
1 kgf = 9.907 N

1 N = 3.597 oz. (f)
= 0.225 lb. (f)
= 0.102 kp
= 0.102 kgf

Energy (Work)

joule (J)

1 btu = 1055.06 J
1 kwh = 3.6 x 10-6 J
1 watt / sec. = 1 J
1 kcal = 4186.8 J



= 9.478 x 10
= 2.778 x 10
= 1 Ws
= 2.389 x 10






watt (W)

1 hp = 746 W

1 W = 1.341 x 10

Qty. of Electricity

coulomb (C)

same as Imperial/Metric

same as Imperial/Metric


volt (V)

same as Imperial/Metric

same as Imperial/Metric


ohm(Ω )

same as Imperial/Metric

same as Imperial/Metric


farad (F)

same as Imperial/Metric

same as Imperial/Metric


henry (H)

same as Imperial/Metric

same as Imperial/Metric



Magnetic Flux

weber (Wb)

1 line = 10 Wb
1 Mx = 10 -8 Wb
1 Vs = 1 Wb

1 Wb = 10 8 lines
= 10 8 lines
= 1 Vs

Magnetic Flux

tesla (T)

1 line / in.2 = 1.55 x 10 -5 T
1 gauss = 10-4 T

1 T = 6.452 x 10 -4 lines / in.2
= 10 4 gauss

Linear Velocity

meter / second
(m / s)

1 inch / sec. = 2.54 x 10 -2m / s
1 mph = 1.609 km / s

1 m / s = 39.37 inches / second
= 3.281 feet / second

Linear Acceleration

meter / second2
(m / s2)

1 inch/second2 = 2.54 x 10 -4 m/s2

1 m / s2 = 39.37 inch / second
= 3.281 feet / second


(No m)

1 lb-ft. = 1.356 N o m
1 oz- in. = 7.062 x 10 -3 N o m
1kilopondmeter = 9.807 N o m
1 lb-in. = 0.113 N o m

1 N o m = 0.738 lb-ft.
= 8.851 lb-in.
= 0.102 kpm
= 141.61 oz. in .


degree Celsuus



F = ( oC x 9/5 ) + 32


C = ( o F-32) x 5/9

Appendix 8
Typical Gearmotor Construction

1. Screw, Shroud (.164-32 X .25 thread forming)
2. Shroud
3. Ring, Fan Retaining
4. Fan
5. Nut, Case Holding Screw (.190-32 X .12 thread
6. Gasket, Aluminum
7. Shield, Front
8. Insulator, Actuator Starting Switch (when required)
9. Switch, Actuator Starting (when required)
10. Screw, Actuator Starting Switch (when required)
11. Washer, Belleville
12. Washer, Steel Spacing (.81 I.D.)
13. Washer, Steel Spacing (1.12 I.D.)
14. Bearing, Ball
15. Ring, Retaining (external)
16. Actuator (when required)
17. Rotor
18. Ring, Retaining (internal)
19. Bearing, Ball
20. Ring, Retaining (bowed, external)
21. Ring and Stator (wound complete)
22. Pin, Nameplate
23. Nameplate
24. Bushing, Threaded
25. Shield, Rear

26. Screw, Gear Housing Holding (.190-32 X .44
thread forming)
27. Screw, Case Holding (48R4-5N) (.190-32 X 4.88)
27. Screw, Case Holding (48R5-5N) (.190-32 X 5.44)
27. Screw, Case Holding (48R6-5N) (.190-32 X 5.91)
28. Seal, Rotor
29. Plug, Breather Hole
30. Housing, Gear
31. Worm
32. Nut, Worm Lock
33. Gasket, Gear Housing End Cap
34. Cap, Gear Housing End
35. Screw, Gear Housing End Cap (.164-32 X .38
thread forming)
36. Screw, Gear Housing End Shield (.190-32 X .56
thread forming)
37. End Shield, Gear Housing (extension end)
38. Seal, Driveshaft
39. “O” Ring
40. Washer, Thrust (nylon)
41. Washer, Thrust (steel)
42. Key
43. Gear and Driveshaft
44. Screw, Oil Level
45. Gasket, Oil Level Screw
46. End Shield, Gear Housing (nonextension end)


Appendix 9
Horsepower/Watts vs. Torque Conversion Chart

at 1125 rpm

at 1200 rpm

at 1425 rpm



mN o m


mN o m


mN o m




































at 3600 rpm

at 5000 rpm

at 7500 rpm



mN o m


mN o m


mN o m




































at 5000 rpm

at 3600 rpm



mN o m


mN o m

at 7500 rpm

mN o m



































at 3600 rpm

at 5000 rpm

at 7500 rpm



mN o m


mN o m


mN o m




































at 10,000 rpm



m No m



















Appendix 10
Specific Resistance of Metals and
Alloys at Ordinary Temperature


Relative Conductance
(% Of Annealed Copper)

Aluminum, 99.57
Cobalt, 99.8%
Copper, annealed
Copper, pure



Silver (18X)
Iron, 99.98
Wrought Iron





Source: U.S. Bureau of Standards



Appendix 11
NEMA Motor Frame Dimensions
Standardized motor dimensions have been established by NEMA for all base mounted
and NEMA Type C face mounted motors which carry a NEMA frame designation (42365U). Since this is a small motor handbook, only 42-56 frames have been listed. It
should be noted that NEMA does not define dimensions for motors smaller than 42.
All dimensions listed below have been excerpted from NEMA Publication No. MG-1
and are shown in inches. As of this writing, metric dimensions are under consideration but
not yet finalized. The latest information can be obtained from NEMA.


Type C

Base Mount or NEMA Type C Face Mount






















NEMA Type C Face Mount Only

















*Dimension D will never be greater than the above values on rigid mount motors, but it may be
less so that shims up to 1/32" thick may be required for coupled or geared machines.


Appendix 12
International Voltage and Frequency Standards
As companies expand into global markets, there is an increasing need to understand
specific regional issues that may differ from country to country. One area that motor application developers must be aware of is the voltage and frequency standards which specific
countries have adopted. Failure to comply with these varying standards can cause severe
damage to motors and their associated controls.
The following table and accompanying socket patterns are designed to assist you in
determining the appropriate voltage and frequency for a given country. The list is based on
information obtained from ”Electric Current Abroad,” 1991 edition, published by the
U.S. Department of Congress.
Every attempt has been made to assure accuracy. However, standards do undergo
periodic review and revision. Therefore it is important, in specific situations, to confirm the
data in this table with the end-user’s requirements.





American Samoa
Burkina Faso
Canary Islands
Cape Verda, Republic of


127 / 220
120 / 240
115 / 127
240 / 250
110 / 220
110 / 220
220 / 230
110 / 220
127 / 220
120 / 208
127 / 220
127 / 220

220 / 380
240 / 480
415 / 440
190 / 380
208 / 240
380 / 400
220 / 440
280 / 240
230 / 380 / 400
220 / 380
208 / 308
220 / 380
230 / 600
220 / 380






Cayman Islands
Central African Republic
Channel Islands
China, People's Repub. of
Commonwealth of
Independent States
(former USSR)
Congo, Republic of
Costa Rica
Cote d'lvoire
Djibouti, Republic of
Dominican Republic
El Salvador
Equatorial Guinea
Faeroe Islands
French Guiana
Gambia, The
Germany, Fed. Republic of
Hong Kong

50 & 60
50 & 60

230 / 240
110 / 120 / 150
120 / 127
110 / 127 / 220
110 / 120

400 / 415
220 / 280 / 260
208 / 220
220 / 220 / 380
220 / 208


Mali, Republic of
Man, Isle of
Netherlands Antilles
Netherlands Antilles
New Caledonia
New Zealand




50 & 60

127 / 220
127 / 220
110 / 220
110 / 220
127 / 230
127 / 220
127 / 220
127 / 220
220 / 230
127 / 220
100 / 120

220 / 380
220 / 380
220 / 380
190 / 380
208 / 240
220 / 400
220 / 380
220 / 380
220 / 380
380 / 400
220 / 380
200 / 240






Papua New Guinea
Puerto Rico
St. Kitts and Nevis
St. Lucia
St. Vincent
Saudi Arabia
Sierra Leone
South Africa
Sri Lanka
Trinidad and Tobago
United Arab Emirates
United Kingdom (England)
United Kingdom (Scotland)
United Kingdom (Wales)
United Kingdom
(Northern Ireland)
United States of America

50 & 60

110 / 115 / 120
110 / 220
110 / 115
110 / 220 / 230
220 / 230 / 250
127 / 220
127 / 220
115 / 230
127 / 220
220 / 230
240 / 480
220 / 230
115 / 230

220 / 230 / 240
220 / 230
220 / 230 / 440
380 / 400 / 430
220 / 380
220 / 380
230/ 400
220 / 380
400 / 415
240 / 415
380 / 400
208 / 230 / 460






USSR (see commonwealth)
Virgin Islands (American)
Western Samoa
Yamen Arab Republic
Zaire, Republic of


120 / 127 / 220
220 / 230

208 / 220 / 380

Information in this chart was compiled from “Electric Current
Abroad,” July 1991 Edition of Commerce.


Glossary of Terms
Acceleration: The time rate of change of velocity; i.e., the rate at which velocity is
changing, expressed as radians per second (radians/sec²). One shaft revolution = 2p
radians. See Torque-to-Inertia Ratio.
Air Gap: The space between the rotating and stationary members of an electric motor.
Alternating Current (AC): A flow of electricity which changes direction on a continuous cycle or frequency. It reaches a maximum in one direction, decreases to zero, then
reverses to reach a maximum in the opposite direction.
Ambient: For air-cooled rotating machinery, ambient is the air which surrounds the motor.
Ampere: The unit of electrical current or rate of electron flow. A voltage drop of one
volt across one ohm of resistance in a closed-loop electrical circuit causes one ampere
of current to flow.
Ampere Turn: The measure of magnetomotive force produced by a current of one
ampere in a coil consisting of one turn.
Angular Velocity: Angular displacement per elapsed unit of time (usually seconds), for
example: degrees/second or radians/second.
Armature: The wound moving element in an electromechanical device such as a generator or motor.
Armature Reaction: The interaction of the magnetic flux produced by current flowing
in the armature winding of a DC motor with the magnetic flux produced by the field
current. The reaction reduces torque capacity, and can affect commutation and the
magnitude of the motor’s generated voltage.
BCD: An acronym for Binary Coded Decimal. A coded direct binary conversion of the
decimal integers from 0 through 9. This conversion is shown in the following table:

Backlash: In a mechanical system where one device is connected to another by a coupler, gear, screw, etc., the motion permitted between one device relative to the other is
called backlash.


Back emf: The voltage produced across a winding of a motor due to the winding turns
being cut by a magnetic field while the motor is operating. This voltage is directly proportional to rotor velocity and is opposite in polarity to the applied voltage. Sometimes
referred to as counter emf.
Bifilar: Furnished or fitted with two windings which are wound simultaneously as one.
Bilevel Drive: A dual voltage drive used to overcome the effects of step motor inductance.
Binary: The base 2 numbering system consisting of only 0’s and 1’s.
Bipolar Drive: A drive which reverses the direction of current flow through a winding,
thus eliminating the need for bifilar windings.
Braking Torque: The torque required to bring a motor down from running speed to a
standstill. The term is also used to describe the torque developed during dynamic
braking conditions.
Breakdown Torque: The maximum torque a motor will develop, at rated voltage,
without a relatively abrupt drop or loss in speed.
Brush: A piece of current conducting material (usually carbon or graphite) which rides
directly on the commutator of a commutated motor, and conducts current from the
power supply to the armature windings.
Buffer: The part of a step motor translator circuit which stores incoming pulse trains.
CMOS: An acronym for Complimentary Metal Oxide Semiconductor. CMOS construction is used in integrated circuit production and is characterized by low power consumption and high speed.
Capacitor: A device which stores electricity, blocks the flow of direct current, and permits the flow of alternating current. In an AC circuit, a capacitor causes the current to
lead the voltage in time phase.
Center Ring: The part of a motor housing which supports the stator, field core or permanent magnet arcs.
Centrifugal Cut-out Switch: An automatic mechanism used in conjunction with
split-phase and other types of induction motors which opens or disconnects the start
winding when the rotor has reached a predetermined speed. Activated by centrifugal
force, the cut-out switch will reconnect the start winding when the motor speed falls
below a certain level. Without these devices, the start winding would be susceptible to
rapid overheating and subsequent burnout.
Chopper Driver: A circuit which limits current to the motor by switching the current
off when it reaches a certain level, and switches it on again when current decays to a
lower level. The switching rate is typically 2 to 20 kHz.
Clock: A circuit which generates periodic signals at regular intervals. Clock circuits are
used in step motor translators to control the step rate of the motor.
Closed-Loop System: A system in which the output is fed back for comparison with
the input, for the purpose of reducing any difference between input command and output response.


Cogging: A term used to describe nonuniform angular velocity. It refers to rotation occurring in jerks or increments rather than smooth continuous motion. Cogging is very
apparent at low speeds. It is due to the interaction of the armature coil as it enters the
magnetic field produced by the field coils or permanent magnets. The armature tends
to speed up and slow down as it cuts through the fields during rotation.
Commutator: A cylindrical device mounted on the armature shaft and consisting of a
number of wedge-shaped copper segments arranged around the shaft. These segments
are insulated from the shaft and from each other. The motor brushes ride on the periphery of the commutator, and electrically connect and switch the armature coils to the
power source.
Compliant Coupling: A coupling which allows limited freedom of movement prior to
transferring torque from the input shaft to the output shaft.
Conductor: Any material such as copper or aluminum, which offers little resistance to
the flow of electric current.
Coupling Angle: The mechanical degree relationship between the rotor and the rotating electrical field in a motor. While present in both synchronous and nonsynchronous
AC motors, it is usually of concern in synchronous applications. At no load, the rotor
poles line up exactly with the field poles, and the coupling angle is considered to be
zero. When a load is applied, the lines of force coupling the rotor with the stator field
are stretched, causing the rotor to fall behind the field. The mechanical angle by which
the rotor lags behind the field is called the coupling angle. The coupling angle will continue to increase with load until it reaches the “pull-out” point. The maximum angle
which is possible prior to pull-out is dependent on the motor type and rotor design.
Damping: The inhibition of oscillation in a system by electrical, magnetic or mechanical
Data Buss: A set of electrical signals whose functions have been predefined to accomplish a data transfer between two or more devices.
Distributed Pole: A motor has distributed poles when its stator or field windings are
distributed in adjacent slots located within the arc of the pole.
Duty Cycle: The relationship between the operating and rest time of a motor. A motor
which can continue to operate within the temperature limits of its insulation system,
after it has reached its normal operating or equilibrium temperature, is considered to
have a continuous duty rating. A motor which never reaches equilibrium temperature
but is permitted to cool down between operations is operating under intermittent duty
Dynamic Unbalance: A vibration-producing condition caused by nonsymmetrical
weight distribution of a rotating member. The lack of uniform wire spacing in a wound
armature or casting voids in a rotor or fan assembly can cause relatively high degrees
of unbalance.
EFSS: Acronym for Error-Free-Stop-Start. The range of motor speeds where a stepper motor can start or stop without losing or gaining steps.
Eddy Current: Localized currents induced in an iron core by alternating magnetic flux.
These currents translate into heat losses. Minimizing eddy currents is an important factor in magnetic core design.


Efficiency: The ratio of mechanical output to electrical input is the measure of a motor’s
efficiency. It is the effectiveness with which a motor can convert electrical energy into
mechanical energy.
Electrical Coupling: When two coils are situated so that some of the flux set up by
either coil links some of the turns of the other, they are said to be electrically coupled.
Electrical Degree: A unit of time measurement applied to alternating current. One
complete cycle = 360 electrical degrees. One cycle in a rotating machine is accomplished when the rotating field moves from one pole to the next pole of the same polarity. There are 360 electrical degrees in this time period. Therefore, in a two pole machine, there are 360 degrees in one revolution, so the electrical and mechanical degrees are equal. In a machine with more than two poles, the number of electrical degrees per revolution is obtained by multiplying the number of pairs of poles by 360.
Electrical Time Constant: The ratio of inductance to resistance, sometimes called
the L/R time constant.
Electromotive Force (emf): A synonym for voltage, usually restricted to generated
Electronic Commutation: The use of logic circuitry to control phase current switching in a motor such as a brushless DC motor control system. The logic circuitry electronically performs the same function as a mechanical commutator. Electronic commutation eliminates the need for brushes in DC motors.
Electronic Interface: The circuitry which matches signal voltage and/or current levels
between two dissimilar devices.
Encapsulated Winding: A motor which has its winding structure completely coated
with an insulating resin such as epoxy. This type of construction is designed for more
severe atmospheric conditions than the normal varnished winding.
Encoder: An electromechanical feedback device connected to a shaft which delivers a
pulse output proportional to the motion of the shaft. Depending on the construction, an
encoder can indicate either shaft position or relative shaft motion.
End Play: Inherent axial motion of the motor shaft under load, due to tolerance build-up
in motor construction and bearing preload system.
End Shield: The part of the motor housing which supports the bearing and acts as a
protective guard to the electrical and rotating parts inside the housing. It may also be
referred to as the end bracket or end bell.
Excitation Current: A term usually applied to the current in the shunt field of a motor
resulting from voltage applied across the field.
Excitation Sequence: In stepper motors, the sequence in which the motor phases
(windings) are energized. This sequence of individual phase excitation establishes both
direction and step size (full or half steps). A specific excitation sequence is required for
each type of drive employed (unipolar or bipolar) as well as each step size required.
Farad: A unit of measure for electrical capacitance. A capacitor has a capacitance of
one farad when a potential difference of one volt will charge it with one coulomb of


Feedback: The return of a signal from the output of a circuit to its input for the purpose
of comparing the output with a reference signal. This is done to automatically compensate the input to maintain a desired output condition. See Closed-Loop System.
Ferromagnetic: A material with high magnetic permeability or one which imposes little
resistance to magnetic orientation of its molecular structure in the presence of a magnetic field. Such materials as iron, steel and nickel are ferromagnetic substances.
Field: A term commonly used to describe the stationary (stator) member of a DC motor.
The field provides the magnetic field with which the mechanically rotating (armature)
member interacts.
Field Weakening: The introduction of resistance in series with the shuntwound field of
a motor to reduce the voltage and current which weakens the magnetic field and thereby increases motor speed.
Flux: The magnetic field which is established around an energized conductor or permanent magnet. The field is represented by flux lines, creating a flux pattern between opposite poles. The density of the flux lines is a measure of the strength of the magnetic
Form Factor: A figure of merit which indicates to what degree rectified current departs
from nonpulsating or pure DC. Pure DC has a form factor of 1.0. A large departure
from unity form factor increases the heating effect of the motor and reduces brush life.
Mathematically, form factor is the ratio of the root-mean-square (rms) value of the
current to the average current or Irms/Iav.
Fractional Horsepower Motor: A motor with a continuous rating of less than one
Frequency: The rate at which alternating current reverses its direction of flow, measured in hertz (Hz). 1 Hz = 1 cycle per second.
Friction (Coulomb): A force of constant magnitude and independent of velocity
which opposes the relative motion of two surfaces. A constant minimum torque is required to overcome friction and produce motion.
Friction (Viscous): A force which opposes the relative motion of two surfaces and is
dependent on the relative velocity of the surfaces, due to the viscosity of the fluid medium separating them.
Full Load Current: The current drawn when the motor is operating at full load torque
and full load speed at rated frequency and voltage.
Full Load Torque: The torque necessary to produce rated horsepower at full load
Full Step (Two Phase On) Drive: A mode of operation in which the windings of a
stepper motor are energized in sequence, maintaining two windings (phases) in the
“on” state at any one time.
Galvanometer: An extremely sensitive instrument used to measure small values of current and voltage in an electrical circuit.
Gearhead: The portion of a gearmotor which contains the actual gearing for converting
the rated motor speed to the rated output speed.


Generated Voltage: A voltage produced whenever conductors of electric current cut
across lines of magnetic force, as in a motor being driven as a generator.
Gravity Load: A load which is produced by gravitational force. A gravity load is seen
by the motor as an inertial load plus a unidirectional torque.
Grounded Motor: A motor with a short circuit between any point in its electrical circuit and its connection to ground.
Half-Step Drive: A mode of operation in which one and two phases of a stepper motor are alternately energized in a particular sequence, resulting in step angles one-half
that of a full step drive. The motor shaft rotates at one half the speed of full step operation at a given pulse rate.
Heat Loss: Losses due to resistance take the form of heat which has to be dissipated
into the air or surrounding cooling medium. Heat loss is also referred to as I²R loss
because the current squared, multiplied by the resistance, will yield the heat loss value
in watts.
Holding Torque: See Static Torque.
Home Position: A known position to which a system (a stepper motor or incremental
encoder) can be set to establish a starting position or reference point.
Hybrid Stepper Motor: A motor combining the properties of both variable reluctance
and permanent magnet stepper motor designs. The rotor includes a cylindrical magnet
captivated within two soft iron-toothed cups. The magnet provides part of the operating flux of the motor.
Hysteresis Loss: The resistance of a material to becoming magnetized (magnetic orientation of molecular structure) results in energy being dissipated and a corresponding
loss. Hysteresis loss in a magnetic circuit is the energy expended to magnetize and demagnetize the core.
Impedance: The total opposition a circuit offers to the flow of alternating current at a
given frequency. It is the vectoral sum of the circuit’s resistance and reactance.
Impedance Protection: A motor which is designed so that it limits current to a value
less than that which would result in overheating under all operating conditions, especially locked rotor conditions, is said to be impedance protected.
Indexer: The part of a stepper motor control system which commands the motor to
rotate through a specific predetermined number of steps.
Inductance: The property of a circuit which opposes any change of current because of
the magnetic field associated with the current itself. The unit of inductance is the henry.
When a current changing at the rate of one ampere per second induces a voltage of
one volt, the inductance of the circuit is one henry. Inductance causes current to lag the
voltage in time phase.
Inertial Load: A load (flywheel, fan, etc.) which tends to cause the motor shaft to continue to rotate after the power has been removed. If this continued rotation cannot be
tolerated, some mechanical or electrical braking must be applied.
Inertial Load-Reflected: The inertia of the load as seen by the motor when driving
the load through a gear reducer or other speed changing system.


Insulator: A material which tends to resist the flow of electric current such as glass,
paper, rubber, etc.
Integral Horsepower Motor: In terms of horsepower, a motor built in a frame having a continuous rating of one horsepower or more. In terms of motor size, an integral
hp motor is usually greater than 9 inches in diameter, although it can be as small as 6
Line Voltage: Voltage supplied by the commercial power company or voltage supplied
as input to the device.
Locked Rotor Current: Steady state current taken from the line with the rotor at
standstill (at rated voltage and frequency).
Locked Rotor Torque: The minimum torque that a motor will develop at rest for all
angular positions of the rotor, with rated voltage applied at rated frequency.
Logic Circuit: A circuit which exchanges and processes information in the form of binary digital data.
Magnetomotive Force (mmf): The magnetic energy supplied with the establishment
of flux between the poles of a magnet. Magnetomotive force is analogous to electromotive force in an electric circuit.
Mechanical Degree: The more popular physical understanding of degrees; i.e., 360
degrees = 1 revolution.
Microprocessor: The control and calculating portion of a small computer system that
is integrated into a single chip.
Mini-Stepping: The process of electronically subdividing the inherent step size of a
stepper motor into smaller increments.
Natural Frequency: The frequency at which a system will oscillate from rest position
when displaced by a momentary force. Stepper motor operation at a natural frequency
is unstable. This instability may be overcome by adding frictional torque to the system.
Open Circuit: Any break in a current path, in an electrical circuit, which causes an interruption of current flow.
Open-Loop System: A control system in which no feedback path exists. The output
has no affect on the input, as in a closed-loop system.
Overhung Load: A load which exerts a force on the motor shaft perpendicular to the
rotational axis of the shaft. Also called radial load.
Overshoot: Motion which is beyond the commanded position. For a stepper motor,
overshoot is the maximum or minimum peak displacement shown on a single step response curve, and is usually dimensioned as a percent of one step.
Phase: In motor terminology, phase indicates the spatial relationship of windings and the
changing values of recurring cycles of AC voltage and current. The positioning of the
windings in a motor (or phase relationship) causes dissimilarities between any given
winding voltage and current at any given instant. Each voltage or current will lead or
lag the other in time.


Phase Displacement: Mechanical or electrical angle by which phases in a polyphase
motor are displaced from each other. It also applies to the mechanical or electrical
angle by which the main winding and the capacitor or start winding are displaced in an
induction motor.
Plug Reversal: Reconnecting a motor’s windings to reverse its direction of rotation
while it is running. Plugging is a very severe method of reversing and should be used
with extreme caution. Other methods of mechanical or dynamic braking should be
Polarities: Terms such as positive, negative, north and south, which indicate the direction of current and magnetic flux flow in electrical and magnetic circuits at any instant in
Polarized Motors: Special motors consisting of hybrid cores which are partially squirrel cage (reluctance type) and partially permanent magnet. Polarized motors can lock
into synchronism in a definite relationship to the stator poles. Two-pole polarized motors have only one lock-in position, while four-pole polarized motors have two lock-in
positions 180° apart. (Standard reluctance type synchronous motors have as many
lock-in points as there are poles in the motor.)
Potentiometer: A variable resistor which, when connected in series with a motor, can
be used to adjust the amount of voltage available to the motor and thereby adjust the
speed of the motor.
Power Factor: A measurement of the time phase difference between the voltage and
the current in an AC circuit. It is represented by the cosine of the angle of this phase
difference. For an angle of 0°, the power factor is 100%, and the voltage / amperes of
the circuit are equal to the watts.
Primary Winding: The winding of a motor, transformer or other electrical device
which is connected to the power source.
Programmable Controller: A solid state digital logic device which allows programmed instructions to control electromechanical devices in a motion control system
via properly timed switch actuations.
Pull-In Torque: The maximum frictional load a motor is capable of bringing to synchronous speed from a standstill. Fhp synchronous motor ratings are based on pull-in
torque measurements.
Pull-Up Torque: The minimum torque developed by an AC motor during the period of
acceleration from zero to the speed at which breakdown occurs. For motors which do
not have a definite breakdown torque, the pull-up torque is the minimum torque developed during the process of getting up to rated speed.
Pulse: An electrical signal of unusually short duration and often square in shape.
Rated Speed: The speed which a motor develops at rated voltage with rated load
Reactance (Inductive): The characteristic of a coil, when connected to alternating
current, which causes the current to lag the voltage in time phase. The current wave
reaches its peak later than the voltage wave.
Rectifier: An electronic circuit which converts alternating current to direct current.


Reluctance: The characteristic of a magnetic material which resists the flow of magnetic lines of force through the material.
Residual Torque: The holding or restoring torque of a nonenergized stepper motor
(all windings open) which tends to restore the rotor to a detent position. Sometimes
referred to as detent torque.
Resilient Mounting: A suspension system or cushioned mounting designed to reduce
the transmission of normal motor noise and vibration to the mounting surface.
Response Time: The time required for a stepper motor to initially reach its next commanded position.
Resonance: In open-loop stepper systems, a speed range in which a low frequency
velocity oscillation occurs around the nominal speed. It grows in amplitude until the
rotor velocity can no longer follow the command pulse train, and the motor stalls.
Rotor: The rotating member of an induction motor, stepper, brushless DC or switched
reluctance motor.
Rotor Inertia: The property of the rotor which resists any change in motion. The inertia
is a function of rotor mass and radius squared, and is expressed as oz-in./sec2** or
Salient Pole: A motor has salient poles when its stator or field poles are concentrated
into confined arcs and the winding is wrapped around them (as opposed to distributing
them in a series of slots).
Silicon Controlled Rectifier (SCR): A semiconductor device which blocks a voltage applied to it in either direction when it is in its normal state. It will conduct in a forward direction when a signal of the proper amplitude is applied to its gate. Once conduction begins, it continues even if the control signal is removed. Conduction will stop
when the anode supply is removed, reversed or reduced sufficiently in amplitude.
Secondary Winding: The secondary winding of a motor (i.e., squirrel cage rotor
conductors) is one which is not connected to the power source, but which carries current induced in it through its magnetic linkage with the primary winding.
Semiconductor: A solid or liquid having a resistive value midway between that of an
insulator and a conductor. Typical semiconductor materials are germanium, silicon,
selenium and lead sulfide. These materials are used to manufacture active electronic
devices such as transistors, diodes, SCRs, and integrated circuits (ICs), which are
used extensively in motion control systems.
Settling Time: The time required for a stepper motor to reach and remain within ±5%
of a single step, after commanded to take a single step.
Service Factor: In motor applications, it is a figure of merit used to adjust measured
loads in an attempt to compensate for conditions which are difficult to measure and
define. Typically, measured loads are multiplied by service factors (experience factors), and the result is an “equivalent required torque” rating of a motor or gearmotor.
Shaft Run-Out: The variation in distance between the surface of a shaft and a fixed
point outside the shaft through one shaft revolution.


Short Circuit: A defect in an electrical circuit which causes part of the circuit to be
bypassed. This frequently results in reducing the resistance to such an extent that excessive current flows in the remaining circuit and results in overheating and subsequent
Skew: The arrangement of laminations on a rotor or armature to provide a slight diagonal pattern of their slots with respect to the shaft axis. This pattern helps eliminate low
speed cogging effects in an armature and minimizes induced vibration in a rotor.
Slew Range: The speed range through which a stepper motor may be operated using
acceleration and deceleration control, without losing or gaining steps.
Slip: The difference between the speed of the rotating magnetic field (which is always
synchronous) and the rotor in a nonsynchronous induction motor. Slip is expressed as
a percentage of synchronous speed. It generally increases with an increase in load.
Starting Current: Amount of current drawn when a motor is initially energized. It usually exceeds the current required for running.
Starting Torque: The torque or twisting force delivered by a motor when initially energized. Starting torque is often higher than rated running torque.
Static Torque: The torque under locked rotor conditions, when one or two of the
phase windings of a stepper motor are excited with a steady state DC current. Static
torque varies as the motor shaft is rotated through one step or more in either direction.
Stator: That part of an induction motor, stepper, brushless DC or switched reluctance
motor which does not rotate.
Step Accuracy: The maximum deviation of a stepper motor from true position under
no-load conditions. Step accuracy is noncummulative in a stepper motor; i.e., the maximum deviation from true position is never more than the maximum single step deviation.
Step Angle: The angle through which a stepper motor shaft rotates to take a single
step. For Bodine stepper motors, the step angle is 1.8°.
Step Rate: The rate in steps per second at which a stepper motor is commanded to
Synchronous Speed: The speed of the rotating magnetic field set up by an energized stator winding. In synchronous motors, the rotor locks into synchronism with the
field and is said to run at synchronous speed.
Tachometer: A small generator normally used as a velocity sensing device. Tachometers are attached to the output shaft of DC motors and typically used as feedback devices. The tachometer feeds its signal to a control which compares it to the reference
signal. The control then adjusts its output accordingly to regulate the speed of the motor to within a predefined tolerance.
Thermal Protection: Motors equipped with devices to disconnect the motor windings
from the line during overheating are said to be thermally protected.


Thermocouple: A temperature sensor containing a junction of two dissimilar materials
which generates a minute voltage in proportion to its temperature. Such devices may
be used as a signal source for control equipment to indicate overheating conditions.
Thrust Load: A load which applies a force to the motor shaft in a direction parallel to
the shaft.
Time Constant: The time interval in which a variable (which is a function of time)
reaches 63% of its maximum value.
Torque: The twisting force of a motor or gearmotor shaft, usually expressed in ounceinches or newton-meters. Torque = force x distance.
Torque-to-Inertia Ratio: The ratio of available torque to the inertia of the rotor. The
ratio T:J is proportional to the acceleration the motor can achieve. The greater the ratio, the greater the motor’s acceleration capability.
Translator: The portion of a stepper motor control which translates a clock signal into
the proper excitation sequence to operate the motor.
Unipolar Drive: A drive in which winding current flows in one direction only.
Voltage: The force which causes current to flow in an electrical circuit. Analogous to
hydraulic pressure, voltage is often referred to as electrical pressure.
Voltage Drop: The loss encountered across a circuit impedance. The voltage drop
across a resistor takes the form of heat released into the air at the point of the resistance.
Watt:The amount of power required to maintain a current of one ampere at a pressure of
one volt. One horsepower = 746 watts.



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