Fluke 190 202 Application Note

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Application Note

From the Fluke Digital Library @ www.fluke.com/library

Variable Speed Drives

Variable Speed Drives (VSDs)

provide a convenient and afford-

able method for varying the

speed of robust ac motors. When

connected to the mains, the

rotation speed of an ac motor

is directly related to the mains

frequency and the number of

poles of the motor. Tradition-

ally an external gearbox was the

only way to work with different

speeds. This all changed with

the introduction of high-power

semiconductors, which made it

possible to build variable speed

drives by electronically creating

a supply voltage that operates

at different frequencies. This

has, however, created the need

for new measurement capa-

bilities, which are provided by

the Fluke 190 Series II portable

oscilloscope.

With their unique triggering and measuring functions,

Fluke ScopeMeter 190 Series II portable oscilloscopes

are ideal for analyzing the voltage-frequency ratio of

pulse-width-modulated variable speed drives.

Pulse Width Modulation

AC drives using Pulse Width

Modulation (PWM) have found

their way into many applica-

tions such as fans, pumps and

conveyor belts driven by squir-

rel-cage asynchronous ac motors.

These motors are robust and

require little maintenance since

they have no brushes that would

need regular replacement.

The basic structure of a

variable speed drive (Figure 1)

includes an input rectifier con-

verting the mains voltage into a

dc voltage that feeds the so-

called dc-bus. This dc voltage is

then converted into a ‘variable

frequency voltage’ using elec-

tronic switches. Since the speed

of the motor can be adjusted

with a simple potentiometer or

control signal from an exter-

nal source, these drives have

become a popular replacement

for gearboxes.

Checking voltage-

frequency ratio on

variable speed drives

with the new Fluke 2-channel ScopeMeter®

190 Series II portable oscilloscope

Figure 1. Basic electric schematics of variable-speed drive.

AC to DC

conversion

Mains

supply

Mechanical

output

Motor (load) Cable interface

DC to AC converterDC lter

and buffer

bus

voltage

L1

T3

L2

T2

L3

Gnd

T1

+Vdc

-Vdc

T3

T2

T1

2 Fluke Corporation Checking voltage-frequency ratio on variable speed drives with the new Fluke 2-channel ScopeMeter® 190 Series II portable oscilloscope

The Motor

AC motors are designed for use

with a rotating magnetic field

of constant strength. The mag-

netic field is generated from the

applied voltage, and its strength

is proportional to the V/Hz ratio.

Normally the motor is designed

to work with the local mains

voltage (230 V/400 V or 120 V

/208 V) and mains frequency

(50 Hz or 60 Hz). The nominal

values are shown on the motor

type plate (Figure 2). When the

motor is connected to a VSD, the

drive unit changes the frequency

of the output voltage, thereby

changing the speed of the rotating

magnetic field and thus the speed

of the motor.

Lowering the frequency only,

however, will generate a higher

magnetic field since the V/Hz

ratio increases. This will result

in magnetic saturation, which

causes unstable running and

generates higher temperatures

in the motor. Likewise, increas-

ing the frequency increases V/Hz

ratio, reducing the magnetic field

and resulting in lower torque.

To overcome these problems,

the VSD also varies the voltage

when the frequency is varied to

maintain a constant V/Hz ratio.

Preferably this is done over the

complete working range of the

VSD. The control type used in

this case is called V/Hz control

which in its simplest form takes

a speed reference command from

an external source and varies the

voltage and frequency applied to

the motor.

Figure 2. Motor type plate.

Figure 3. Selecting the V/Hz ratio for

measurements carried out on input A.

Figure 4. Measuring Vpwm and Hz, and

displaying the V/Hz ratio.

Measurement requirements

To be able to verify if the V/

Hz ratio is constant over the

working range of the VSD, the

output voltage and frequency

of the drive must be measured

simultaneously. The challenge

here, however, is that the output

waveform of a PWM drive is far

from a sine wave, since it con-

sists of pulses with varying width

to create a motor drive current

that resembles a sine wave. This

is accomplished by varying the

duty cycle of these pulses such

that the current (but not the volt-

age) through the motor winding

resembles a sine waveform.

In effect, the motor wind-

ings function as a low-pass filter

through which the pulse-width-

modulated voltage causes a

current resembling a sine wave

to flow. A true rms voltmeter

used in this situation would give

erroneous readings since these

meters give the rms voltage of

the full bandwidth signal. This

application requires a meter

that is capable of measuring the

effective voltage of the funda-

mental component only, since

this is what the motor actually

‘sees’. Another complication is

that the complex modulated

waveforms often make it difficult

to obtain a stable picture and

readings of the signal.

The new Fluke 2-channel

ScopeMeter 190 Series II is ide-

ally suited for this application as

it can immediately display the

V/Hz ratio after this option is

chosen. No further adjustments

are required and the technician

can concentrate on the job as

there is no need to spend time

adjusting the ScopeMeter for

optimal settings. Figure 3 shows

the set-up display for choosing

the V/Hz ratio.

The ScopeMeter also provides

Connect-and-View triggering

which automatically displays the

stable picture.

Making measurements

The Fluke 190 Series II Scope-

Meter test tool is certified up to

a 600 V CAT IV/1000 V CAT III

safety rating, making the series

an extremely safe instrument

for professional and industrial

applications.

Connect the ScopeMeter

directly to the motor terminals

using the 10:1 VPS410 probe

supplied as standard with the

instrument. From the SCOPE

READINGS menu select the read-

ing V/Hz (Figure 3). This new

option assures a direct read-out

of the ratio allowing the operator

to continue to focus on the ratio

rather than on the correct set-

tings. The automatic triggering of

the ScopeMeter assures a stable

waveform and readings display,

allowing measurements to be

taken over the whole operating

range of the drive unit.

Figure 4 shows the value of

the V/Hz ratio calculated from

a the input values measured on

input A whereby the actual values

are displayed in the banner.

3 Fluke Corporation Checking voltage-frequency ratio on variable speed drives with the new Fluke 2-channel ScopeMeter® 190 Series II portable oscilloscope

The measurements show that

the V/Hz ratio is relatively con-

stant over the working range for

frequencies up to 50 Hz, where

the voltage reaches the level of

the input voltage of the drive

(Figure 5). The readings match

the nominal values for the motor,

i.e. 220 V/50 Hz = 4.4. When

the frequency is increased above

50 Hz, the drive unit can no

longer increase the output volt-

age since it is limited by the

drive input of 230 V. This gives

a lower V/Hz ratio resulting in a

lower magnetic field and hence

lower torque when running at

high speed.

When reducing the speed,

the drive unit increases the

V/Hz ratio slightly to improve

the torque at lower speed. This

technique is commonly referred

to as Voltage Boost. Normally a

motor gives less torque when

running at lower speed, an effect

known as ‘Ohms loss’. This is

caused by the resistance of the

Figure 5. V - Hz graph.

Fluke Corporation

PO Box 9090, Everett, WA 98206 U.S.A.

Fluke Europe B.V.

PO Box 1186, 5602 BD

Eindhoven, The Netherlands

For more information call:

In the U.S.A. (800) 443-5853 or

Fax (425) 446-5116

In Europe/M-East/Africa +31 (0) 40 2675 200 or

Fax +31 (0) 40 2675 222

In Canada (800)-36-FLUKE or

Fax (905) 890-6866

From other countries +1 (425) 446-5500 or

Fax +1 (425) 446-5116

Web access: http://www.fluke.com

©2005-2011 Fluke Corporation.

Specifications subject to change without notice.

Printed in U.S.A. 6/2011 2543364B A-EN-N

Pub-ID 10668-eng, rev 01

Modification of this document is not permitted

without written permission from Fluke Corporation.

Fluke. Keeping your world

up and running.®

motor windings having greater

influence on the total imped-

ance of the windings at lower

speed since the inductance of

the windings decreases with

frequency while the resistance

remains constant. The part of the

voltage that contributes to the

inductance is consequently lower

causing a relatively lower mag-

netic field at lower speeds.

Conclusion

The new 2-channel Fluke 190

Series II ScopeMeter is par-

ticularly well suited for this

application as with one press of

the button it is possible to get

the required reading. Adjusting

specific settings is no longer the

case.

With this new feature it

makes analyzing the V/Hz ratio

easier than ever and makes it

possible to analyze the behavior

of the system and find possible

causes for unstable motor behav-

ior in the VSD configuration.