ISM For Electrical Machinery Fundamentals 4e (Instructor's Manual)

i

Instructor’s Manual

to accompany

Chapman

Electric Machinery Fundamentals

Fourth Edition

Stephen J. Chapman

BAE SYSTEMS Australia

ii

Instructor’s Manual to accompany Electric Machinery Fundamentals, Fourth Edition

Copyright  2004 McGraw-Hill, Inc.

any manner whatsoever without written permission, with the following exception: homework solutions may be

copied for classroom use.

ISBN: ???

iii

TABLE OF CONTENTS

CHAPTER 1: INTRODUCTION TO MACHINERY PRINCIPLES 1

CHAPTER 2: TRANSFORMERS 23

CHAPTER 3: INTRODUCTION TO POWER ELECTRONICS 63

CHAPTER 4: AC MACHINERY FUNDAMENTALS 103

CHAPTER 5: SYNCHRONOUS GENERATORS 109

CHAPTER 6: SYNCHRONOUS MOTORS 149

CHAPTER 7: INDUCTION MOTORS 171

CHAPTER 8: DC MACHINERY FUNDAMENTALS 204

CHAPTER 9: DC MOTORS AND GENERATORS 214

CHAPTER 10: SINGLE-PHASE AND SPECIAL-PURPOSE MOTORS 270

APPENDIX A: REVIEW OF THREE-PHASE CIRCUITS 280

APPENDIX B: COIL PITCH AND DISTRIBUTED WINDINGS 288

APPENDIX C: SALIENT POLE THEORY OF SYNCHRONOUS MACHINES 295

APPENDIX D: ERRATA FOR ELECTRIC MACHINERY FUNDAMENTALS 4/E 301

iv

PREFACE

TO THE INSTRUCTOR

This Instructor’s Manual is intended to accompany the fourth edition of Electric Machinery Fundamentals. To

make this manual easier to use, it has been made self-contained. Both the original problem statement and the

problem solution are given for each problem in the book. This structure should make it easier to copy pages from

the manual for posting after problems have been assigned.

Many of the problems in Chapters 2, 5, 6, and 9 require that a student read one or more values from a

magnetization curve. The required curves are given within the textbook, but they are shown with relatively few

vertical and horizontal lines so that they will not appear too cluttered. Electronic copies of the corresponding open-

circuit characteristics, short-circuit characteristics, and magnetization curves as also supplied with the book. They

are supplied in two forms, as MATLAB MAT-files and as ASCII text files. Students can use these files for

electronic solutions to homework problems. The ASCII files are supplied so that the information can be used with

non-MATLAB software.

Please note that the file extent of the magnetization curves and open-circuit characteristics have changed in this

edition. In the Third Edition, I used the file extent *.mag for magnetization curves. Unfortunately, after the book

was published, Microsoft appropriated that extent for a new Access table type in Office 2000. That made it hard

for users to examine and modify the data in the files. In this edition, all magnetization curves, open-circuit

characteristics, short-circuit characteristics, etc. use the file extent *.dat to avoid this problem.

Each curve is given in ASCII format with comments at the beginning. For example, the magnetization curve in

Figure P9-1 is contained in file p91_mag.dat. Its contents are shown below:

% This is the magnetization curve shown in Figure

% P9-1. The first column is the field current in

% amps, and the second column is the internal

% generated voltage in volts at a speed of 1200 r/min.

% To use this file in MATLAB, type "load p91_mag.dat".

% The data will be loaded into an N x 2 array named

% "p91_mag", with the first column containing If and

% the second column containing the open-circuit voltage.

% MATLAB function "interp1" can be used to recover

% a value from this curve.

0 0

0.0132 6.67

0.03 13.33

0.033 16

0.067 31.30

0.1 45.46

0.133 60.26

0.167 75.06

0.2 89.74

v

0.233 104.4

0.267 118.86

0.3 132.86

0.333 146.46

0.367 159.78

0.4 172.18

0.433 183.98

0.467 195.04

0.5 205.18

0.533 214.52

0.567 223.06

0.6 231.2

0.633 238

0.667 244.14

0.7 249.74

0.733 255.08

0.767 259.2

0.8 263.74

0.833 267.6

0.867 270.8

0.9 273.6

0.933 276.14

0.966 278

1 279.74

1.033 281.48

1.067 282.94

1.1 284.28

1.133 285.48

1.167 286.54

1.2 287.3

1.233 287.86

1.267 288.36

1.3 288.82

1.333 289.2

1.367 289.375

1.4 289.567

1.433 289.689

1.466 289.811

1.5 289.950

To use this curve in a MATLAB program, the user would include the following statements in the program:

% Get the magnetization curve. Note that this curve is

% defined for a speed of 1200 r/min.

load p91_mag.dat

if_values = p91_mag(:,1);

ea_values = p91_mag(:,2);

n_0 = 1200;

Unfortunately, an error occurred during the production of this book, and the values (resistances, voltages, etc.) in

some end-of-chapter artwork are not the same as the values quoted in the end-of-chapter problem text. I have

attached corrected pages showing each discrepancy in Appendix D of this manual. Please print these pages and

distribute them to your students before assigning homework problems. (Note that this error will be corrected at the

second printing, so it may not be present in your student’s books.)

vi

The solutions in this manual have been checked carefully, but inevitably some errors will have slipped through. If

you locate errors which you would like to see corrected, please feel free to contact me at the address shown below,

or at my email address schapman@tpgi.com.au. I greatly appreciate your input! My physical and email

addresses may change from time to time, but my contact details will always be available at the book’s Web site,

which is http://www.mhhe.com/engcs/electrical/chapman/.

I am also contemplating a homework problem refresh, with additional problems added on the book’s Web site mid-

way through the life of this edition. If that feature would be useful to you, please provide me with feedback about

which problems that you actually use, and the areas where you would like to have additional exercises. This

information can be passed to the email address given below, or alternately via you McGraw-Hill representative.

Thank you.

Stephen J. Chapman

Melbourne, Australia

January 4, 2004

Stephen J. Chapman

278 Orrong Road

Caulfield North, VIC 3161

Australia

Phone +61-3-9527-9372

1

Chapter 1: Introduction to Machinery Principles

1-1. A motor’s shaft is spinning at a speed of 3000 r/min. What is the shaft speed in radians per second?

S

OLUTION The speed in radians per second is

()

1 min 2 rad

3000 r/min 314.2 rad/s

60 s 1 r

π

ω

 

==

 

 

1-2. A flywheel with a moment of inertia of 2 kg ⋅ m2 is initially at rest. If a torque of 5 N ⋅ m

(counterclockwise) is suddenly applied to the flywheel, what will be the speed of the flywheel after 5 s?

Express that speed in both radians per second and revolutions per minute.

S

OLUTION The speed in radians per second is:

()

2

5 N m

5 s 12.5 rad/s

2 kg m

tt

J

τ

ωα

⋅



== = =

 ⋅



The speed in revolutions per minute is:

()

1 r 60 s

12.5 rad/s 119.4 r/min

2 rad 1 min

n

π



==





1-3. A force of 5 N is applied to a cylinder, as shown in Figure P1-1. What are the magnitude and direction of

the torque produced on the cylinder? What is the angular acceleration α of the cylinder?

S

OLUTION The magnitude and the direction of the torque on this cylinder is:

CCW ,sin

ind

θτ

rF=

()()

ind 0.25 m 10 N sin 30 1.25 N m, CCW

τ

=°=⋅

The resulting angular acceleration is:

2

1.25 N m 0.25 rad/s

5 kg mJ

τ

α

⋅

== =

⋅

1-4. A motor is supplying 60 N ⋅ m of torque to its load. If the motor’s shaft is turning at 1800 r/min, what is

the mechanical power supplied to the load in watts? In horsepower?

S

OLUTION The mechanical power supplied to the load is

()( )

1 min 2 rad

60 N m 1800 r/min 11,310 W

60 s 1 r

P

π

τω

== ⋅ =

2

()

1 hp

11,310 W 15.2 hp

746 W

P

==

1-5. A ferromagnetic core is shown in Figure P1-2. The depth of the core is 5 cm. The other dimensions of the

core are as shown in the figure. Find the value of the current that will produce a flux of 0.005 Wb. With

this current, what is the flux density at the top of the core? What is the flux density at the right side of the

core? Assume that the relative permeability of the core is 1000.

S

OLUTION There are three regions in this core. The top and bottom form one region, the left side forms a

second region, and the right side forms a third region. If we assume that the mean path length of the flux is

in the center of each leg of the core, and if we ignore spreading at the corners of the core, then the path

lengths are 1

l = 2(27.5 cm) = 55 cm, 2

l = 30 cm, and 3

l = 30 cm. The reluctances of these regions are:

()

()()

17

0.55 m 58.36 kA t/Wb

1000 4 10 H/m 0.05 m 0.15 m

ro

ll

AA

µµµ π

−

== = = ⋅

×

R

()

()()

27

0.30 m 47.75 kA t/Wb

1000 4 10 H/m 0.05 m 0.10 m

ro

ll

AA

µµµ π

−

== = = ⋅

×

R

()

()()

37

0.30 m 95.49 kA t/Wb

1000 4 10 H/m 0.05 m 0.05 m

ro

ll

AA

µµµ π

−

== = = ⋅

×

R

The total reluctance is thus

TOT 1 2 3 58.36 47.75 95.49 201.6 kA t/Wb=++= + + = ⋅RRRR

and the magnetomotive force required to produce a flux of 0.003 Wb is

(

)

(

)

0.005 Wb 201.6 kA t/Wb 1008 A t

φ

== ⋅ = ⋅FR

and the required current is

1008 A t 2.52 A

400 t

iN

⋅

== =

F

The flux density on the top of the core is

()()

0.005 Wb 0.67 T

0.15 m 0.05 m

BA

φ

== =

3

The flux density on the right side of the core is

()()

0.005 Wb 2.0 T

0.05 m 0.05 m

BA

φ

== =

1-6. A ferromagnetic core with a relative permeability of 1500 is shown in Figure P1-3. The dimensions are as

shown in the diagram, and the depth of the core is 7 cm. The air gaps on the left and right sides of the core

are 0.070 and 0.020 cm, respectively. Because of fringing effects, the effective area of the air gaps is 5

percent larger than their physical size. If there are 4001 turns in the coil wrapped around the center leg of

the core and if the current in the coil is 1.0 A, what is the flux in each of the left, center, and right legs of

the core? What is the flux density in each air gap?

S

OLUTION This core can be divided up into five regions. Let 1

R be the reluctance of the left-hand portion

of the core, 2

R be the reluctance of the left-hand air gap, 3

R be the reluctance of the right-hand portion of

the core, 4

R be the reluctance of the right-hand air gap, and 5

R be the reluctance of the center leg of the

core. Then the total reluctance of the core is

()

1234

TOT 5

1234

++

=+ +++

RRRR

RR

RRRR

()

()()

1

17

01

1.11 m 90.1 kA t/Wb

2000 4 10 H/m 0.07 m 0.07 m

r

l

A

µµ π

−

== =⋅

×

R

()

()()()

2

27

02

0.0007 m 108.3 kA t/Wb

4 10 H/m 0.07 m 0.07 m 1.05

l

A

µπ

−

== = ⋅

×

R

()

()()

3

37

03

1.11 m 90.1 kA t/Wb

2000 4 10 H/m 0.07 m 0.07 m

r

l

A

µµ π

−

== =⋅

×

R

()

()()()

4

47

04

0.0005 m 77.3 kA t/Wb

4 10 H/m 0.07 m 0.07 m 1.05

l

A

µπ

−

== = ⋅

×

R

()

()()

5

57

05

0.37 m 30.0 kA t/Wb

2000 4 10 H/m 0.07 m 0.07 m

r

l

A

µµ π

−

== =⋅

×

R

The total reluctance is

1 In the first printing, this value was given incorrectly as 300.

4

()

(

)

(

)

1234

TOT 5

12 34

90.1 108.3 90.1 77.3

30.0 120.8 kA t/Wb

90.1 108.3 90.1 77.3

++ ++

=+ = + = ⋅

+++ + + +

RRRR

RR

RR RR

The total flux in the core is equal to the flux in the center leg:

(

)

(

)

center TOT

TOT

400 t 1.0 A 0.0033 Wb

120.8 kA t/Wb

φφ

== = =

⋅

F

R

The fluxes in the left and right legs can be found by the “flux divider rule”, which is analogous to the

current divider rule.

()

(

)

()

34

left TOT

1234

90.1 77.3 0.0033 Wb 0.00193 Wb

90.1 108.3 90.1 77.3

φφ

++

== =

+++ + + +

RR

RR RR

()

(

)

()

12

right TOT

12 34

90.1 108.3 0.0033 Wb 0.00229 Wb

90.1 108.3 90.1 77.3

φφ

++

== =

+++ + + +

RR

RR RR

The flux density in the air gaps can be determined from the equation BA

φ

=:

()()()

left

eff

0.00193 Wb 0.375 T

0.07 cm 0.07 cm 1.05

BA

φ

== =

()()()

right

eff

0.00229 Wb 0.445 T

0.07 cm 0.07 cm 1.05

BA

φ

== =

1-7. A two-legged core is shown in Figure P1-4. The winding on the left leg of the core (N1) has 400 turns, and

the winding on the right (N2) has 300 turns. The coils are wound in the directions shown in the figure. If

the dimensions are as shown, then what flux would be produced by currents i1 = 0.5 A and i2 = 0.75 A?

Assume r

µ

= 1000 and constant.

5

S

OLUTION The two coils on this core are would so that their magnetomotive forces are additive, so the total

magnetomotive force on this core is

(

)

(

)

(

)

(

)

TOT 1 1 2 2 400 t 0.5 A 300 t 0.75 A 425 A tNi Ni=+ = + = ⋅F

The total reluctance in the core is

()

()()

TOT 7

0

2.60 m 92.0 kA t/Wb

1000 4 10 H/m 0.15 m 0.15 m

r

l

A

µµ π

−

== = ⋅

×

R

and the flux in the core is:

TOT

425 A t 0.00462 Wb

92.0 kA t/Wb

φ

⋅

== =

⋅

F

R

1-8. A core with three legs is shown in Figure P1-5. Its depth is 5 cm, and there are 200 turns on the leftmost

leg. The relative permeability of the core can be assumed to be 1500 and constant. What flux exists in

each of the three legs of the core? What is the flux density in each of the legs? Assume a 4% increase in

the effective area of the air gap due to fringing effects.

S

OLUTION This core can be divided up into four regions. Let 1

R be the reluctance of the left-hand portion

of the core, 2

R be the reluctance of the center leg of the core, 3

R be the reluctance of the center air gap,

and 4

R be the reluctance of the right-hand portion of the core. Then the total reluctance of the core is

()

234

TOT 1

234

+

=+++

RRR

RR

RRR

()

()()

1

17

01

1.08 m 127.3 kA t/Wb

1500 4 10 H/m 0.09 m 0.05 m

r

l

A

µµ π

−

== = ⋅

×

R

()

()()

2

27

02

0.34 m 24.0 kA t/Wb

1500 4 10 H/m 0.15 m 0.05 m

r

l

A

µµ π

−

== =⋅

×

R

()

()()()

3

37

03

0.0004 m 40.8 kA t/Wb

4 10 H/m 0.15 m 0.05 m 1.04

l

A

µπ

−

== = ⋅

×

R

()

()()

4

47

04

1.08 m 127.3 kA t/Wb

1500 4 10 H/m 0.09 m 0.05 m

r

l

A

µµ π

−

== = ⋅

×

R

The total reluctance is

6

()

(

)

234

TOT 1

234

24.0 40.8 127.3

127.3 170.2 kA t/Wb

24.0 40.8 127.3

++

=+ = + = ⋅

++ + +

RRR

RR

RRR

The total flux in the core is equal to the flux in the left leg:

(

)

(

)

left TOT

TOT

200 t 2.0 A 0.00235 Wb

170.2 kA t/Wb

φφ

== = =

⋅

F

R

The fluxes in the center and right legs can be found by the “flux divider rule”, which is analogous to the

current divider rule.

()

4

center TOT

234

127.3 0.00235 Wb 0.00156 Wb

24.0 40.8 127.3

φφ

== =

++ + +

R

RRR

()

23

right TOT

234

24.0 40.8 0.00235 Wb 0.00079 Wb

24.0 40.8 127.3

φφ

++

== =

++ + +

RR

RRR

The flux density in the legs can be determined from the equation BA=

φ

:

()()

left

0.00235 Wb 0.522 T

0.09 cm 0.05 cm

BA

φ

== =

()()

center

0.00156 Wb 0.208 T

0.15 cm 0.05 cm

BA

φ

== =

()()

left

right

0.00079 Wb 0.176 T

0.09 cm 0.05 cm

BA

φ

== =

1-9. A wire is shown in Figure P1-6 which is carrying 5.0 A in the presence of a magnetic field. Calculate the

magnitude and direction of the force induced on the wire.

S

OLUTION The force on this wire can be calculated from the equation

(

)

(

)

(

)

(

)

5 A 1 m 0.25 T 1.25 N, into the pagei ilB=×= = =FlB

7

1-10. The wire is shown in Figure P1-7 is moving in the presence of a magnetic field. With the information given

in the figure, determine the magnitude and direction of the induced voltage in the wire.

S

OLUTION The induced voltage on this wire can be calculated from the equation shown below. The voltage

on the wire is positive downward because the vector quantity Bv × points downward.

() ()( )( )

ind cos 45 5 m/s 0.25 T 0.50 m cos 45 0.442 V, positive downevBl= × ⋅ = °= °=vBl

1-11. Repeat Problem 1-10 for the wire in Figure P1-8.

S

OLUTION The induced voltage on this wire can be calculated from the equation shown below. The total

voltage is zero, because the vector quantity Bv × points into the page, while the wire runs in the plane of

the page.

() ()()( )

ind cos 90 1 m/s 0.5 T 0.5 m cos 90 0 VevBl= × ⋅ = °= °=vBl

1-12. The core shown in Figure P1-4 is made of a steel whose magnetization curve is shown in Figure P1-9.

Repeat Problem 1-7, but this time do not assume a constant value of µr. How much flux is produced in the

core by the currents specified? What is the relative permeability of this core under these conditions? Was

the assumption in Problem 1-7 that the relative permeability was equal to 1000 a good assumption for these

conditions? Is it a good assumption in general?

8

S

OLUTION The magnetization curve for this core is shown below:

The two coils on this core are wound so that their magnetomotive forces are additive, so the total

magnetomotive force on this core is

()( )()( )

TOT 1 1 2 2 400 t 0.5 A 300 t 0.75 A 425 A tNi Ni=+ = + = ⋅F

Therefore, the magnetizing intensity H is

9

425 A t 163 A t/m

2.60 m

c

Hl

⋅

== = ⋅

F

From the magnetization curve,

0.15 TB=

and the total flux in the core is

(

)

(

)

(

)

TOT 0.15 T 0.15 m 0.15 m 0.0033 WbBA

φ

== =

The relative permeability of the core can be found from the reluctance as follows:

A

l

r0TOT

TOT

µµφ

== F

R

Solving for µr yields

(

)

(

)

()

()()

TOT

-7

TOT 0

0.0033 Wb 2.6 m

714

425 A t 4 10 H/m 0.15 m 0.15 m

r

l

A

φ

µµπ

== =

⋅×

F

The assumption that

r

µ

= 1000 is not very good here. It is not very good in general.

1-13. A core with three legs is shown in Figure P1-10. Its depth is 8 cm, and there are 400 turns on the center

leg. The remaining dimensions are shown in the figure. The core is composed of a steel having the

magnetization curve shown in Figure 1-10c. Answer the following questions about this core:

(a) What current is required to produce a flux density of 0.5 T in the central leg of the core?

(b) What current is required to produce a flux density of 1.0 T in the central leg of the core? Is it twice the

current in part (a)?

(c) What are the reluctances of the central and right legs of the core under the conditions in part (a)?

(d) What are the reluctances of the central and right legs of the core under the conditions in part (b)?

(e) What conclusion can you make about reluctances in real magnetic cores?

10

S

OLUTION The magnetization curve for this core is shown below:

(a) A flux density of 0.5 T in the central core corresponds to a total flux of

(

)

(

)

(

)

TOT 0.5 T 0.08 m 0.08 m 0.0032 WbBA

φ

== =

By symmetry, the flux in each of the two outer legs must be 12

0.0016 Wb

φφ

== , and the flux density in

the other legs must be

()()

12

0.0016 Wb 0.25 T

0.08 m 0.08 m

BB== =

The magnetizing intensity H required to produce a flux density of 0.25 T can be found from Figure 1-10c.

It is 50 A·t/m. Similarly, the magnetizing intensity H required to produce a flux density of 0.50 T is 70

A·t/m. Therefore, the total MMF needed is

TOT center center outer outer

Hl Hl=+F

()()()()

TOT 70 A t/m 0.24 m 50 A t/m 0.72 m 52.8 A t=⋅ +⋅ = ⋅F

and the required current is

TOT 52.8 A t 0.13 A

400 t

iN

⋅

== =

F

(b) A flux density of 1.0 T in the central core corresponds to a total flux of

()()()

TOT 1.0 T 0.08 m 0.08 m 0.0064 WbBA

φ

== =

By symmetry, the flux in each of the two outer legs must be 12

0.0032 Wb

φφ

== , and the flux density in

the other legs must be

()()

12

0.0032 Wb 0.50 T

0.08 m 0.08 m

BB== =

11

The magnetizing intensity H required to produce a flux density of 0.50 T can be found from Figure 1-10c.

It is 70 A·t/m. Similarly, the magnetizing intensity H required to produce a flux density of 1.00 T is about

160 A·t/m. Therefore, the total MMF needed is

TOT center center outer outer

HI HI=+F

(

)

(

)

(

)

(

)

TOT 160 A t/m 0.24 m 70 A t/m 0.72 m 88.8 A t=⋅ +⋅ =⋅F

and the required current is

TOT 88.8 A t 0.22 A

400 t

iN

φ

⋅

== =

This current is less not twice the current in part (a).

(c) The reluctance of the central leg of the core under the conditions of part (a) is:

(

)

(

)

TOT

cent

TOT

70 A t/m 0.24 m 5.25 kA t/Wb

0.0032 Wb

φ

⋅

== = ⋅

F

R

The reluctance of the right leg of the core under the conditions of part (a) is:

()()

TOT

right

TOT

50 A t/m 0.72 m 22.5 kA t/Wb

0.0016 Wb

φ

⋅

== = ⋅

F

R

(d) The reluctance of the central leg of the core under the conditions of part (b) is:

(

)

(

)

TOT

cent

TOT

160 A t/m 0.24 m 6.0 kA t/Wb

0.0064 Wb

φ

⋅

== = ⋅

F

R

The reluctance of the right leg of the core under the conditions of part (b) is:

()()

TOT

right

TOT

70 A t/m 0.72 m 15.75 kA t/Wb

0.0032 Wb

φ

⋅

== = ⋅

F

R

(e) The reluctances in real magnetic cores are not constant.

1-14. A two-legged magnetic core with an air gap is shown in Figure P1-11. The depth of the core is 5 cm, the

length of the air gap in the core is 0.06 cm, and the number of turns on the coil is 1000. The magnetization

curve of the core material is shown in Figure P1-9. Assume a 5 percent increase in effective air-gap area to

account for fringing. How much current is required to produce an air-gap flux density of 0.5 T? What are

the flux densities of the four sides of the core at that current? What is the total flux present in the air gap?

12

S

OLUTION The magnetization curve for this core is shown below:

An air-gap flux density of 0.5 T requires a total flux of

(

)

(

)

(

)

(

)

eff 0.5 T 0.05 m 0.05 m 1.05 0.00131 WbBA

φ

== =

This flux requires a flux density in the right-hand leg of

()()

right

0.00131 Wb 0.524 T

0.05 m 0.05 m

BA

φ

== =

The flux density in the other three legs of the core is

()()

top left bottom

0.00131 Wb 0.262 T

0.10 m 0.05 m

BBB A

φ

== == =

13

The magnetizing intensity required to produce a flux density of 0.5 T in the air gap can be found from the

equation ag ago

BH

µ

=:

ag

ag 7

0

0.5 T 398 kA t/m

410 H/m

B

H

µπ

−

== = ⋅

×

The magnetizing intensity required to produce a flux density of 0.524 T in the right-hand leg of the core can

be found from Figure P1-9 to be

right 410 A t/mH=⋅

The magnetizing intensity required to produce a flux density of 0.262 T in the top, left, and bottom legs of

the core can be found from Figure P1-9 to be

top left bottom 240 A t/mHHH== = ⋅

The total MMF required to produce the flux is

TOT ag ag right right top top left left bottom bottom

Hl H l H l H l H l=+ + + +F

(

)

(

)

(

)

(

)

(

)

(

)

TOT 398 kA t/m 0.0006 m 410 A t/m 0.40 m 3 240 A t/m 0.40 m=⋅ +⋅ +⋅F

TOT 278.6 164 288 691 A t=++= ⋅F

and the required current is

TOT 691 A t 0.691 A

1000 t

iN

⋅

== =

F

The flux densities in the four sides of the core and the total flux present in the air gap were calculated

above.

1-15. A transformer core with an effective mean path length of 10 in has a 300-turn coil wrapped around one leg.

Its cross-sectional area is 0.25 in2, and its magnetization curve is shown in Figure 1-10c. If current of 0.25

A is flowing in the coil, what is the total flux in the core? What is the flux density?

S

OLUTION The magnetizing intensity applied to this core is

14

(

)

(

)

()( )

300 t 0.25 A 295 A t/m

10 in 0.0254 m/in

cc

Ni

Hll

== = = ⋅

F

From the magnetization curve, the flux density in the core is

1.27 T

B=

The total flux in the core is

()

2

20.0254 m

1.27 T 0.25 in 0.000205 Wb

1 in

BA

φ

== =

1-16. The core shown in Figure P1-2 has the flux

φ

shown in Figure P1-12. Sketch the voltage present at the

terminals of the coil.

S

OLUTION By Lenz’ Law, an increasing flux in the direction shown on the core will produce a voltage that

tends to oppose the increase. This voltage will be the same polarity as the direction shown on the core, so it

will be positive. The induced voltage in the core is given by the equation

ind

d

eN

dt

φ

=

so the voltage in the windings will be

15

Time

dt

d

N

φ

ind

e

0 < t < 2 s

()

0.010 Wb

500 t 2 s 2.50 V

2 < t < 5 s

()

0.020 Wb

500 t 3 s

− -3.33 V

5 < t < 7 s

()

0.010 Wb

500 t 2 s 2.50 V

7 < t < 8 s

()

0.010 Wb

500 t 1 s 5.00 V

The resulting voltage is plotted below:

1-17. Figure P1-13 shows the core of a simple dc motor. The magnetization curve for the metal in this core is

given by Figure 1-10c and d. Assume that the cross-sectional area of each air gap is 18 cm2 and that the

width of each air gap is 0.05 cm. The effective diameter of the rotor core is 4 cm.

16

S

OLUTION The magnetization curve for this core is shown below:

The relative permeability of this core is shown below:

Note: This is a design problem, and the answer presented here is not unique. Other

values could be selected for the flux density in part (a), and other numbers of turns

could be selected in part (c). These other answers are also correct if the proper steps

were followed, and if the choices were reasonable.

(a) From Figure 1-10c, a reasonable maximum flux density would be about 1.2 T. Notice that the

saturation effects become significant for higher flux densities.

(b) At a flux density of 1.2 T, the total flux in the core would be

(1.2 T)(0.04 m)(0.04 m) 0.00192 WbBA

φ

== =

(c) The total reluctance of the core is:

17

TOT stator air gap 1 rotor air gap 2

=+ ++RRR RR

At a flux density of 1.2 T, the relative permeability r

µ

of the stator is about 3800, so the stator reluctance

is

()

()()

stator

stator 7

stator stator

0.48 m 62.8 kA t/Wb

3800 4 10 H/m 0.04 m 0.04 m

l

A

µπ

−

== =⋅

×

R

At a flux density of 1.2 T, the relative permeability r

µ

of the rotor is 3800, so the rotor reluctance is

()

()()

rotor

rotor 7

stator rotor

0.04 m 5.2 kA t/Wb

3800 4 10 H/m 0.04 m 0.04 m

l

A

µπ

−

== =⋅

×

R

The reluctance of both air gap 1 and air gap 2 is

()()

air gap

air gap 1 air gap 2 72

air gap air gap

0.0005 m 221 kA t/Wb

4 10 H/m 0.0018 m

l

A

µπ

−

== = =⋅

×

RR

Therefore, the total reluctance of the core is

TOT stator air gap 1 rotor air gap 2

=+ ++RRR RR

TOT 62.8 221 5.2 221 510 kA t/Wb=+++= ⋅R

The required MMF is

(

)

(

)

TOT TOT 0.00192 Wb 510 kA t/Wb 979 A t

φ

== ⋅=⋅FR

Since

Ni=F, and the current is limited to 1 A, one possible choice for the number of turns is N = 1000.

1-18. Assume that the voltage applied to a load is 208 30 V=∠−°V and the current flowing through the load is

515 A=∠ °

I.

(a) Calculate the complex power S consumed by this load.

(b) Is this load inductive or capacitive?

(c) Calculate the power factor of this load?

(d) Calculate the reactive power consumed or supplied by this load. Does the load consume reactive power

from the source or supply it to the source?

S

OLUTION

(a) The complex power S consumed by this load is

(

)

(

)

(

)

(

)

*

208 30 V 5 15 A 208 30 V 5 15 A= = ∠− ° ∠ ° = ∠− ° ∠− °

*

SVI

1040 45 VA=∠−°S

(b) This is a capacitive load.

(c) The power factor of this load is

()

PF cos 45 0.707 leading=−°=

(d) This load supplies reactive power to the source. The reactive power of the load is

(

)

(

)

(

)

sin 208 V 5 A sin 45 735 varQVI

θ

== −°=−

1-19. Figure P1-14 shows a simple single-phase ac power system with three loads. The voltage source is

120 0 V=∠°

V, and the three loads are

1530 =∠ °ΩZ 2545 =∠ °ΩZ 3590 =∠− °ΩZ

18

Answer the following questions about this power system.

(a) Assume that the switch shown in the figure is open, and calculate the current I, the power factor, and

the real, reactive, and apparent power being supplied by the source.

(b) Assume that the switch shown in the figure is closed, and calculate the current I, the power factor, and

the real, reactive, and apparent power being supplied by the source.

(c) What happened to the current flowing from the source when the switch closed? Why?

+

-

I

V

+

-

+

-

+

-

1

Z2

Z3

Z

120 0 V=∠°V

S

OLUTION

(a) With the switch open, only loads 1 and 2 are connected to the source. The current 1

I in Load 1 is

1

120 0 V 24 30 A

530 A

∠°

==∠−°

∠°

I

The current

2

I in Load 2 is

2

120 0 V 24 45 A

545 A

∠°

==∠−°

∠°

I

Therefore the total current from the source is

12

24 30 A 24 45 A 47.59 37.5 A=+=∠−° + ∠−°= ∠− °II I

The power factor supplied by the source is

(

)

PF cos cos 37.5 0.793 lagging

θ

==−°=

The real, reactive, and apparent power supplied by the source are

(

)

(

)

(

)

cos 120 V 47.59 A cos 37.5 4531 WPVI

θ

== −°=

()( )( )

cos 120 V 47.59 A sin 37.5 3477 varQVI

θ

== −°=−

(

)

(

)

120 V 47.59 A 5711 VASVI== =

(b) With the switch open, all three loads are connected to the source. The current in Loads 1 and 2 is the

same as before. The current 3

I in Load 3 is

3

120 0 V 24 90 A

590 A

∠°

==∠°

∠− °

I

Therefore the total current from the source is

123

24 30 A 24 45 A 24 90 A 38.08 7.5 A= + + = ∠− ° + ∠− ° + ∠ ° = ∠− °II I I

The power factor supplied by the source is

(

)

PF cos cos 7.5 0.991 lagging

θ

==−°=

The real, reactive, and apparent power supplied by the source are

(

)

(

)

(

)

cos 120 V 38.08 A cos 7.5 4531 WPVI

θ

== −°=

19

(

)

(

)

(

)

cos 120 V 38.08 A sin 7.5 596 varQVI

θ

== −°=−

(

)

(

)

120 V 38.08 A 4570 VASVI== =

(c) The current flowing decreased when the switch closed, because most of the reactive power being

consumed by Loads 1 and 2 is being supplied by Load 3. Since less reactive power has to be supplied by

the source, the total current flow decreases.

1-20. Demonstrate that Equation (1-59) can be derived from Equation (1-58) using simple trigonometric

identities:

()

() () () 2 cos cospt vt it VI t t

ωωθ

== − (1-58)

()

() cos 1 cos2 sin sin2pt VI t VI t

θω θω

=++ (1-59)

S

OLUTION

The first step is to apply the following identity:

()()

1

cos cos cos cos

2

αβ αβ αβ

=

The result is

()

() () () 2 cos cospt vt it VI t t

ωωθ

== −

)

()()

1

() 2 cos cos

2

pt VI tt tt

ωωθ ωωθ

=

()

() cos cos 2pt VI t

θωθ

=

Now we must apply the angle addition identity to the second term:

()

cos cos cos sin sin

αβ α β α β

−= +

The result is

[]

() cos cos2 cos sin2 sinpt VI t t

θωθωθ

=+ +

Collecting terms yields the final result:

(

)

() cos 1 cos2 sin sin2pt VI t VI t

θω θω

=++

1-21. A linear machine has a magnetic flux density of 0.5 T directed into the page, a resistance of 0.25 Ω, a bar

length l = 1.0 m, and a battery voltage of 100 V.

(a) What is the initial force on the bar at starting? What is the initial current flow?

(b) What is the no-load steady-state speed of the bar?

(c) If the bar is loaded with a force of 25 N opposite to the direction of motion, what is the new steady-

state speed? What is the efficiency of the machine under these circumstances?

20

S

OLUTION

(a) The current in the bar at starting is

100 V 400 A

0.25

B

V

iR

== =

Ω

Therefore, the force on the bar at starting is

(

)

(

)

(

)

(

)

400 A 1 m 0.5 T 200 N, to the righti=×= =FlB

(b) The no-load steady-state speed of this bar can be found from the equation

vBleVB== ind

()()

100 V 200 m/s

0.5 T 1 m

B

V

vBl

== =

(c) With a load of 25 N opposite to the direction of motion, the steady-state current flow in the bar will

be given by

ilBFF == indapp

()()

app 25 N 50 A

0.5 T 1 m

F

iBl

== =

The induced voltage in the bar will be

(

)

(

)

ind 100 V - 50 A 0.25 87.5 V

B

eViR=−= Ω=

and the velocity of the bar will be

()()

87.5 V 175 m/s

0.5 T 1 m

B

V

vBl

== =

The

input power to the linear machine under these conditions is

(

)

(

)

in 100 V 50 A 5000 W

B

PVi== =

The

output power from the linear machine under these conditions is

(

)

(

)

out 87.5 V 50 A 4375 W

B

PVi== =

Therefore, the efficiency of the machine under these conditions is

out

in

4375 W

100% 100% 87.5%

5000 W

P

η

=× = × =

1-22. A linear machine has the following characteristics:

0.33 T into pageB= 0.50 R=Ω

21

0.5 m

l= 120 V

B

V=

(a) If this bar has a load of 10 N attached to it opposite to the direction of motion, what is the steady-state

speed of the bar?

(b) If the bar runs off into a region where the flux density falls to 0.30 T, what happens to the bar? What

is its final steady-state speed?

(c) Suppose B

V is now decreased to 80 V with everything else remaining as in part (b). What is the new

steady-state speed of the bar?

(d) From the results for parts (b) and (c), what are two methods of controlling the speed of a linear

machine (or a real dc motor)?

SOLUTION

(a) With a load of 20 N opposite to the direction of motion, the steady-state current flow in the bar will

be given by

ilBFF == indapp

()()

app 10 N 60.5 A

0.33 T 0.5 m

F

iBl

== =

The induced voltage in the bar will be

(

)

(

)

ind 120 V - 60.5 A 0.50 89.75 V

B

eViR=−= Ω=

and the velocity of the bar will be

()()

ind 89.75 V 544 m/s

0.33 T 0.5 m

e

vBl

== =

(b) If the flux density drops to 0.30 T while the load on the bar remains the same, there will be a speed

transient until app ind 10 NFF== again. The new steady state current will be

app ind

F F ilB==

()()

app 10 N 66.7 A

0.30 T 0.5 m

F

iBl

== =

The induced voltage in the bar will be

(

)

(

)

ind 120 V - 66.7 A 0.50 86.65 V

B

eViR=−= Ω=

and the velocity of the bar will be

()()

ind 86.65 V 577 m/s

0.30 T 0.5 m

e

vBl

== =

(c) If the battery voltage is decreased to 80 V while the load on the bar remains the same, there will be a

speed transient until app ind 10 NFF== again. The new steady state current will be

app ind

F F ilB==

()()

app 10 N 66.7 A

0.30 T 0.5 m

F

iBl

== =

The induced voltage in the bar will be

22

(

)

(

)

ind 80 V - 66.7 A 0.50 46.65 V

B

eViR=−= Ω=

and the velocity of the bar will be

()()

ind 46.65 V 311 m/s

0.30 T 0.5 m

e

vBl

== =

(d) From the results of the two previous parts, we can see that there are two ways to control the speed of

a linear dc machine. Reducing the flux density B of the machine increases the steady-state speed, and

reducing the battery voltage VB decreases the stead-state speed of the machine. Both of these speed control

methods work for real dc machines as well as for linear machines.

23

Chapter 2: Transformers

2-1. The secondary winding of a transformer has a terminal voltage of ( ) 282.8 sin 377 V

s

vt t=. The turns

ratio of the transformer is 100:200 (a = 0.50). If the secondary current of the transformer is

()

( ) 7.07 sin 377 36.87 A

s

it t=−°, what is the primary current of this transformer? What are its voltage

regulation and efficiency? The impedances of this transformer referred to the primary side are

eq 0.20 R=Ω 300

C

R=Ω

eq 0.750 X=Ω 80

M

X=Ω

S

OLUTION The equivalent circuit of this transformer is shown below. (Since no particular equivalent circuit

was specified, we are using the approximate equivalent circuit referred to the primary side.)

The secondary voltage and current are

282.8 0 V 200 0 V

2

S=∠°=∠°V

7.07 36.87 A 5 -36.87 A

2

S=∠− °=∠ °I

The secondary voltage referred to the primary side is

100 0 V

SS

a

′==∠°VV

The secondary current referred to the primary side is

10 36.87 A

S

Sa

′==∠− °

I

The primary circuit voltage is given by

()

eq eqPSS

RjX

′′

=+ +VVI

(

)

(

)

100 0 V 10 36.87 A 0.20 0.750 106.2 2.6 V

Pj=∠°+∠− ° Ω+ Ω= ∠°V

The excitation current of this transformer is

EX

106.2 2.6 V 106.2 2.6 V 0.354 2.6 1.328 87.4

300 80

CM j

∠° ∠°

=+=+=∠°+∠−°

ΩΩ

III

EX 1.37 72.5 A=∠−°I

24

Therefore, the total primary current of this transformer is

EX 10 36.87 1.37 72.5 11.1 41.0 A

PS

′

=+ =∠− °+ ∠− °= ∠− °II I

The voltage regulation of the transformer at this load is

106.2 100

VR 100% 100% 6.2%

100

PS

S

VaV

aV

−−

=×= ×=

The input power to this transformer is

()() ()

IN cos 106.2 V 11.1 A cos 2.6 41.0

PP

PVI

θ

==

()()

IN 106.2 V 11.1 A cos 43.6 854 WP=°=

The output power from this transformer is

(

)

(

)

(

)

OUT cos 200 V 5 A cos 36.87 800 W

SS

PVI

θ

== °=

Therefore, the transformer’s efficiency is

OUT

IN

800 W

100% 100% 93.7%

854 W

P

η

=× = × =

2-2. A 20-kVA 8000/480-V distribution transformer has the following resistances and reactances:

Ω= 32

P

R Ω= 05.0

S

R

Ω= 45

P

X 0.06

S

X=Ω

Ω= k 250

C

R Ω= k 30

M

X

The excitation branch impedances are given referred to the high-voltage side of the transformer.

(a) Find the equivalent circuit of this transformer referred to the high-voltage side.

(b) Find the per-unit equivalent circuit of this transformer.

(c) Assume that this transformer is supplying rated load at 480 V and 0.8 PF lagging. What is this

transformer’s input voltage? What is its voltage regulation?

(d) What is the transformer’s efficiency under the conditions of part (c)?

S

OLUTION

(a) The turns ratio of this transformer is a = 8000/480 = 16.67. Therefore, the secondary impedances

referred to the primary side are

()( )

2

216.67 0.05 13.9

SS

RaR

′== Ω=Ω

()( )

2

216.67 0.06 16.7

SS

XaX

′== Ω=Ω

25

The resulting equivalent circuit is

32

Ω

250 k

Ω

j

45

Ω

j

30 k

Ω

j

16.7

Ω

13.9

Ω

(b) The rated kVA of the transformer is 20 kVA, and the rated voltage on the primary side is 8000 V, so

the rated current in the primary side is 20 kVA/8000 V = 2.5 A. Therefore, the base impedance on the

primary side is

Ω=== 3200

A 2.5

V 8000

base

base I

V

Z

Since baseactualpu /ZZZ =, the resulting per-unit equivalent circuit is as shown below:

0.01

78.125

j

0.0141

j

9.375

0.0043

j

0.0052

(c) To simplify the calculations, use the simplified equivalent circuit referred to the primary side of the

transformer:

32

Ω

250 k

Ω

j

45

Ω

j

30 k

Ω

j

16.7

Ω

13.9

Ω

The secondary current in this transformer is

20 kVA 36.87 A 41.67 36.87 A

480 V

S=∠−°=∠−°I

The secondary current referred to the primary side is

41.67 36.87 A 2.50 36.87 A

16.67

S

Sa

∠− °

′== = ∠− °

I

26

Therefore, the primary voltage on the transformer is

()

′

++

′

=SSP jXRIVV

EQEQ

(

)

(

)

8000 0 V 45.9 61.7 2.50 36.87 A 8185 0.38 V

Pj=∠°+ + ∠− °=∠°V

The voltage regulation of the transformer under these conditions is

8185-8000

VR 100% 2.31%

8000

=×=

(d) Under the conditions of part (c), the transformer’s output power copper losses and core losses are:

()()

OUT cos 20 kVA 0.8 16 kWPS

θ

== =

()

()( )

22

CU EQ 2.5 45.9 287 W

S

PIR

′

== =

22

core

8185 268 W

250,000

S

C

V

PR

′

== =

The efficiency of this transformer is

OUT

OUT CU core

16,000

100% 100% 96.6%

16,000 287 268

P

PPP

η

=×= ×=

++ ++

2-3. A 1000-VA 230/115-V transformer has been tested to determine its equivalent circuit. The results of the

tests are shown below.

Open-circuit test Short-circuit test

VOC = 230 V VSC = 19.1 V

IOC = 0.45 A ISC = 8.7 A

POC = 30 W PSC = 42.3 W

All data given were taken from the primary side of the transformer.

(a) Find the equivalent circuit of this transformer referred to the low-voltage side of the transformer.

(b) Find the transformer’s voltage regulation at rated conditions and (1) 0.8 PF lagging, (2) 1.0 PF, (3) 0.8

PF leading.

(c) Determine the transformer’s efficiency at rated conditions and 0.8 PF lagging.

S

OLUTION

(a) OPEN CIRCUIT TEST:

001957.0

V 230

A 45.0

EX ==−= MC jBGY

()( )

°=== −− 15.73

A 45.0V 230

W 30

coscos 1

OCOC

OC

1

IV

P

θ

mho 0.001873-0.000567 mho 15.73001957.0

EX jjBGY MC =°−∠=−=

Ω== 1763

1

C

CG

R

Ω== 534

1

M

MB

X

27

SHORT CIRCUIT TEST:

EQ EQ EQ

19.1 V 2.2

8.7 A

ZRjX=+ = =Ω

()()

11

SC

SC SC

42.3 W

cos cos 75.3

19.1 V 8.7 A

P

VI

θ

−−

== =°

Ω+=Ω°∠=+= 128.2558.0 3.7520.2

EQEQEQ jjXRZ

Ω= 558.0

EQ

R

Ω= 128.2

EQ jX

To convert the equivalent circuit to the secondary side, divide each impedance by the square of the turns

ratio (a = 230/115 = 2). The resulting equivalent circuit is shown below:

Ω= 140.0

sEQ,

R Ω= 532.0

sEQ, jX

Ω= 441

,sC

R Ω= 134

,sM

X

(b) To find the required voltage regulation, we will use the equivalent circuit of the transformer referred

to the secondary side. The rated secondary current is

A 70.8

V 115

VA 1000 ==

S

I

We will now calculate the primary voltage referred to the secondary side and use the voltage regulation

equation for each power factor.

(1) 0.8 PF Lagging:

()()

EQ 115 0 V 0.140 0.532 8.7 36 87 A

PS S

Zj.

′=+ =∠°+ + Ω ∠− °VV I

118.8 1.4 V

P

′=∠°

V

118.8-115

VR 100% 3.3%

115

=×=

(2) 1.0 PF:

()()

EQ 115 0 V 0.140 0.532 8.7 0 A

PS S

Zj

′=+ =∠°+ + Ω ∠°VV I

116.3 2.28 V

P

′=∠°

V

28

116.3-115

VR 100% 1.1%

115

=×=

(3) 0.8 PF Leading:

()()

EQ 115 0 V 0.140 0.532 8.7 36 87 A

PS S

Zj.

′=+ =∠°+ + Ω ∠ °VV I

113.3 2.24 V

P

′=∠°

V

113.3-115

VR 100% 1.5%

115

=×=−

(c) At rated conditions and 0.8 PF lagging, the output power of this transformer is

(

)

(

)

(

)

OUT cos 115 V 8.7 A 0.8 800 W

SS

PVI

θ

== =

The copper and core losses of this transformer are

(

)

(

)

2

CU EQ,

8.7 A 0.140 10.6 W

SS

PIR== Ω=

()

2

core

118.8 V

32.0 W

441

P

C

V

PR

′

== =

Ω

Therefore the efficiency of this transformer at these conditions is

OUT

OUT CU core

800 W

100% 94.9%

800 W 10.6 W 32.0 W

P

PPP

η

=×= =

++ + +

2-4. A single-phase power system is shown in Figure P2-1. The power source feeds a 100-kVA 14/2.4-kV

transformer through a feeder impedance of 40.0 + j150 Ω. The transformer’s equivalent series impedance

referred to its low-voltage side is 0.12 + j0.5 Ω. The load on the transformer is 90 kW at 0.80 PF lagging

and 2300 V.

(a) What is the voltage at the power source of the system?

(b) What is the voltage regulation of the transformer?

(c) How efficient is the overall power system?

S

OLUTION

To solve this problem, we will refer the circuit to the secondary (low-voltage) side. The feeder’s impedance

referred to the secondary side is

()

2

line

2.4 kV 40 150 1.18 4.41

14 kV

Zjj

′=Ω+Ω=+Ω

29

The secondary current S

I is given by

()()

90 kW 46.03 A

2300 V 0.85

S

I==

46.03 31.8 A

S=∠−°I

(a) The voltage at the power source of this system (referred to the secondary side) is

EQlinesource ZZ SSS IIVV +

′

+=

′

()()()()

source 2300 0 V 46.03 31.8 A 1.18 4.11 46.03 31.8 A 0.12 0.5 jj

′= ∠ ° + ∠− ° + Ω + ∠− ° + ΩV

source 2467 3.5 V

′=∠°V

Therefore, the voltage at the power source is

()

source

14 kV

2467 3.5 V 14.4 3.5 kV

2.4 kV

=∠° =∠°

V

(b) To find the voltage regulation of the transformer, we must find the voltage at the primary side of the

transformer (referred to the secondary side) under full load conditions:

EQ

Z

SSP IVV +=

′

()()

2300 0 V 46.03 31.8 A 0.12 0.5 2317 0.41 V

Pj

′=∠°+ ∠−° +Ω=∠°V

There is a voltage drop of 17 V under these load conditions. Therefore the voltage regulation of the

transformer is

2317 2300

VR 100% 0.74%

2300

−

=×=

(c) The overall efficiency of the power system will be the ratio of the output power to the input power.

The output power supplied to the load is POUT = 90 kW. The input power supplied by the source is

()( )

IN source cos 2467 V 46.03 A cos 35.3 92.68 kW

S

PV I

θ

′

== °=

Therefore, the efficiency of the power system is

OUT

IN

90 kW

100% 100% 97.1%

92.68 kW

P

η

=× = × =

2-5. When travelers from the USA and Canada visit Europe, they encounter a different power distribution

system. Wall voltages in North America are 120 V rms at 60 Hz, while typical wall voltages in Europe are

220 to 240 V at 50 Hz. Many travelers carry small step-up / step-down transformers so that they can use

their appliances in the countries that they are visiting. A typical transformer might be rated at 1-kVA and

120/240 V. It has 500 turns of wire on the 120-V side and 1000 turns of wire on the 240-V side. The

magnetization curve for this transformer is shown in Figure P2-2, and can be found in file p22_mag.dat

at this book’s Web site.

30

(a) Suppose that this transformer is connected to a 120-V, 60 Hz power source with no load connected

to the 240-V side. Sketch the magnetization current that would flow in the transformer. (Use

MATLAB to plot the current accurately, if it is available.) What is the rms amplitude of the

magnetization current? What percentage of full-load current is the magnetization current?

(b) Now suppose that this transformer is connected to a 240-V, 50 Hz power source with no load

connected to the 120-V side. Sketch the magnetization current that would flow in the transformer. (Use

MATLAB to plot the current accurately, if it is available.) What is the rms amplitude of the

magnetization current? What percentage of full-load current is the magnetization current?

(c) In which case is the magnetization current a higher percentage of full-load current? Why?

Note: An electronic version of this magnetization curve can be found in file

p22_mag.dat, which can be used with MATLAB programs. Column 1

contains the MMF in A ⋅ turns, and column 2 contains the resulting flux in

webers.

S

OLUTION

(a) When this transformer is connected to a 120-V 60 Hz source, the flux in the core will be given by the

equation

cos )( t

N

V

t

P

M

ω

φ

−= (2-101)

The magnetization current required for any given flux level can be found from Figure P2-2, or alternately

from the equivalent table in file p22_mag.dat. The MATLAB program shown below calculates the flux

level at each time, the corresponding magnetization current, and the rms value of the magnetization current.

% M-file: prob2_5a.m

% M-file to calculate and plot the magnetization

% current of a 120/240 transformer operating at

% 120 volts and 60 Hz. This program also

31

% calculates the rms value of the mag. current.

% Load the magnetization curve. It is in two

% columns, with the first column being mmf and

% the second column being flux.

load p22_mag.dat;

mmf_data = p22(:,1);

flux_data = p22(:,2);

% Initialize values

S = 1000; % Apparent power (VA)

Vrms = 120; % Rms voltage (V)

VM = Vrms * sqrt(2); % Max voltage (V)

NP = 500; % Primary turns

% Calculate angular velocity for 60 Hz

freq = 60; % Freq (Hz)

w = 2 * pi * freq;

% Calculate flux versus time

time = 0:1/3000:1/30; % 0 to 1/30 sec

flux = -VM/(w*NP) * cos(w .* time);

% Calculate the mmf corresponding to a given flux

% using the MATLAB interpolation function.

mmf = interp1(flux_data,mmf_data,flux);

% Calculate the magnetization current

im = mmf / NP;

% Calculate the rms value of the current

irms = sqrt(sum(im.^2)/length(im));

disp(['The rms current at 120 V and 60 Hz is ', num2str(irms)]);

% Calculate the full-load current

i_fl = S / Vrms;

% Calculate the percentage of full-load current

percnt = irms / i_fl * 100;

disp(['The magnetization current is ' num2str(percnt) ...

'% of full-load current.']);

% Plot the magnetization current.

figure(1)

plot(time,im);

title ('\bfMagnetization Current at 120 V and 60 Hz');

xlabel ('\bfTime (s)');

ylabel ('\bf\itI_{m} \rm(A)');

axis([0 0.04 -0.5 0.5]);

grid on;

When this program is executed, the results are

» prob2_5a

The rms current at 120 V and 60 Hz is 0.31863

The magnetization current is 3.8236% of full-load current.

32

The rms magnetization current is 0.318 A. Since the full-load current is 1000 VA / 120 V = 8.33 A, the

magnetization current is 3.82% of the full-load current. The resulting plot is

(b) When this transformer is connected to a 240-V 50 Hz source, the flux in the core will be given by the

equation

cos )( t

N

V

t

S

M

ω

φ

−=

The magnetization current required for any given flux level can be found from Figure P2-2, or alternately

from the equivalent table in file p22_mag.dat. The MATLAB program shown below calculates the flux

level at each time, the corresponding magnetization current, and the rms value of the magnetization current.

% M-file: prob2_5b.m

% M-file to calculate and plot the magnetization

% current of a 120/240 transformer operating at

% 240 volts and 50 Hz. This program also

% calculates the rms value of the mag. current.

% Load the magnetization curve. It is in two

% columns, with the first column being mmf and

% the second column being flux.

load p22_mag.dat;

mmf_data = p22(:,1);

flux_data = p22(:,2);

% Initialize values

S = 1000; % Apparent power (VA)

Vrms = 240; % Rms voltage (V)

VM = Vrms * sqrt(2); % Max voltage (V)

NP = 1000; % Primary turns

% Calculate angular velocity for 50 Hz

freq = 50; % Freq (Hz)

w = 2 * pi * freq;

% Calculate flux versus time

time = 0:1/2500:1/25; % 0 to 1/25 sec

33

flux = -VM/(w*NP) * cos(w .* time);

% Calculate the mmf corresponding to a given flux

% using the MATLAB interpolation function.

mmf = interp1(flux_data,mmf_data,flux);

% Calculate the magnetization current

im = mmf / NP;

% Calculate the rms value of the current

irms = sqrt(sum(im.^2)/length(im));

disp(['The rms current at 50 Hz is ', num2str(irms)]);

% Calculate the full-load current

i_fl = S / Vrms;

% Calculate the percentage of full-load current

percnt = irms / i_fl * 100;

disp(['The magnetization current is ' num2str(percnt) ...

'% of full-load current.']);

% Plot the magnetization current.

figure(1);

plot(time,im);

title ('\bfMagnetization Current at 240 V and 50 Hz');

xlabel ('\bfTime (s)');

ylabel ('\bf\itI_{m} \rm(A)');

axis([0 0.04 -0.5 0.5]);

grid on;

When this program is executed, the results are

» prob2_5b

The rms current at 50 Hz is 0.22973

The magnetization current is 5.5134% of full-load current.

The rms magnetization current is 0.318 A. Since the full-load current is 1000 VA / 240 V = 4.17 A, the

magnetization current is 5.51% of the full-load current. The resulting plot is

34

(c) The magnetization current is a higher percentage of the full-load current for the 50 Hz case than for

the 60 Hz case. This is true because the peak flux is higher for the 50 Hz waveform, driving the core

further into saturation.

2-6. A 15-kVA 8000/230-V distribution transformer has an impedance referred to the primary of 80 + j300 Ω.

The components of the excitation branch referred to the primary side are Ω= k 350

C

R and

Ω= k 70

M

X.

(a) If the primary voltage is 7967 V and the load impedance is ZL = 3.2 + j1.5 Ω, what is the secondary

voltage of the transformer? What is the voltage regulation of the transformer?

(b) If the load is disconnected and a capacitor of –j3.5 Ω is connected in its place, what is the secondary

voltage of the transformer? What is its voltage regulation under these conditions?

S

OLUTION

(a) The easiest way to solve this problem is to refer all components to the primary side of the

transformer. The turns ratio is a = 8000/230 = 34.78. Thus the load impedance referred to the primary

side is

()( )

2

34.78 3.2 1.5 3871 1815

L

Zjj

′=+Ω=+Ω

The referred secondary current is

()( )

7967 0 V 7967 0 V 1.78 28.2 A

80 300 3871 1815 4481 28.2

Sjj

∠° ∠°

′===∠−°

+Ω+ + Ω ∠°Ω

I

and the referred secondary voltage is

()()

1.78 28.2 A 3871 1815 7610 3.1 V

SSL

Zj

′′′

= = ∠− ° + Ω = ∠− °VI

The actual secondary voltage is thus

7610 3.1 V 218.8 3.1 V

34.78

S

Sa

′∠− °

== = ∠−°

V

The voltage regulation is

7967-7610

VR 100% 4.7%

7610

=×=

(b) The easiest way to solve this problem is to refer all components to the primary side of the

transformer. The turns ratio is again a = 34.78. Thus the load impedance referred to the primary side is

()( )

2

34.78 3.5 4234

L

Zjj

′=−Ω=−Ω

The referred secondary current is

()()

7967 0 V 7967 0 V 2.025 88.8 A

80 300 4234 3935 88.8

Sjj

∠° ∠°

′===∠°

+Ω+− Ω ∠−°Ω

I

and the referred secondary voltage is

()()

2.25 88.8 A 4234 8573 1.2 V

SSL

Zj

′′′

==∠°−Ω=∠−°VI

The actual secondary voltage is thus

35

8573 1.2 V 246.5 1.2 V

34.78

S

Sa

′∠− °

== = ∠−°

V

The voltage regulation is

7967 8573

VR 100% 7.07%

8573

−

=×=−

2-7. A 5000-kVA 230/13.8-kV single-phase power transformer has a per-unit resistance of 1 percent and a per-

unit reactance of 5 percent (data taken from the transformer’s nameplate). The open-circuit test performed

on the low-voltage side of the transformer yielded the following data:

VOC kV=138. A 1.15

OC =I kW 9.44

OC =P

(a) Find the equivalent circuit referred to the low-voltage side of this transformer.

(b) If the voltage on the secondary side is 13.8 kV and the power supplied is 4000 kW at 0.8 PF

lagging, find the voltage regulation of the transformer. Find its efficiency.

S

OLUTION

(a) The open-circuit test was performed on the low-voltage side of the transformer, so it can be used to

directly find the components of the excitation branch relative to the low-voltage side.

EX

15.1 A 0.0010942

13.8 kV

CM

YGjB=− = =

()()

11

OC

OC OC

44.9 kW

cos cos 77.56

13.8 kV 15.1 A

P

VI

θ

−−

== =°

EX 0.0010942 77.56 S 0.0002358 0.0010685 S

CM

YGjB j=− = ∠− °= −

14240

C

RG

== Ω

1936

M

XB

==Ω

The base impedance of this transformer referred to the secondary side is

()

2

base

13.8 kV 38.09

5000 kVA

V

ZS

== = Ω

so

(

)

(

)

EQ 0.01 38.09 0.38 R=Ω=Ω and

(

)

(

)

EQ 0.05 38.09 1.9 X=Ω=Ω. The resulting equivalent circuit

is shown below:

36

Ω= 38.0

sEQ,

R Ω= 9.1

sEQ, jX

Ω= 4240

,sC

R Ω= 936

,sM

X

(b) If the load on the secondary side of the transformer is 4000 kW at 0.8 PF lagging and the secondary

voltage is 13.8 kV, the secondary current is

()()

LOAD 4000 kW 362.3 A

PF 13.8 kV 0.8

S

P

IV

== =

362.3 36.87 A

S=∠−°I

The voltage on the primary side of the transformer (referred to the secondary side) is

EQPSS

Z

′=+

VVI

()()

13,800 0 V 362.3 36.87 A 0.38 1.9 14,330 1.9 V

Pj

′=∠°+∠−° +Ω=∠°V

There is a voltage drop of 14 V under these load conditions. Therefore the voltage regulation of the

transformer is

14,330 13,800

VR 100% 3.84%

13,800

−

=×=

The transformer copper losses and core losses are

()()

2

CU EQ,

362.3 A 0.38 49.9 kW

SS

PIR== Ω=

()

2

core

14,330 V

48.4 kW

4240

P

C

V

PR

′

== =

Ω

Therefore the efficiency of this transformer at these conditions is

OUT

OUT CU core

4000 kW

100% 97.6%

4000 kW 49.9 kW 48.4 kW

P

PPP

η

=×= =

++ + +

2-8. A 200-MVA 15/200-kV single-phase power transformer has a per-unit resistance of 1.2 percent and a per-

unit reactance of 5 percent (data taken from the transformer’s nameplate). The magnetizing impedance is

j80 per unit.

(a) Find the equivalent circuit referred to the low-voltage side of this transformer.

(b) Calculate the voltage regulation of this transformer for a full-load current at power factor of 0.8

lagging.

(c) Assume that the primary voltage of this transformer is a constant 15 kV, and plot the secondary voltage

as a function of load current for currents from no-load to full-load. Repeat this process for power

factors of 0.8 lagging, 1.0, and 0.8 leading.

SOLUTION

(a) The base impedance of this transformer referred to the primary (low-voltage) side is

()

2

base

15 kV 1.125

200 MVA

V

ZS

== =Ω

so

(

)

(

)

EQ 0.012 1.125 0.0135 R=Ω=Ω

()( )

EQ 0.05 1.125 0.0563 X=Ω=Ω

37

()( )

100 1.125 112.5

M

X=Ω=Ω

The equivalent circuit is

EQ, 0.0135

P

R=Ω EQ, 0.0563

P

Xj=Ω

not specified

C

R= 112.5

M

X=Ω

(b) If the load on the secondary side of the transformer is 200 MVA at 0.8 PF lagging, and the referred

secondary voltage is 15 kV, then the referred secondary current is

()()

LOAD 200 MVA 16,667 A

PF 15 kV 0.8

S

P

IV

′== =

16,667 36.87 A

S

′=∠−°

I

The voltage on the primary side of the transformer is

EQ,PSSP

Z

′′

=+

VV I

()()

15,000 0 V 16,667 36.87 A 0.0135 0.0563 15, 755 2.24 V

Pj=∠°+ ∠−° + Ω=∠°V

Therefore the voltage regulation of the transformer is

15,755-15,000

VR 100% 5.03%

15,000

=×=

(c) This problem is repetitive in nature, and is ideally suited for MATLAB. A program to calculate the

secondary voltage of the transformer as a function of load is shown below:

% M-file: prob2_8.m

% M-file to calculate and plot the secondary voltage

% of a transformer as a function of load for power

% factors of 0.8 lagging, 1.0, and 0.8 leading.

% These calculations are done using an equivalent

% circuit referred to the primary side.

% Define values for this transformer

VP = 15000; % Primary voltage (V)

amps = 0:166.67:16667; % Current values (A)

Req = 0.0135; % Equivalent R (ohms)

Xeq = 0.0563; % Equivalent X (ohms)

% Calculate the current values for the three

% power factors. The first row of I contains

% the lagging currents, the second row contains

% the unity currents, and the third row contains

38

% the leading currents.

I(1,:) = amps .* ( 0.8 - j*0.6); % Lagging

I(2,:) = amps .* ( 1.0 ); % Unity

I(3,:) = amps .* ( 0.8 + j*0.6); % Leading

% Calculate VS referred to the primary side

% for each current and power factor.

aVS = VP - (Req.*I + j.*Xeq.*I);

% Refer the secondary voltages back to the

% secondary side using the turns ratio.

VS = aVS * (200/15);

% Plot the secondary voltage (in kV!) versus load

plot(amps,abs(VS(1,:)/1000),'b-','LineWidth',2.0);

hold on;

plot(amps,abs(VS(2,:)/1000),'k--','LineWidth',2.0);

plot(amps,abs(VS(3,:)/1000),'r-.','LineWidth',2.0);

title ('\bfSecondary Voltage Versus Load');

xlabel ('\bfLoad (A)');

ylabel ('\bfSecondary Voltage (kV)');

legend('0.8 PF lagging','1.0 PF','0.8 PF leading');

grid on;

hold off;

The resulting plot of secondary voltage versus load is shown below:

2-9. A three-phase transformer bank is to handle 600 kVA and have a 34.5/13.8-kV voltage ratio. Find the

rating of each individual transformer in the bank (high voltage, low voltage, turns ratio, and apparent

power) if the transformer bank is connected to (a) Y-Y, (b) Y-∆, (c) ∆-Y, (d) ∆-∆, (e) open-∆, (f) open-

Y—open-∆.

S

OLUTION For the first four connections, the apparent power rating of each transformer is 1/3 of the total

apparent power rating of the three-phase transformer. For the open-∆ and open-Y—open-∆ connections,

the apparent power rating is a bit more complicated. The 600 kVA must be 86.6% of the total apparent

39

power rating of the two transformers, implying that the apparent power rating of each transformer must be

231 kVA.

The ratings for

each transformer in the bank for each connection are given below:

Connection Primary Voltage Secondary Voltage Apparent Power Turns Ratio

Y-Y 19.9 kV 7.97 kV 200 kVA 2.50:1

Y-∆ 19.9 kV 13.8 kV 200 kVA 1.44:1

∆-Y 34.5 kV 7.97 kV 200 kVA 4.33:1

∆-∆ 34.5 kV 13.8 kV 200 kVA 2.50:1

open-∆ 34.5 kV 13.8 kV 346 kVA 2.50:1

open-Y—open-∆ 19.9 kV 13.8 kV 346 kVA 1.44:1

Note: The open-Y—open-∆ answer assumes that the Y is on the high-voltage side; if the Y is on the low-

voltage side, the turns ratio would be 4.33:1, and the apparent power rating would be unchanged.

2-10. A 13,800/480 V three-phase Y-∆-connected transformer bank consists of three identical 100-kVA

7967/480-V transformers. It is supplied with power directly from a large constant-voltage bus. In the

short-circuit test, the recorded values on the high-voltage side for one of these transformers are

VSC V=560 A 6.12

SC =I W 3300

SC =P

(a) If this bank delivers a rated load at 0.85 PF lagging and rated voltage, what is the line-to-line voltage on

the primary of the transformer bank?

(b) What is the voltage regulation under these conditions?

(c) Assume that the primary voltage of this transformer bank is a constant 13.8 kV, and plot the secondary

voltage as a function of load current for currents from no-load to full-load. Repeat this process for

power factors of 0.85 lagging, 1.0, and 0.85 leading.

(d) Plot the voltage regulation of this transformer as a function of load current for currents from no-load to

full-load. Repeat this process for power factors of 0.85 lagging, 1.0, and 0.85 leading.

S

OLUTION From the short-circuit information, it is possible to determine the per-phase impedance of the

transformer bank referred to the high-voltage side. The primary side of this transformer is Y-connected, so

the short-circuit phase voltage is

SC

,SC

560 V 323.3 V

33

V

φ

== =

the short-circuit phase current is

,SC SC 12.6 AII

φ

==

and the power per phase is

SC

,SC 1100 W

3

P

φ

==

Thus the per-phase impedance is

EQ EQ EQ

323.3 V 25.66

12.6 A

ZRjX=+ = = Ω

()()

11

SC

SC SC

1100 W

cos cos 74.3

323.3 V 12.6 A

P

VI

θ

−−

== =°

EQ EQ EQ 25.66 74.3 6.94 24.7 ZRjX j=+ = ∠°Ω= + Ω

40

EQ 6.94 R=Ω

EQ 24.7 Xj=Ω

(a) If this Y-∆ transformer bank delivers rated kVA (300 kVA) at 0.85 power factor lagging while the

secondary voltage is at rated value, then each transformer delivers 100 kVA at a voltage of 480 V and 0.85

PF lagging. Referred to the primary side of one of the transformers, the load on each transformer is

equivalent to 100 kVA at 7967 V and 0.85 PF lagging. The equivalent current flowing in the secondary of

one transformer referred to the primary side is

,

100 kVA 12.55 A

7967 V

S

I

φ

′==

,12.55 31.79 A

S

φ

′=∠−°I

The voltage on the primary side of a single transformer is thus

PSSP ZEQ,,,,

′

+

′

=

φφφ

IVV

()()

,7967 0 V 12.55 31.79 A 6.94 24.7 8207 1.52 V

Pj

φ

=∠°+ ∠− ° + Ω=∠°V

The line-to-line voltage on the primary of the transformer is

(

)

LL, ,

3 3 8207 V 14.22 kV

PP

VV

φ

== =

(b) The voltage regulation of the transformer is

8207-7967

VR 100% 3.01%

7967

=×=

Note: It is much easier to solve problems of this sort in the per-unit

system, as we shall see in the next problem.

(c) This sort of repetitive operation is best performed with MATLAB. A suitable MATLAB program is

shown below:

% M-file: prob2_10c.m

% M-file to calculate and plot the secondary voltage

% of a three-phase Y-delta transformer bank as a

% function of load for power factors of 0.85 lagging,

% 1.0, and 0.85 leading. These calculations are done

% using an equivalent circuit referred to the primary side.

% Define values for this transformer

VL = 13800; % Primary line voltage (V)

VPP = VL / sqrt(3); % Primary phase voltage (V)

amps = 0:0.0126:12.6; % Phase current values (A)

Req = 6.94; % Equivalent R (ohms)

Xeq = 24.7; % Equivalent X (ohms)

% Calculate the current values for the three

% power factors. The first row of I contains

% the lagging currents, the second row contains

% the unity currents, and the third row contains

% the leading currents.

41

re = 0.85;

im = sin(acos(re));

I(1,:) = amps .* ( re - j*im); % Lagging

I(2,:) = amps .* ( 1.0 ); % Unity

I(3,:) = amps .* ( re + j*im); % Leading

% Calculate secondary phase voltage referred

% to the primary side for each current and

% power factor.

aVSP = VPP - (Req.*I + j.*Xeq.*I);

% Refer the secondary phase voltages back to

% the secondary side using the turns ratio.

% Because this is a delta-connected secondary,

% this is also the line voltage.

VSP = aVSP * (480/7967);

% Plot the secondary voltage versus load

plot(amps,abs(VSP(1,:)),'b-','LineWidth',2.0);

hold on;

plot(amps,abs(VSP(2,:)),'k--','LineWidth',2.0);

plot(amps,abs(VSP(3,:)),'r-.','LineWidth',2.0);

title ('\bfSecondary Voltage Versus Load');

xlabel ('\bfLoad (A)');

ylabel ('\bfSecondary Voltage (V)');

legend('0.85 PF lagging','1.0 PF','0.85 PF leading');

grid on;

hold off;

The resulting plot is shown below:

(d) This sort of repetitive operation is best performed with MATLAB. A suitable MATLAB program is

shown below:

% M-file: prob2_10d.m

42

% M-file to calculate and plot the voltage regulation

% of a three-phase Y-delta transformer bank as a

% function of load for power factors of 0.85 lagging,

% 1.0, and 0.85 leading. These calculations are done

% using an equivalent circuit referred to the primary side.

% Define values for this transformer

VL = 13800; % Primary line voltage (V)

VPP = VL / sqrt(3); % Primary phase voltage (V)

amps = 0:0.0126:12.6; % Phase current values (A)

Req = 6.94; % Equivalent R (ohms)

Xeq = 24.7; % Equivalent X (ohms)

% Calculate the current values for the three

% power factors. The first row of I contains

% the lagging currents, the second row contains

% the unity currents, and the third row contains

% the leading currents.

re = 0.85;

im = sin(acos(re));

I(1,:) = amps .* ( re - j*im); % Lagging

I(2,:) = amps .* ( 1.0 ); % Unity

I(3,:) = amps .* ( re + j*im); % Leading

% Calculate secondary phase voltage referred

% to the primary side for each current and

% power factor.

aVSP = VPP - (Req.*I + j.*Xeq.*I);

% Calculate the voltage regulation.

VR = (VPP - abs(aVSP)) ./ abs(aVSP) .* 100;

% Plot the voltage regulation versus load

plot(amps,VR(1,:),'b-','LineWidth',2.0);

hold on;

plot(amps,VR(2,:),'k--','LineWidth',2.0);

plot(amps,VR(3,:),'r-.','LineWidth',2.0);

title ('\bfVoltage Regulation Versus Load');

xlabel ('\bfLoad (A)');

ylabel ('\bfVoltage Regulation (%)');

legend('0.85 PF lagging','1.0 PF','0.85 PF leading');

grid on;

hold off;

43

The resulting plot is shown below:

2-11. A 100,000-kVA 230/115-kV ∆-∆ three-phase power transformer has a per-unit resistance of 0.02 pu and a

per-unit reactance of 0.055 pu. The excitation branch elements are pu 110=

C

R and pu 20=

M

X.

(a) If this transformer supplies a load of 80 MVA at 0.85 PF lagging, draw the phasor diagram of one

phase of the transformer.

(b) What is the voltage regulation of the transformer bank under these conditions?

(c) Sketch the equivalent circuit referred to the low-voltage side of one phase of this transformer.

Calculate all of the transformer impedances referred to the low-voltage side.

S

OLUTION

(a) The transformer supplies a load of 80 MVA at 0.85 PF lagging. Therefore, the secondary line

current of the transformer is

()

80,000,000 VA 402 A

3 3 115, 000 V

LS

S

IV

== =

The base value of the secondary line current is

()

base

,base

100,000,000 VA 502 A

3 3 115,000 V

LS

S

IV

== =

so the per-unit secondary current is

()

1

,pu

402 A cos 0.85 0.8 31.8

502 A

LS

I

−

== ∠ =∠−°I

44

The per-unit phasor diagram is shown below:

I

= 0.8

∠

-31.8°

V

= 1.0

∠

0°

S

V

P

θ

(b) The per-unit primary voltage of this transformer is

()( )

EQ

1.0 0 0.8 31.8 0.02 0.055 1.037 1.6

PS

Zj=+ =∠°+ ∠− ° + = ∠°VVI

and the voltage regulation is

1.037 1.0

VR 100% 3.7%

1.0

−

=×=

(c) The base impedance of the transformer referred to the low-voltage side is:

()

2

,base

base

3 3 115 kV 397

100 MVA

V

ZS

φ

== =Ω

Each per-unit impedance is converted to actual ohms referred to the low-voltage side by multiplying it by

this base impedance. The resulting equivalent circuit is shown below:

(

)

(

)

EQ, 0.02 397 7.94

S

R=Ω=Ω

(

)

(

)

EQ, 0.055 397 21.8

S

X=Ω=Ω

(

)

(

)

110 397 43.7 k

C

R=Ω=Ω

(

)

(

)

20 397 7.94 k

M

X=Ω=Ω

Note how easy it was to solve this problem in per-unit, compared with Problem 2-10 above.

2-12. An autotransformer is used to connect a 13.2-kV distribution line to a 13.8-kV distribution line. It must be

capable of handling 2000 kVA. There are three phases, connected Y-Y with their neutrals solidly

grounded.

(a) What must the SE

/

C

NN turns ratio be to accomplish this connection?

(b) How much apparent power must the windings of each autotransformer handle?

(c) If one of the autotransformers were reconnected as an ordinary transformer, what would its ratings be?

45

S

OLUTION

(a) The transformer is connected Y-Y, so the primary and secondary phase voltages are the line voltages

divided by 3. The turns ratio of each autotransformer is given by

SE 13.8 kV/ 3

13.2 kV/ 3

HC

LC

VNN

VN

+

==

SE

13.2 13.2 13.8

CC

NN N+=

SE

13.2 0.6

C

NN=

Therefore,

SE

/

C

NN = 22.

(b) The power advantage of this autotransformer is

IO SE 22 23

CCC

WC C

SNNN N

SN N

++

== =

so 1/22 of the power in each transformer goes through the windings. Since 1/3 of the total power is

associated with each phase, the windings in each autotransformer must handle

()( )

2000 kVA 30.3 kVA

322

W

S==

(c) The voltages across each phase of the autotransformer are 13.8/ 3 = 7967 V and 13.2 / 3 = 7621

V. The voltage across the common winding ( C

N) is 7621 kV, and the voltage across the series winding

(SE

N) is 7967 kV – 7621 kV = 346 V. Therefore, a single phase of the autotransformer connected as an

ordinary transformer would be rated at 7621/346 V and 30.3 kVA.

2-13. Two phases of a 13.8-kV three-phase distribution line serve a remote rural road (the neutral is also

available). A farmer along the road has a 480 V feeder supplying 120 kW at 0.8 PF lagging of three-phase

loads, plus 50 kW at 0.9 PF lagging of single-phase loads. The single-phase loads are distributed evenly

among the three phases. Assuming that the open-Y—open-∆ connection is used to supply power to his

farm, find the voltages and currents in each of the two transformers. Also find the real and reactive powers

supplied by each transformer. Assume the transformers are ideal.

S

OLUTION The farmer’s power system is illustrated below:

Load 1 Load 2

VLL,P VLL,S

+

-

IL,P IL,S

The loads on each phase are balanced, and the total load is found as:

1120 kWP=

46

(

)

(

)

-1

11

tan 120 kW tan cos 0.8 90 kvarQP

θ

== =

250 kWP=

(

)

(

)

-1

22

tan 50 kW tan cos 0.9 24.2 kvarQP

θ

== =

TOT 170 kW P=

TOT 114.2 kvarQ=

11

TOT

114.2 kvar

PF cos tan cos tan 0.830 lagging

170 kW

Q

P

−−

== =

The line current on the secondary side of the transformer bank is

()()

TOT 170 kW 246.4 A

3 PF 3 480 V 0.830

LS

P

IV

== =

The open-Y—open ∆ connection is shown below. From the figure, it is obvious that the secondary voltage

across the transformer is 480 V, and the secondary current in each transformer is 246 A. The primary

voltages and currents are given by the transformer turns ratios to be 7967 V and 14.8 A, respectively. If

the voltage of phase A of the primary side is arbitrarily taken as an angle of 0°, then the voltage of phase B

will be at an angle of –120°, and the voltages of phases A and B on the secondary side will be

V 0480 °∠=

AS

V and V 120480 °−∠=

BS

V respectively.

Note that line currents are shifted by 30° due to the difference between line and phase quantities, and by a

further 33.9° due to the power factor of the load.

-

--

++

+

-

VA = 7967∠0° V

VB = 7967 ∠-120° V

VAS = 480∠0° V VBS = 480∠-120° V

.

IAS = 246∠-63.9° A

IBS = 246∠-183.9° A

ICS = 246∠56.1° A

I

φ

B = 246∠-123.9° A

A

B

n

A

B

C

+

IAP = 14.8∠-63.9° A

IBP = 14.8∠-183.9° A

In = 14.8∠56.1° A

The real and reactive powers supplied by each transformer are calculated below:

(

)

(

)

(

)

cos 480 V 246.4 A cos 0 63.9 52.0 kW

AASA

PVI

θ

==

(

)

(

)

(

)

sin 480 V 246.4 A sin 0 63.9 106.2 kvar

AASA

QVI

θ

==

(

)

(

)

(

)

cos 480 V 246.4 A cos 120 123.9 118 kW

BBSB

PVI

φ

θ

==

(

)

(

)

(

)

sin 480 V 246.4 A sin 120 123.9 8.04 kvar

BBSB

QVI

φ

θ

==

Notice that the real and reactive powers supplied by the two transformers are radically different, put the

apparent power supplied by each transformer is the same. Also, notice that the total power AB

PP+

supplied by the transformers is equal to the power consumed by the loads (within roundoff error), while the

total reactive power AB

QQ+ supplied by the transformers is equal to the reactive power consumed by the

loads.

47

2-14. A 13.2-kV single-phase generator supplies power to a load through a transmission line. The load’s

impedance is load 500 36.87 Z=∠ °Ω, and the transmission line’s impedance is line 60 53.1 Z=∠ °Ω.

(a) If the generator is directly connected to the load (Figure P2-3a), what is the ratio of the load

voltage to the generated voltage? What are the transmission losses of the system?

(b) If a 1:10 step-up transformer is placed at the output of the generator and a 10:1 transformer is

placed at the load end of the transmission line, what is the new ratio of the load voltage to the generated

voltage? What are the transmission losses of the system now? (Note: The transformers may be

assumed to be ideal.)

S

OLUTION

(a) In the case of the directly-connected load, the line current is

line load

13.2 0 kV 23.66 38.6 A

60 53.1 500 36.87

∠°

== = ∠−°

∠°Ω+∠ °Ω

II

The load voltage is

(

)

(

)

load load load 23.66 38.6 A 500 36.87 11.83 1.73 kVZ==∠−°∠°Ω=∠−°VI

The ratio of the load voltage to the generated voltage is 11.83/13.2 = 0.896. The resistance in the

transmission line is

(

)

line line cos 60cos 53.1 36 RZ

θ

== °=Ω

so the transmission losses in the system are

(

)

(

)

2

loss line line 23.66 A 36 20.1 kWPIR== Ω=

(b) In this case, a 1:10 step-up transformer precedes the transmission line and a 10:1 step-down

transformer follows the transmission line. If the transformers are removed by referring the transmission

line to the voltage levels found on either end, then the impedance of the transmission line becomes

48

()

22

line line

11

60 53.1 0.60 53.1

10 10

ZZ

′==∠°Ω=∠°Ω

The current in the referred transmission line and in the load becomes

line load

13.2 0 kV 26.37 36.89 A

0.60 53.1 500 36.87

∠°

′== = ∠− °

∠°Ω+∠ °Ω

II

The load voltage is

(

)

(

)

load load load 26.37 36.89 A 500 36.87 13.185 0.02 kVZ==∠−°∠°Ω=∠−°VI

The ratio of the load voltage to the generated voltage is 13.185/13.2 = 0.9989. Also, the transmission

losses in the system are reduced. The current in the transmission line is

()

line load

11

26.37 A 2.637 A

10 10

II

== =

and the losses in the transmission line are

()()

2

loss line line 2.637 A 36 250 WPIR== Ω=

Transmission losses have decreased by a factor of more than 80.

2-15. A 5000-VA 480/120-V conventional transformer is to be used to supply power from a 600-V source to a

120-V load. Consider the transformer to be ideal, and assume that all insulation can handle 600 V.

(a) Sketch the transformer connection that will do the required job.

(b) Find the kilovoltampere rating of the transformer in the configuration.

(c) Find the maximum primary and secondary currents under these conditions.

S

OLUTION (a) For this configuration, the common winding must be the smaller of the two windings, and

SE 4C

NN=. The transformer connection is shown below:

+

-

+

-

120 V

600 V

N

C

N

SE

(b) The kVA rating of the autotransformer can be found from the equation

()

SE

IO

SE

45000 VA 6250 VA

4

CCC

W

C

NN NN

SS

NN

++

== =

(c) The maximum primary current for this configuration will be

6250 VA 10.4 A

600 V

P

S

IV

== =

and the maximum secondary current is

49

6250 VA 52.1 A

120 V

S

IV

== =

2-16. A 5000-VA 480/120-V conventional transformer is to be used to supply power from a 600-V source to a

480-V load. Consider the transformer to be ideal, and assume that all insulation can handle 600 V.

Answer the questions of Problem 2-15 for this transformer.

S

OLUTION (a) For this configuration, the common winding must be the larger of the two windings, and

SE

4

C

NN=. The transformer connection is shown below:

+

-

+

-

480 V

600 V

N

C

N

SE

(b) The kVA rating of the autotransformer can be found from the equation

()

SE SE SE

IO

SE SE

45000 VA 25,000 VA

C

W

NN N N

SS

NN

++

== =

(c) The maximum primary current for this configuration will be

25,000 VA 41.67 A

600 V

P

S

IV

== =

and the maximum secondary current is

25,000 VA 52.1 A

480 V

S

IV

== =

Note that the apparent power handling capability of the autotransformer is much higher when there is only

a small difference between primary and secondary voltages. Autotransformers are normally only used

when there is a small difference between the two voltage levels.

2-17. Prove the following statement: If a transformer having a series impedance eq

Z

is connected as an

autotransformer, its per-unit series impedance eq

Z

′ as an autotransformer will be

SE

eq eq

SE C

N

Z

NN

=

′+

Note that this expression is the reciprocal of the autotransformer power advantage.

S

OLUTION The impedance of a transformer can be found by shorting the secondary winding and

determining the ratio of the voltage to the current of its primary winding. For the transformer connected as

an ordinary transformer, the impedance referred to the primary ( C

N) is:

50

+

-

+

-

N

C

N

SE

V

1

V

2

Z

1

Z

2

eq 1 2

C

SE

N

Z

ZZ

N

=+

The corresponding equivalent circuit is:

+

-

+

-

N

C

N

SE

V

1

V

2

Z

eq

When this transformer is connected as an autotransformer, the circuit is as shown below. If the output

windings of the autotransformer are shorted out, the voltages H

V will be zero, and the voltage L

V will be

+

-

+

-

N

C

N

SE

V

L

V

H

Z

eq

.

+

-

+

V

C

V

SE

I

SE

I

L

I

C

eq

Z

CL IV =

where

eq

Z is the impedance of the ordinary transformer. However,

C

SE

CSE

C

SE

C

CSECL N

NN

N

NIIIIII +

=+=+=

or L

C

CNN

NII +

=

SE

so the input voltage can be expressed in terms of the input current as:

eqeq Z

NN

N

ZL

CSE

SE

CL IIV +

==

The input impedance of the autotransformer is defined as LL

ZIV /

eq =, so

51

eqeq Z

NN

N

Z

CSE

SE

L

+

==

′

I

V

This is the expression that we were trying to prove.

2-18. Three 25-kVA 24,000/277-V distribution transformers are connected in ∆-Y. The open-circuit test was

performed on the low-voltage side of this transformer bank, and the following data were recorded:

line,OC 480 VV= line,OC 4.10 AI= 3,OC 945 WPφ=

The short-circuit test was performed on the high-voltage side of this transformer bank, and the following

data were recorded:

line,SC 1600 VV= line,SC 2.00 AI= 3,SC 1150 WPφ=

(a) Find the per-unit equivalent circuit of this transformer bank.

(b) Find the voltage regulation of this transformer bank at the rated load and 0.90 PF lagging.

(c) What is the transformer bank’s efficiency under these conditions?

S

OLUTION (a) The equivalent of this three-phase transformer bank can be found just like the equivalent

circuit of a single-phase transformer if we work on a per-phase bases. The open-circuit test data on the

low-voltage side can be used to find the excitation branch impedances referred to the secondary side of the

transformer bank. Since the low-voltage side of the transformer is Y-connected, the per-phase open-circuit

quantities are:

,OC 277 VV

φ

= ,OC 4.10 AI

φ

= ,OC 315 WP

φ

=

The excitation admittance is given by

,

4.10 A 0.01480 S

277 V

OC

EX

OC

I

YV

φ

== =

The admittance angle is

()( )

,

11

,,

315 W

cos cos 73.9

277 V 4.10 A

OC

OC OC

P

VI

φ

φφ

θ

−−

=− =− =− °

Therefore,

0.01483 73.9 0.00410 0.01422

EX C M

YGjB j=− = ∠− °= −

1 / 244

CC

RG==Ω

1/ 70.3

MM

XB==Ω

The base impedance for a single transformer referred to the low-voltage side is

()

22

,

base,

277 V 3.069

25 kVA

S

V

ZS

φ

== =Ω

so the excitation branch elements can be expressed in per-unit as

244 79.5 pu

3.069

C

RΩ

==

Ω 70.3 22.9 pu

3.069

M

XΩ

==

Ω

52

The short-circuit test data can be used to find the series impedances referred to the high-voltage side, since

the short-circuit test data was taken on the high-voltage side. Note that the high-voltage is ∆-connected, so

,SC SC 1600 VVV

φ

== , ,SC SC

/

3 1.1547 AII

φ

== , and ,SC SC

/

3 383 WPP

φ

== .

,

1600 V 1385

1.155 A

SC

EQ

SC

V

ZI

φ

== = Ω

()( )

,

11

,,

383 W

cos cos 78.0

1600 V 1.155 A

SC

SC SC

P

VI

φ

φφ

θ

−−

== =°

1385 78.0 288 1355

EQ EQ EQ

ZRjX j=+ = ∠°=+ Ω

The base impedance referred to the high-voltage side is

()

22

,

base,

24,000 V 23,040

25 kVA

P

V

ZS

φ

== =Ω

The resulting per-unit impedances are

288 0.0125 pu

23,040

EQ

RΩ

==

Ω 1355 0.0588 pu

23,040

EQ

XΩ

==

Ω

The per-unit, per-phase equivalent circuit of the transformer bank is shown below:

+

-

VS

VP

IS

+

-

IP

RCjXM

REQ jXEQ

0.0125 j0.0588

79.5 j22.9

(b) If this transformer is operating at rated load and 0.90 PF lagging, then current flow will be at an

angle of

()

9.0cos 1−

−, or –25.8°. The per-unit voltage at the primary side of the transformer will be

(

)

(

)

EQ 1.0 0 1.0 25.8 0.0125 0.0588 1.038 2.62

PSS

Zj=+ =∠°+ ∠− ° + = ∠ °VVI

The voltage regulation of this transformer bank is

1.038 1.0

VR 100% 3.8%

1.0

−

=×=

(c) The output power of this transformer bank is

(

)

(

)

(

)

OUT cos 1.0 1.0 0.9 0.9 pu

SS

PVI

θ

== =

The copper losses are

(

)

(

)

2

CU EQ 1.0 0.0125 0.0125 pu

S

PIR== =

53

The core losses are

()

2

core

1.038 0.0136 pu

79.5

P

C

V

PR

== =

Therefore, the total input power to the transformer bank is

IN OUT CU core 0.9 0.0125 0.0136 0.926PP PP=++=+ + =

and the efficiency of the transformer bank is

OUT

IN

0.9

100% 100% 97.2%

0.926

P

η

=× = × =

2-19. A 20-kVA 20,000/480-V 60-Hz distribution transformer is tested with the following results:

Open-circuit test

(measured from secondary side)

Short-circuit test

(measured from primary side)

VOC = 480 V VSC = 1130 V

IOC = 1.60 A ISC = 1.00 A

VOC = 305 W PSC = 260 W

(a) Find the per-unit equivalent circuit for this transformer at 60 Hz.

(b) What would the rating of this transformer be if it were operated on a 50-Hz power system?

(c) Sketch the equivalent circuit of this transformer referred to the primary side if it is operating at 50 Hz.

S

OLUTION

(a) The base impedance of this transformer referred to the primary side is

(

)

()

22

base,

20,000 V 20 k

20 kVA

P

V

ZS

== =Ω

The base impedance of this transformer referred to the secondary side is

()

22

base,

480 V 11.52

20 kVA

S

V

ZS

== =Ω

The open circuit test yields the values for the excitation branch (referred to the secondary side):

,

1.60 A 0.00333 S

480 V

OC

EX

OC

I

YV

φ

== =

()( )

11

305 W

cos cos 66.6

480 V 1.60 A

OC

OC OC

P

VI

θ

−−

=− =− =− °

0.00333 66.6 0.00132 0.00306

EX C M

YGjB j=− = ∠− °= −

1/ 757

CC

RG==Ω

1/ 327

MM

XB==Ω

The excitation branch elements can be expressed in per-unit as

757 65.7 pu

11.52

C

RΩ

==

Ω 327 28.4 pu

11.52

M

XΩ

==

Ω

The short circuit test yields the values for the series impedances (referred to the primary side):

54

1130 V 1130

1.00 A

SC

EQ

SC

V

ZI

== = Ω

()()

11

260 W

cos cos 76.7

1130 V 1.00 A

SC

SC SC

P

VI

θ

−−

== =°

1130 76.7 260 1100

EQ EQ EQ

ZRjX j=+ = ∠°=+ Ω

The resulting per-unit impedances are

260 0.013 pu

20,000

EQ

RΩ

==

Ω 1100 0.055 pu

20,000

EQ

XΩ

==

Ω

The per-unit equivalent circuit is

+

-

VS

VP

IS

+

-

IP

RCjXM

REQ jXEQ

0.013 j0.055

65.7 j28.4

(b) If this transformer were operated at 50 Hz, both the voltage and apparent power would have to be

derated by a factor of 50/60, so its ratings would be 16.67 kVA, 16,667/400 V, and 50 Hz.

(c) The transformer parameters referred to the primary side at 60 Hz are:

()()

base ,pu 20 k 65.7 1.31 M

CC

RZR==Ω=Ω

()()

base ,pu 20 k 28.4 568 k

MM

XZX==Ω=Ω

()()

EQ base E ,pu 20 k 0.013 260

Q

RZR==Ω=Ω

()()

EQ base E ,pu 20 k 0.055 1100

Q

XZX==Ω=Ω

At 50 Hz, the resistance will be unaffected but the reactances are reduced in direct proportion to the

decrease in frequency. At 50 Hz, the reactances are

()

50 Hz 568 k 473 k

60 Hz

M

X

=Ω=Ω

()

EQ

50 Hz 1100 917

60 Hz

X

=Ω=Ω

55

The resulting equivalent circuit referred to the primary at 50 Hz is shown below:

+

-

VS

VP

IS

+

-

IP

RCjXM

REQ jXEQ

1.31 MΩj473 kΩ

260 Ωj917 Ω

2-20. Prove that the three-phase system of voltages on the secondary of the Y-∆ transformer shown in Figure 2-

38b lags the three-phase system of voltages on the primary of the transformer by 30°.

S

OLUTION The figure is reproduced below:

V

C

V

B

V

A

V

C

'

V

A

'

V

B

'

++

+

--

-

--

+

++

V

A

'

V

B

'

V

C

'

V

A

V

B

V

C

56

Assume that the phase voltages on the primary side are given by

°∠= 0

PA V

φ

V °−∠= 120

PB V

φ

V °∠= 120

PC V

φ

V

Then the phase voltages on the secondary side are given by

°∠=

′0

SA V

φ

V °−∠=

′120

SB V

φ

V °∠=

′120

SC V

φ

V

where aVV PS /

φφ

=. Since this is a Y-∆ transformer bank, the line voltage ab

V on the primary side is

°∠=°−∠−°∠=−= 3031200PPPBAab VVV

φφφ

VVV

and the voltage °∠=

′

=

′′ 0

SAba V

φ

VV . Note that the line voltage on the secondary side lags the line

voltage on the primary side by 30

°

.

2-21. Prove that the three-phase system of voltages on the secondary of the ∆-Y transformer shown in Figure 2-

38c lags the three-phase system of voltages on the primary of the transformer by 30°.

S

OLUTION The figure is reproduced below:

++

--

V

A

V

A

'

V

B

V

B

'

V

C

V

C

'

Assume that the phase voltages on the primary side are given by

°∠= 0

PA V

φ

V °−∠= 120

PB V

φ

V °∠= 120

PC V

φ

V

57

Then the phase voltages on the secondary side are given by

°∠=

′0

SA V

φ

V °−∠=

′120

SB V

φ

V °∠=

′120

SC V

φ

V

where aVV PS /

φφ

=. Since this is a ∆-Y transformer bank, the line voltage ab

V on the primary side is just

equal to °∠= 0

PA V

φ

V. The line voltage on the secondary side is given by

°−∠=°∠−°∠=−=

′′ 3031200PPPCAba VVV

φφφ

VVV

Note that the line voltage on the secondary side lags the line voltage on the primary side by 30

°

.

2-22. A single-phase 10-kVA 480/120-V transformer is to be used as an autotransformer tying a 600-V

distribution line to a 480-V load. When it is tested as a conventional transformer, the following values are

measured on the primary (480-V) side of the transformer:

Open-circuit test Short-circuit test

VOC = 480 V VSC = 10.0 V

IOC = 0.41 A ISC = 10.6 A

VOC = 38 W PSC = 26 W

(a) Find the per-unit equivalent circuit of this transformer when it is connected in the conventional manner.

What is the efficiency of the transformer at rated conditions and unity power factor? What is the

voltage regulation at those conditions?

(b) Sketch the transformer connections when it is used as a 600/480-V step-down autotransformer.

(c) What is the kilovoltampere rating of this transformer when it is used in the autotransformer connection?

(d) Answer the questions in (a) for the autotransformer connection.

S

OLUTION

(a) The base impedance of this transformer referred to the primary side is

()

22

base,

480 V 23.04

10 kVA

P

V

ZS

== =Ω

The open circuit test yields the values for the excitation branch (referred to the primary side):

,

0.41 A 0.000854 S

480 V

OC

EX

OC

I

YV

φ

== =

()( )

11

38 W

cos cos 78.87

480 V 0.41 A

OC

OC OC

P

VI

θ

−−

=− =− =− °

0.000854 78.87 0.000165 0.000838

EX C M

YGjB j=− = ∠− °= −

1/ 6063

CC

RG==Ω

1/ 1193

MM

XB==Ω

The excitation branch elements can be expressed in per-unit as

6063 263 pu

23.04

C

RΩ

==

Ω 1193 51.8 pu

23.04

M

XΩ

==

Ω

The short circuit test yields the values for the series impedances (referred to the primary side):

10.0 V 0.943

10.6 A

SC

EQ

SC

V

ZI

== = Ω

58

()()

11

26 W

cos cos 75.8

10.0 V 10.6 A

SC

SC SC

P

VI

θ

−−

== =°

0.943 75.8 0.231 0.915

EQ EQ EQ

ZRjX j=+ = ∠°= + Ω

The resulting per-unit impedances are

0.231 0.010 pu

23.04

EQ

RΩ

==

Ω 0.915 0.0397 pu

23.04

EQ

XΩ

==

Ω

The per-unit equivalent circuit is

+

-

V

S

V

P

I

S

+

-

I

P

R

C

jX

M

R

EQ

jX

EQ

0.010

j

0.0397

263

j

51.8

At rated conditions and unity power factor, the input power to this transformer would be IN

P = 1.0 pu.

The core losses (in resistor C

R) would be

()

2

core

1.0 0.00380 pu

263

C

V

PR

== =

The copper losses (in resistor EQ

R) would be

()( )

2

CU EQ 1.0 0.010 0.010 puPIR== =

The output power of the transformer would be

OUT OUT CU core 1.0 0.010 0.0038 0.986PPPP=−−=− − =

and the transformer efficiency would be

OUT

IN

0.986

100% 100% 98.6%

1.0

P

η

=× = × =

The output voltage of this transformer is

()( )

OUT IN EQ 1.0 1.0 0 0.01 0.0397 0.991 2.3Zj=− =−∠° + = ∠−°VVI

The voltage regulation of the transformer is

1.0 0.991

VR 100% 0.9%

0.991

−

=×=

(b) The autotransformer connection for 600/480 V stepdown operation is

59

+

-

+

-

480 V

600 V

N

C

N

SE

V

SE

V

C

+

-

(c) When used as an autotransformer, the kVA rating of this transformer becomes:

()

SE

IO

SE

41

10 kVA 50 kVA

1

C

W

NN

SS

N

++

== =

(d) As an autotransformer, the per-unit series impedance EQ

Z is decreased by the reciprocal of the power

advantage, so the series impedance becomes

0.010 0.002 pu

5

EQ

R==

0.0397 0.00794 pu

5

EQ

X==

while the magnetization branch elements are basically unchanged. At rated conditions and unity power

factor, the input power to this transformer would be IN

P = 1.0 pu. The core losses (in resistor C

R) would

be

()

2

core

1.0 0.00380 pu

263

C

V

PR

== =

The copper losses (in resistor EQ

R) would be

()( )

2

CU EQ 1.0 0.002 0.002 puPIR== =

The output power of the transformer would be

OUT OUT CU core 1.0 0.002 0.0038 0.994PPPP=−−=− − =

and the transformer efficiency would be

OUT

IN

0.994

100% 100% 99.4%

1.0

P

η

=× = × =

The output voltage of this transformer is

()( )

OUT IN EQ 1.0 1.0 0 0.002 0.00794 0.998 0.5Zj=− =−∠° + = ∠−°VVI

The voltage regulation of the transformer is

1.0 0.998

VR 100% 0.2%

0.998

−

=×=

2-23. Figure P2-4 shows a power system consisting of a three-phase 480-V 60-Hz generator supplying two loads

through a transmission line with a pair of transformers at either end.

(a) Sketch the per-phase equivalent circuit of this power system.

60

(b) With the switch opened, find the real power P, reactive power Q, and apparent power S supplied by the

generator. What is the power factor of the generator?

(c) With the switch closed, find the real power P, reactive power Q, and apparent power S supplied by the

generator. What is the power factor of the generator?

(d) What are the transmission losses (transformer plus transmission line losses) in this system with the

switch open? With the switch closed? What is the effect of adding Load 2 to the system?

Region 1 Region 2 Region 3

base1

S = 1000 kVA base2

S = 1000 kVA base3

S= 1000 kVA

base2,L

V= 480 V base2,L

V = 14,400 V base3,L

V = 480 V

S

OLUTION This problem can best be solved using the per-unit system of measurements. The power system

can be divided into three regions by the two transformers. If the per-unit base quantities in Region 1 are

chosen to be base1

S = 1000 kVA and base1,L

V = 480 V, then the base quantities in Regions 2 and 3 will be as

shown above. The base impedances of each region will be:

()

2

1

base1

33 277 V 0.238

1000 kVA

V

ZS

φ

== = Ω

()

2

base2

338314 V 207.4

1000 kVA

V

ZS

φ

== =Ω

()

2

3

base3

33277 V 0.238

1000 kVA

V

ZS

φ

== =Ω

(a) To get the per-unit, per-phase equivalent circuit, we must convert each impedance in the system to

per-unit on the base of the region in which it is located. The impedance of transformer 1

T is already in per-

unit to the proper base, so we don’t have to do anything to it:

010.0

pu,1 =R

040.0

pu,1 =X

The impedance of transformer 2

T is already in per-unit, but it is per-unit to the base of transformer 2

T, so

it must be converted to the base of the power system.

()()

2

base 1 base 2

pu on base 2 pu on base 1 2

base 2 base 1

( , , ) ( , , ) VS

RX Z RX Z

VS

= (2-60)

()( )

2

2,pu 2

8314 V 1000 kVA

0.020 0.040

8314 V 500 kVA

R==

()( )

2

2,pu 2

8314 V 1000 kVA

0.085 0.170

8314 V 500 kVA

X==

61

The per-unit impedance of the transmission line is

line

line,pu

base2

1.5 10 0.00723 0.0482

207.4

Zj

Z

+Ω

== = +

Ω

The per-unit impedance of Load 1 is

load1

load1,pu

base3

0.45 36.87 1.513 1.134

0.238

Z

Zj

Z

∠°Ω

== =+

Ω

The per-unit impedance of Load 2 is

load2

load2,pu

base3

0.8 3.36

0.238

Zj

Z

−Ω

== =−

Ω

The resulting per-unit, per-phase equivalent circuit is shown below:

+

-

1∠0°

T1T2

Line

L1L2

0.010 j0.040 0.00723 j0.0482 0.040 j0.170

1.513

j1.134 -j3.36

(b) With the switch opened, the equivalent impedance of this circuit is

EQ 0.010 0.040 0.00723 0.0482 0.040 0.170 1.513 1.134Zj j j j=+ + + ++ ++

EQ 1.5702 1.3922 2.099 41.6Zj=+ =∠°

The resulting current is

10 0.4765 41.6

2.099 41.6

∠°

==∠−°

∠°

I

The load voltage under these conditions would be

()()

Load,pu Load

0.4765 41.6 1.513 1.134 0.901 4.7Zj== ∠−°+ =∠−°VI

()( )

Load Load,pu base3 0.901 480 V 432 VVVV== =

The power supplied to the load is

()()

2

Load,pu Load 0.4765 1.513 0.344PIR== =

()( )

Load Load,pu base 0.344 1000 kVA 344 kWPPS== =

The power supplied by the generator is

()( )

,pu cos 1 0.4765 cos41.6 0.356

G

PVI

θ

== °=

()( )

,pu sin 1 0.4765 sin 41.6 0.316

G

QVI

θ

== °=

()( )

,pu 1 0.4765 0.4765

G

SVI== =

()( )

,pu base 0.356 1000 kVA 356 kW

GG

PPS== =

()( )

,pu base 0.316 1000 kVA 316 kVAR

GG

QQS== =

()( )

,pu base 0.4765 1000 kVA 476.5 kVA

GG

SSS== =

The power factor of the generator is

62

PF cos 41.6 0.748 lagging=°=

(c) With the switch closed, the equivalent impedance of this circuit is

()()

EQ

1.513 1.134 3.36

0.010 0.040 0.00723 0.0482 0.040 0.170 1.513 1.134 3.36

jj

Zj j j jj

+−

=+ + + ++ + ++−

EQ 0.010 0.040 0.00788 0.0525 0.040 0.170 (2.358 0.109)Zj j j j=+++++++

EQ 2.415 0.367 2.443 8.65Zj=+ =∠°

The resulting current is

10 0.409 8.65

2.443 8.65

∠°

==∠−°

∠°

I

The load voltage under these conditions would be

()()

Load,pu Load

0.409 8.65 2.358 0.109 0.966 6.0Zj==∠−°+ =∠−°VI

()( )

Load Load,pu base3 0.966 480 V 464 VVVV== =

The power supplied to the two loads is the power supplied to the resistive component of the parallel

combination of the two loads: 2.358 pu.

()()

2

Load,pu Load 0.409 2.358 0.394PIR== =

()( )

Load Load,pu base 0.394 1000 kVA 394 kWPPS== =

The power supplied by the generator is

()( )

,pu cos 1 0.409 cos6.0 0.407

G

PVI

θ

== °=

()( )

,pu sin 1 0.409 sin 6.0 0.0428

G

QVI

θ

== °=

()( )

,pu 1 0.409 0.409

G

SVI== =

()( )

,pu base 0.407 1000 kVA 407 kW

GG

PPS== =

()( )

,pu base 0.0428 1000 kVA 42.8 kVAR

GG

QQS== =

()( )

,pu base 0.409 1000 kVA 409 kVA

GG

SSS== =

The power factor of the generator is

PF cos 6.0 0.995 lagging=°=

(d) The transmission losses with the switch open are:

()( )

2

line,pu line 0.4765 0.00723 0.00164PIR== =

()( )

line l ,pu base 0.00164 1000 kVA 1.64 kW

ine

PPS== =

The transmission losses with the switch closed are:

()( )

2

line,pu line 0.409 0.00723 0.00121PIR== =

()( )

line l ,pu base 0.00121 1000 kVA 1.21 kW

ine

PPS== =

Load 2 improved the power factor of the system, increasing the load voltage and the total power supplied to

the loads, while simultaneously decreasing the current in the transmission line and the transmission line

losses. This problem is a good example of the advantages of power factor correction in power systems.

63

Chapter 3: Introduction to Power Electronics

3-1. Calculate the ripple factor of a three-phase half-wave rectifier circuit, both analytically and using

MATLAB.

S

OLUTION A three-phase half-wave rectifier and its output voltage are shown below

π

/6 5

π

/6

2

π

/3

()

sin

AM

vt V t

ω

=

() ( )

sin 2 / 3

BM

vt V t

ωπ

=−

() ( )

sin 2 / 3

CM

vt V t

ωπ

=+

S

OLUTION If we find the average and rms values over the interval from π/6 to 5π/6 (one period), these

values will be the same as the average and rms values of the entire waveform, and they can be used to

calculate the ripple factor. The average voltage is

()

5/6

/6

13

() sin

2

DC M

VvtdtVtdt

T

π

ωω

π

==

5

6

333333

cos 0.8270

22222

MM

DC MM

VV

Vt VV

π

ω

ππ π

=− =− − − = =

The rms voltage is

()

5/6

222

rms

/6

13

() sin

2M

VvtdtVtdt

T

π

ωω

π

==

5/6

2

rms

/

6

311

sin 2

22 4

M

V

Vtt

π

ωω

π

=−

64

2

rms

315 15

sin sin

22664 3 3

M

V

ππ π π

π

=−−−

22

rms

315 3133

sin sin

234 3 3 23422

MM

VV

V

πππ π

ππ

=−−=−−−

22

rms

31333 3

0.8407

23422 234

MM

M

VV

ππ

=−−−=+=

The resulting ripple factor is

22

rms

DC

0.8407

1 100% 1 100% 18.3%

0.8270

M

VV

rVV

=−×= −×=

The ripple can be calculated with MATLAB using the ripple function developed in the text. We must

right a new function halfwave3 to simulate the output of a three-phase half-wave rectifier. This output

is just the largest voltage of

()

tvA,

()

tvB, and

()

tvC at any particular time. The function is shown below:

function volts = halfwave3(wt)

% Function to simulate the output of a three-phase

% half-wave rectifier.

% wt = Phase in radians (=omega x time)

% Convert input to the range 0 <= wt < 2*pi

while wt >= 2*pi

wt = wt - 2*pi;

end

while wt < 0

wt = wt + 2*pi;

end

% Simulate the output of the rectifier.

a = sin(wt);

b = sin(wt - 2*pi/3);

c = sin(wt + 2*pi/3);

volts = max( [ a b c ] );

The function

ripple is reproduced below. It is identical to the one in the textbook.

function r = ripple(waveform)

% Function to calculate the ripple on an input waveform.

% Calculate the average value of the waveform

nvals = size(waveform,2);

temp = 0;

for ii = 1:nvals

temp = temp + waveform(ii);

end

average = temp/nvals;

% Calculate rms value of waveform

65

temp = 0;

for ii = 1:nvals

temp = temp + waveform(ii)^2;

end

rms = sqrt(temp/nvals);

% Calculate ripple factor

r = sqrt((rms / average)^2 - 1) * 100;

Finally, the test driver program is shown below.

% M-file: test_halfwave3.m

% M-file to calculate the ripple on the output of a

% three phase half-wave rectifier.

% First, generate the output of a three-phase half-wave

% rectifier

waveform = zeros(1,128);

for ii = 1:128

waveform(ii) = halfwave3(ii*pi/64);

end

% Now calculate the ripple factor

r = ripple(waveform);

% Print out the result

string = ['The ripple is ' num2str(r) '%.'];

disp(string);

When this program is executed, the results are

» test_halfwave3

The ripple is 18.2759%.

This answer agrees with the analytical solution above.

3-2. Calculate the ripple factor of a three-phase full-wave rectifier circuit, both analytically and using

MATLAB.

S

OLUTION A three-phase half-wave rectifier and its output voltage are shown below

66

T

/12

()

sin

AM

vt V t

ω

=

() ( )

sin 2 / 3

BM

vt V t

ωπ

=−

() ( )

sin 2 / 3

CM

vt V t

ωπ

=+

S

OLUTION By symmetry, the rms voltage over the interval from 0 to T/12 will be the same as the rms

voltage over the whole interval. Over that interval, the output voltage is:

() () ()

22

sin sin

33

CB M M

vt vt vt V t V t

ππ

ωω

=−= +− −

()

22 22

sin cos cos sin sin cos cos sin

33 33

MM

vt V t t V t t

ππ ππ

ωω ωω

=+−−

()

2

2cos sin 3cos

3

M

vt V t t

π

ωω

==

Note that the period of the waveform is 2/T

πω

=, so T/12 is

/

6

πω

. The average voltage over the

interval from 0 to T/12 is

/6

/

6

0

16 63

() 3 cos sin

DC M M

Vvtdt VtdtVt

T

πω πω

ωωω

ππ

== =

33 1.6540

DC M M

VV V

π

==

The rms voltage is

/6

222

rms

0

16

() 3 cos

M

Vvtdt Vtdt

T

πω

ωω

π

==

67

/

6

2

rms

0

18 1 1 sin 2

24

M

VVt t

πω

ωω

πω

=+

rms

18 1 3 9 3

sin 1.6554

12 4 3 2 4

MMM

VV V V

ωπ π

πωω π

=+=+=

The resulting ripple factor is

22

rms

DC

1.6554

1 100% 1 100% 4.2%

1.6540

M

VV

rVV

=−×= −×=

The ripple can be calculated with MATLAB using the ripple function developed in the text. We must

right a new function fullwave3 to simulate the output of a three-phase half-wave rectifier. This output

is just the largest voltage of

()

tvA,

()

tvB, and

()

tvC at any particular time. The function is shown below:

function volts = fullwave3(wt)

% Function to simulate the output of a three-phase

% full-wave rectifier.

% wt = Phase in radians (=omega x time)

% Convert input to the range 0 <= wt < 2*pi

while wt >= 2*pi

wt = wt - 2*pi;

end

while wt < 0

wt = wt + 2*pi;

end

% Simulate the output of the rectifier.

a = sin(wt);

b = sin(wt - 2*pi/3);

c = sin(wt + 2*pi/3);

volts = max( [ a b c ] ) - min( [ a b c ] );

The test driver program is shown below.

% M-file: test_fullwave3.m

% M-file to calculate the ripple on the output of a

% three phase full-wave rectifier.

% First, generate the output of a three-phase full-wave

% rectifier

waveform = zeros(1,128);

for ii = 1:128

waveform(ii) = fullwave3(ii*pi/64);

end

% Now calculate the ripple factor

r = ripple(waveform);

% Print out the result

string = ['The ripple is ' num2str(r) '%.'];

disp(string);

68

When this program is executed, the results are

» test_fullwave3

The ripple is 4.2017%.

This answer agrees with the analytical solution above.

3-3. Explain the operation of the circuit shown in Figure P3-1. What would happen in this circuit if switch S1

were closed?

S

OLUTION Diode D1 and D2 together with the transformer form a full-wave rectifier. Therefore, a voltage

oriented positive-to-negative as shown will be applied to the SCR and the control circuit on each half cycle.

(1) Initially, the SCR is an open circuit, since v1 < VBO for the SCR. Therefore, no current flows to the

load and vLOAD = 0.

(2) Voltage v1 is applied to the control circuit, charging capacitor C1 with time constant RC1.

(3) When vC > VBO for the DIAC, it conducts, supplying a gate current to the SCR.

(4) The gate current in the SCR lowers its breakover voltage, and the SCR fires. When the SCR fires,

current flows through the SCR and the load.

(5) The current flow continues until iD falls below IH for the SCR (at the end of the half cycle). The

process starts over in the next half cycle.

69

If switch S1 is shut, the charging time constant is increased, and the DIAC fires later in each half cycle.

Therefore, less power is supplied to the load.

3-4. What would the rms voltage on the load in the circuit in Figure P3-1 be if the firing angle of the SCR were

(a) 0°, (b) 30°, (c) 90°?

S

OLUTION The input voltage to the circuit of Figure P3-1 is

()

ttv

ω

sin339

ac =, where rad/s 377=

ω

Therefore, the voltage on the secondary of the transformer will be

()

ttv

ω

sin5.169

ac =

(a) The average voltage applied to the load will be the integral over the conducting portion of the half

cycle divided by π/ω, the period of a half cycle. For a firing angle of 0°, the average voltage will be

/

ave

0

00

11

() sin cos

T

MM

VvtdtVtdtVt

T

πω

ωωω

ππ

== =−

[]

()( )

ave

12

1 1 0.637 169.5 V 108 V

MM

VV V

ππ

=− − − = = =

(b) For a firing angle of 30°, the average voltage will be

/

ave

/

6

/6 /6

11

() sin cos

T

MM

VvtdtVtdtVt

T

πω

π

ππ

ωωω

ππ

== =−

()( )

ave

1323

1 0.594 169.5 V 101 V

22

MM

VV V

ππ

+

=− − − = = =

(c) For a firing angle of 90°, the average voltage will be

/

ave

/

2

/2 /2

11

() sin cos

T

MM

VvtdtVtdtVt

T

πω

π

ππ

ωωω

ππ

== =−

70

[]

()( )

ave

11

1 0.318 169.5 V 54 V

MM

VV V

ππ

=− − = = =

3-5. For the circuit in Figure P3-1, assume that VBO for the DIAC is 30 V, C1 is 1 µF, R is adjustable in the

range 1-20 kΩ, and that switch S1 is open. What is the firing angle of the circuit when R is 10 kΩ? What

is the rms voltage on the load under these conditions?

Note: Problem 3-5 is significantly harder for many students, since it involves solving

a differential equation with a forcing function. This problem should only be

assigned if the class has the mathematical sophistication to handle it.

S

OLUTION At the beginning of each half cycle, the voltages across the DIAC and the SCR will both be

smaller then their respective breakover voltages, so no current will flow to the load (except for the very tiny

current charging capacitor C), and vload(t) will be 0 volts. However, capacitor C charges up through

resistor R, and when the voltage vC(t) builds up to the breakover voltage of D1, the DIAC will start to

conduct. This current flows through the gate of SCR1, turning the SCR ON. When it turns ON, the

voltage across the SCR will drop to 0, and the full source voltage vS(t) will be applied to the load,

producing a current flow through the load. The SCR continues to conduct until the current through it falls

below IH, which happens at the very end of the half cycle.

Note that after D1 turns on, capacitor C discharges through it and the gate of the SCR. At the end of the

half cycle, the voltage on the capacitor is again essentially 0 volts, and the whole process is ready to start

over again at the beginning of the next half cycle.

To determine when the DIAC and the SCR fire in this circuit, we must determine when vC(t) exceeds VBO

for D1. This calculation is much harder than in the examples in the book, because in the previous problems

the source was a simple DC voltage source, while here the voltage source is sinusoidal. However, the

principles are identical.

(a) To determine when the SCR will turn ON, we must calculate the voltage vC(t), and then solve for the time

at which vC(t) exceeds VBO for D1. At the beginning of the half cycle, D1 and SCR1 are OFF, and the

voltage across the load is essentially 0, so the entire source voltage vS(t) is applied to the series RC circuit.

To determine the voltage vC(t) on the capacitor, we can write a Kirchhoff's Current Law equation at the

node above the capacitor and solve the resulting equation for vC(t).

0

21 =+ ii (since the DIAC is an open circuit at this time)

0

1=+

−

C

Cv

dt

d

C

R

vv

71

1

11 v

RC

v

RC

v

dt

d

CC =+

t

R

C

V

v

R

C

v

dt

dM

CC

ω

sin

1=+

The solution can be divided into two parts, a natural response and a forced response. The natural response

is the solution to the differential equation

10

CC

dvv

dt RC

+=

The solution to the natural response differential equation is

()

, e

t

RC

Cn

vtA

−

=

where the constant A must be determined from the initial conditions in the system. The forced response is

the steady-state solution to the equation

1sin

M

CC

dV

vv t

dt RC RC

ω

+=

It must have a form similar to the forcing function, so the solution will be of the form

(

)

,1 2

sin cos

Cf

vtB tB t

ωω

=+

where the constants 1

B and 2

B must be determined by substitution into the original equation. Solving for

1

B and 2

B yields:

()()

12 12

1

sin cos sin cos sin

M

dV

BtBt BtBt t

dt RC RC

ωω ωω ω

++ +=

()()

12 12

1

cos in sin cos sin

M

V

BtBst B tB t t

RC RC

ωωω ω ω ω ω

−+ +=

cosine equation:

12

10

BB

RC

ω

+= ⇒ 21

BRCB

ω

=−

sine equation:

21

1M

V

BB

RC RC

ω

−+ =

2

11

1

M

V

RC B B

RC RC

ω

+=

2

1

M

V

RC B

RC RC

ω

+=

222

1

1M

RC V

B

RC RC

ω

+=

Finally,

72

1222

1

M

V

BRC

ω

=+ and 2222

1

M

RC V

BRC

ω

−

=+

Therefore, the forced solution to the equation is

()

,222 222

sin cos

11

MM

Cf

VRCV

vt t t

RC RC

ω

ωω

=−

++

and the total solution is

(

)

(

)

(

)

,,CCnCf

vt v t v t=+

()

222 222

sin cos

11

t

MM

RC

C

VRCV

vt Ae t t

RC RC

ω

ωω

−

=+ −

++

The initial condition for this problem is that the voltage on the capacitor is zero at the beginning of the half-

cycle:

()

0

222 222

0 sin 0 cos 00

11

MM

RC

C

VRCV

vAe RC RC

ω

ωω

−

=+ − =

++

22 2

0

1

M

RC V

ARC

ω

−=

+

222

1

M

RC V

ARC

ω

=+

Therefore, the voltage across the capacitor as a function of time before the DIAC fires is

()

222 222 222

sin cos

11 1

t

MM M

RC

C

RC V V RC V

vt e t t

RC RC RC

ωω

ωω ω

−

=+ −

++ +

If we substitute the known values for R, C,

ω

, and VM, this equation becomes

(

)

100

42 11.14 sin 42 cos

t

C

vt e t t

ωω

−

=+ −

This equation is plotted below:

It reaches a voltage of 30 V at a time of 3.50 ms. Since the frequency of the waveform is 60 Hz, the

waveform there are 360° in 1/60 s, and the firing angle α is

73

()

360

3.50 ms 75.6

1/60 s

α

°

==°

or 1.319 radians

Note: This problem could also have been solved using Laplace Transforms, if desired.

(b) The rms voltage applied to the load is

/

222

rms

1() sin

M

VvtdtVtdt

T

πω

α

ωω

π

==

/

2

rms

11

sin 2

24

M

V

Vtt

πω

α

ωω

π

=−

()()

2

rms

11

sin 2 sin 2

24

M

V

πα π α

π

=−−−

rms 0.3284 0.573 97.1 V

MM

VV V===

3-6. One problem with the circuit shown in Figure P3-1 is that it is very sensitive to variations in the input

voltage vt

ac (). For example, suppose the peak value of the input voltage were to decrease. Then the time

that it takes capacitor C1 to charge up to the breakover voltage of the DIAC will increase, and the SCR will

be triggered later in each half cycle. Therefore, the rms voltage supplied to the load will be reduced both by

the lower peak voltage and by the later firing. This same effect happens in the opposite direction if vt

ac ()

increases. How could this circuit be modified to reduce its sensitivity to variations in input voltage?

S

OLUTION If the voltage charging the capacitor could be made constant or nearly so, then the feedback

effect would be stopped and the circuit would be less sensitive to voltage variations. A common way to do

this is to use a zener diode that fires at a voltage greater than BO

V for the DIAC across the RC charging

circuit. This diode holds the voltage across the RC circuit constant, so that the capacitor charging time is

not much affected by changes in the power supply voltage.

v

C

R

74

3-7. Explain the operation of the circuit shown in Figure P3-2, and sketch the output voltage from the circuit.

S

OLUTION This circuit is a single-phase voltage source inverter.

(1) Initially, suppose that both SCRs are OFF. Then the voltage on the transformer T3 will be 0, and

voltage VDC will be dropped across SCR1 and SCR2 as shown.

(2) Now, apply a pulse to transformer T1 that turns on SCR1. When that happens, the circuit looks like:

Since the top of the transformer is now grounded, a voltage VDC appears across the upper winding as

shown. This voltage induces a corresponding voltage on the lower half of the winding, charging capacitor

C1 up to a voltage of 2VDC, as shown.

Now, suppose that a pulse is applied to transformer T2. When that occurs, SCR2 becomes a short circuit,

and SCR1 is turned OFF by the reverse voltage applied to it by capacitor C1 (forced commutation). At that

time, the circuit looks like:

Now the voltages on the transformer are reversed, charging capacitor C1 up to a voltage of 2VDC in the

opposite direction. When SCR1 is triggered again, the voltage on C1 will turn SCR2 OFF.

The output voltage from this circuit would be roughly a square wave, except that capacitor C2 filters it

somewhat.

75

(

Note: The above discussion assumes that transformer T3 is never in either state long enough for it to

saturate.)

3-8. Figure P3-3 shows a relaxation oscillator with the following parameters:

R1=variable Ω= 1500

2

R

1.0 FC

µ

= V 100

DC =V

BO 30 VV= 0.5 mA

H

I=

(a) Sketch the voltages vt

C(), vt

D(), and vt

o() for this circuit.

(b) If R1 is currently set to 500 kΩ, calculate the period of this relaxation oscillator.

S

OLUTION

(a) The voltages vC(t), vD(t) and vo(t) are shown below. Note that vC(t) and vD(t) look the same during

the rising portion of the cycle. After the PNPN Diode triggers, the voltage across the capacitor decays with

time constant τ2 =

R1R2

R1 + R2

C, while the voltage across the diode drops immediately.

76

(b) When voltage is first applied to the circuit, the capacitor C charges with a time constant

τ

1 = R1 C =

(500 kΩ)(1.00 µF) = 0.50 s. The equation for the voltage on the capacitor as a function of time during the

charging portion of the cycle is

()

1

t

RC

C

vt ABe

−

=+

where A and B are constants depending upon the initial conditions in the circuit. Since vC(0) = 0 V and

vC(∞) = 100 V, it is possible to solve for A and B.

A = vC(∞) = 100 V

A + B = vC(0) = 0 V ⇒ B = -100 V

Therefore,

()

0.50

100 100 V

t

C

vt e

−

=−

The time at which the capacitor will reach breakover voltage is found by setting vC(t) = VBO and solving for

time t1:

77

1

100 V 30 V

0.50 ln 178 ms

100 V

t−

=− =

Once the PNPN Diode fires, the capacitor discharges through the parallel combination of R1 and R2, so the

time constant for the discharge is

(

)

(

)

()

12

2

12

500 k 1.5 k 1.0 F 0.0015 s

500 k 1.5 k

RR C

RR

τµ

ΩΩ

== =

+Ω+Ω

The equation for the voltage on the capacitor during the discharge portion of the cycle is

()

2

t

C

vt ABe

τ

−

=+

()

2

BO

t

C

vt V e

τ

−

=

The current through the PNPN diode is given by

()

2

BO

2

t

D

V

it e

R

τ

−

=

If we ignore the continuing trickle of current from R1, the time at which iD(t) reaches IH is

()

(

)

(

)

2

22

BO

0.0005 A 1500

ln 0.0015 ln 5.5 ms

30 V

H

IR

tRC

V

Ω

=− =− =

Therefore, the period of the relaxation oscillator is T = 178 ms + 5.5 ms = 183.5 ms, and the frequency of

the relaxation oscillator is f = 1/T = 5.45 Hz.

3-9. In the circuit in Figure P3-4, T1 is an autotransformer with the tap exactly in the center of its winding.

Explain the operation of this circuit. Assuming that the load is inductive, sketch the voltage and current

applied to the load. What is the purpose of SCR2? What is the purpose of D2? (This chopper circuit

arrangement is known as a Jones circuit.)

S

OLUTION First, assume that SCR1 is triggered. When that happens, current will flow from the power

supply through SCR1 and the bottom portion of transformer T1 to the load. At that time, a voltage will be

applied to the bottom part of the transformer which is positive at the top of the winding with respect to the

bottom of the winding. This voltage will induce an equal voltage in the upper part of the autotransformer

78

winding, forward biasing diode D1 and causing the current to flow up through capacitor C. This current

causes C to be charged with a voltage that is positive at its bottom with respect to its top. (This condition

is shown in the figure above.)

Now, assume that SCR2 is triggered. When SCR2 turns ON, capacitor C applies a reverse-biased voltage

to SCR1, shutting it off. Current then flow through the capacitor, SCR2, and the load as shown below.

This current charges C with a voltage of the opposite polarity, as shown.

SCR2 will cut off when the capacitor is fully charged. Alternately, it will be cut off by the voltage across

the capacitor if SCR1 is triggered before it would otherwise cut off.

In this circuit, SCR1 controls the power supplied to the load, while SCR2 controls when SCR1 will be

turned off. Diode D2 in this circuit is a free-wheeling diode, which allows the current in the load to

continue flowing for a short time after SCR1 turns off.

79

3-10. A series-capacitor forced commutation chopper circuit supplying a purely resistive load is shown in Figure

P3-5.

DC 120 VV= 20 kR=Ω

8 mA

H

I=2 LOAD 250 R=Ω

BO 200 VV= 150 FC

µ

=

(a) When SCR1is turned on, how long will it remain on? What causes it to turn off?

(b) When SCR1 turns off, how long will it be until the SCR can be turned on again? (Assume that three

time constants must pass before the capacitor is discharged.)

(c) What problem or problems do these calculations reveal about this simple series-capacitor forced

commutation chopper circuit?

(d) How can the problem(s) described in part (c) be eliminated?

Solution

(a) When the SCR is turned on, it will remain on until the current flowing through it drops below IH.

This happens when the capacitor charges up to a high enough voltage to decrease the current below IH. If

we ignore resistor R (because it is so much larger than RLOAD), the capacitor charges through resistor

RLOAD with a time constant

τ

LOAD = RLOADC = (250 Ω)(150

µ

F) = 0.0375 s. The equation for the voltage

on the capacitor as a function of time during the charging portion of the cycle is

()

LOAD

t

RC

C

vt ABe

−

=+

where A and B are constants depending upon the initial conditions in the circuit. Since vC(0) = 0 V and

vC(∞) = VDC, it is possible to solve for A and B.

A = vC(∞) = VDC

A + B = vC(0) = VDC ⇒ B = -VDC

Therefore,

2 The first printing of this book incorrectly stated that IH is 6 mA.

80

()

LOAD

DC DC V

t

RC

C

vt V V e

−

=−

The current through the capacitor is

() ()

CC

d

it C vt

dt

=

()

LOAD

DC DC

t

RC

C

d

it C V V e

dt

−

=−

()

LOAD

DC

LOAD

A

t

RC

C

V

it e

R

−

=

Solving for time yields

(

)

(

)

22

LOAD

DC DC

ln 0.0375 ln

CC

itR itR

tRC VV

=− =−

The current through the SCR consists of the current through resistor R plus the current through the

capacitor. The current through resistor R is 120 V / 20 kΩ = 6 mA, and the holding current of the SCR is

8 mA, so the SCR will turn off when the current through the capacitor drops to 2 mA. This occurs at time

(

)

(

)

2 mA 250

0.0375 ln 0.206 s

120 V

tΩ

=− =

(b) The SCR can be turned on again once the capacitor has discharged. The capacitor discharges

through resistor R. It can be considered to be completely discharged after three time constants. Since

τ

=

RC = (20 kΩ)(150

µ

F) = 3 s, the SCR will be ready to fire again after 9 s.

(c) In this circuit, the ON time of the SCR is much shorter than the reset time for the SCR, so power can

flow to the load only a very small fraction of the time. (This effect would be less exaggerated if the ratio of

R to RLOAD were smaller.)

(d) This problem can be eliminated by using one of the more complex series commutation circuits

described in Section 3-5. These more complex circuits provide special paths to quickly discharge the

capacitor so that the circuit can be fired again soon.

3-11. A parallel-capacitor forced commutation chopper circuit supplying a purely resistive load is shown in

Figure P3-6.

DC 120 VV= 120 kR=Ω

5 mA

H

I= load 250 R=Ω

81

BO 250 VV= 15 FC

µ

=

(a) When SCR1 is turned on, how long will it remain on? What causes it to turn off?

(b) What is the earliest time that SCR1 can be turned off after it is turned on? (Assume that three time

constants must pass before the capacitor is charged.)

(c) When SCR1 turns off, how long will it be until the SCR can be turned on again?

(d) What problem or problems do these calculations reveal about this simple parallel-capacitor forced

commutation chopper circuit?

(e) How can the problem(s) describe in part (d) be eliminated?

S

OLUTION

(a) When SCR1 is turned on, it will remain on indefinitely until it is forced to turn off. When SCR1 is turned

on, capacitor C charges up to VDC volts with the polarity shown in the figure above. Once it is charged,

SCR1 can be turned off at any time by triggering SCR2. When SCR2 is triggered, the voltage across it

drops instantaneously to about 0 V, which forces the voltage at the anode of SCR1 to be -VDC volts, turning

SCR1 off. (Note that SCR2 will spontaneously turn off after the capacitor discharges, since VDC / R1 < IH

for SCR2.)

(b) If we assume that the capacitor must be fully charged before SCR1 can be forced to turn off, then the time

required would be the time to charge the capacitor. The capacitor charges through resistor R1, and the time

constant for the charging is

τ

= R1C = (20 kΩ)(15

µ

F) = 0.3 s. If we assume that it takes 3 time constants

to fully charge the capacitor, then the time until SCR1 can be turned off is 0.9 s.

(Note that this is not a very realistic assumption. In real life, it is possible to turn off SCR1 with less than a

full VDC volts across the capacitor.)

(c) SCR1 can be turned on again after the capacitor charges up and SCR2 turns off. The capacitor charges

through RLOAD, so the time constant for charging is

τ

= RLOADC = (250 Ω)(15

µ

F) = 0.00375 s

and SCR2 will turn off in a few milliseconds.

(d) In this circuit, once SCR1 fires, a substantial period of time must pass before the power to the load can be

turned off. If the power to the load must be turned on and off rapidly, this circuit could not do the job.

82

(e) This problem can be eliminated by using one of the more complex parallel commutation circuits described

in Section 3-5. These more complex circuits provide special paths to quickly charge the capacitor so that

the circuit can be turned off quickly after it is turned on.

3-12. Figure P3-7 shows a single-phase rectifier-inverter circuit. Explain how this circuit functions. What are

the purposes of C1 and C2? What controls the output frequency of the inverter?

S

OLUTION The last element in the filter of this rectifier circuit is an inductor, which keeps the current flow

out of the rectifier almost constant. Therefore, this circuit is a current source inverter. The rectifier and

filter together produce an approximately constant dc voltage and current across the two SCRs and diodes at

the right of the figure. The applied voltage is positive at the top of the figure with respect to the bottom of

the figure. To understand the behavior of the inverter portion of this circuit, we will step through its

operation.

(1) First, assume that SCR1 and SCR4 are triggered. Then the voltage V will appear across the load

positive-to-negative as shown in Figure (a). At the same time, capacitor C1 will charge to V volts through

diode D3, and capacitor C2 will charge to V volts through diode D2.

(a)

(2) Now, assume that SCR2 and SCR3 are triggered. At the instant they are triggered, the voltage across

capacitors C1 and C2 will reverse bias SCR1 and SCR4, turning them OFF. Then a voltage of V volts will

appear across the load positive-to-negative as shown in Figure (b). At the same time, capacitor C1 will

charge to V volts with the opposite polarity from before, and capacitor C2 will charge to V volts with the

opposite polarity from before.

83

Figure (b)

(3) If SCR1 and SCR4 are now triggered again, the voltages across capacitors C1 and C2 will force

SCR2 and SCR3 to turn OFF. The cycle continues in this fashion.

Capacitors C1 and C2 are called commutating capacitors. Their purpose is to force one set of SCRs to turn

OFF when the other set turns ON.

The output frequency of this rectifier-inverter circuit is controlled by the rates at which the SCRs are

triggered. The resulting voltage and current waveforms (assuming a resistive load) are shown below.

3-13. A simple full-wave ac phase angle voltage controller is shown in Figure P3-8. The component values in

this circuit are:

R = 20 to 300 kΩ, currently set to 80 kΩ

C = 0.15

µ

F

84

BO

V = 40 V (for PNPN Diode D1)

BO

V = 250 V (for SCR1)

( ) sin volts

sM

vt V t

ω

=

where

M

V = 169.7 V and

ω

= 377 rad/s

(a) At what phase angle do the PNPN diode and the SCR turn on?

(b) What is the rms voltage supplied to the load under these circumstances?

Note: Problem 3-13 is significantly harder for many students, since it involves

solving a differential equation with a forcing function. This problem should

only be assigned if the class has the mathematical sophistication to handle it.

S

OLUTION At the beginning of each half cycle, the voltages across the PNPN diode and the SCR will both be

smaller then their respective breakover voltages, so no current will flow to the load (except for the very tiny

current charging capacitor C), and vload(t) will be 0 volts. However, capacitor C charges up through

resistor R, and when the voltage vC(t) builds up to the breakover voltage of D1, the PNPN diode will start to

conduct. This current flows through the gate of SCR1, turning the SCR ON. When it turns ON, the

voltage across the SCR will drop to 0, and the full source voltage vS(t) will be applied to the load,

producing a current flow through the load. The SCR continues to conduct until the current through it falls

below IH, which happens at the very end of the half cycle.

Note that after D1 turns on, capacitor C discharges through it and the gate of the SCR. At the end of the

half cycle, the voltage on the capacitor is again essentially 0 volts, and the whole process is ready to start

over again at the beginning of the next half cycle.

To determine when the PNPN diode and the SCR fire in this circuit, we must determine when vC(t) exceeds

VBO for D1. This calculation is much harder than in the examples in the book, because in the previous

problems the source was a simple DC voltage source, while here the voltage source is sinusoidal. However,

the principles are identical.

(a) To determine when the SCR will turn ON, we must calculate the voltage vC(t), and then solve for the time

at which vC(t) exceeds VBO for D1. At the beginning of the half cycle, D1 and SCR1 are OFF, and the

voltage across the load is essentially 0, so the entire source voltage vS(t) is applied to the series RC circuit.

To determine the voltage vC(t) on the capacitor, we can write a Kirchhoff's Current Law equation at the

node above the capacitor and solve the resulting equation for vC(t).

12

0ii+= (since the PNPN diode is an open circuit at this time)

85

10

C

vv d

Cv

Rdt

−+=

1

11

CC

dvv v

dt RC RC

+=

1sin

M

CC

dV

vv t

dt RC RC

ω

+=

The solution can be divided into two parts, a natural response and a forced response. The natural response

is the solution to the equation

10

CC

dvv

dt RC

+=

The solution to the natural response equation is

()

, e

t

RC

Cn

vtA

−

=

where the constant A must be determined from the initial conditions in the system. The forced response is

the steady-state solution to the equation

1sin

M

CC

dV

vv t

dt RC RC

ω

+=

It must have a form similar to the forcing function, so the solution will be of the form

(

)

,1 2

sin cos

Cf

vtB tB t

ωω

=+

where the constants 1

B and 2

B must be determined by substitution into the original equation. Solving for

1

B and 2

B yields:

()()

12 12

1

sin cos sin cos sin

M

dV

BtBt BtBt t

dt RC RC

ωω ωω ω

++ +=

()()

12 12

1

cos in sin cos sin

M

V

BtBst B tB t t

RC RC

ωωω ω ω ω ω

−+ +=

cosine equation:

12

10BB

RC

ω

+= ⇒ 21

BRCB

ω

=−

sine equation:

21

1M

V

BB

RC RC

ω

−+ =

2

11

1

M

V

RC B B

RC RC

ω

+=

2

1

M

V

RC B

RC RC

ω

+=

222

1

1M

RC V

B

RC RC

ω

+=

Finally,

86

1222

1

M

V

BRC

ω

=+ and 2222

1

M

RC V

BRC

ω

−

=+

Therefore, the forced solution to the equation is

()

,222 222

sin cos

11

MM

Cf

VRCV

vt t t

RC RC

ω

ωω

=−

++

and the total solution is

(

)

(

)

(

)

,,CCnCf

vt v t v t=+

()

222 222

sin cos

11

t

MM

RC

C

VRCV

vt Ae t t

RC RC

ω

ωω

−

=+ −

++

The initial condition for this problem is that the voltage on the capacitor is zero at the beginning of the half-

cycle:

()

0

222 222

0 sin 0 cos 00

11

MM

RC

C

VRCV

vAe RC RC

ω

ωω

−

=+ − =

++

22 2

0

1

M

RC V

ARC

ω

−=

+

222

1

M

RC V

ARC

ω

=+

Therefore, the voltage across the capacitor as a function of time before the PNPN diode fires is

()

222 222 222

sin cos

11 1

t

MM M

RC

C

RC V V RC V

vt e t t

RC RC RC

ωω

ωω ω

−

=+ −

++ +

If we substitute the known values for R, C, ω, and VM, this equation becomes

(

)

83.3

35.76 7.91 sin 35.76 cos

t

C

vt e t t

ωω

−

=+−

This equation is plotted below:

87

It reaches a voltage of 40 V at a time of 4.8 ms. Since the frequency of the waveform is 60 Hz, the

waveform there are 360° in 1/60 s, and the firing angle α is

()

360

4.8 ms 103.7

1/ 60 s

α

°

==°

or 1.810 radians

Note: This problem could also have been solved using Laplace Transforms, if desired.

(b) The rms voltage applied to the load is

/

222

rms

1() sin

M

VvtdtVtdt

T

πω

α

ωω

π

==

/

2

rms

11

sin 2

24

M

V

Vtt

πω

α

ωω

π

=−

()()











−−−=

απαπ

π

2sin2sin

4

1

2

1

2

rms

M

V

Since

α

= 1.180 radians, the rms voltage is

rms 0.1753 0.419 71.0 V

MM

VV V===

3-14. Figure P3-9 shows a three-phase full-wave rectifier circuit supplying power to a dc load. The circuit uses

SCRs instead of diodes as the rectifying elements.

(a) What will the load voltage and ripple be if each SCR is triggered as soon as it becomes forward biased?

At what phase angle should the SCRs be triggered in order to operate this way? Sketch or plot the

output voltage for this case.

(b) What will the rms load voltage and ripple be if each SCR is triggered at a phase angle of 90° (that is,

half way through the half-cycle in which it is forward biased)? Sketch or plot the output voltage for

this case.

88

S

OLUTION Assume that the three voltages applied to this circuit are:

(

)

sin

AM

vt V t

ω

=

(

)

(

)

sin 2 / 3

BM

vt V t

ωπ

=−

(

)

(

)

sin 2 / 3

CM

vt V t

ωπ

=+

The period of the input waveforms is T, where 2/T

πω

=. For the purpose of the calculations in this

problem, we will assume that

ω

is 377 rad/s (60 Hz).

(a) The when the SCRs start to conduct as soon as they are forward biased, this circuit is just a three-

phase full-wave bridge, and the output voltage is identical to that in Problem 3-2. The sketch of output

voltage is reproduced below, and the ripple is 4.2%. The following table shows which SCRs must conduct

in what order to create the output voltage shown below. The times are expressed as multiples of the period

T of the input waveforms, and the firing angle is in degrees relative to time zero.

Start Time

(

ω

t)

Stop Time

(

ω

t)

Positive

Phase

Negative

Phase

Conducting

SCR

(Positive)

Conducting

SCR

(Negative)

Triggered

SCR

Firing

Angle

12/T− 12/T c b SCR3 SCR5 SCR5 -30°

12/T 12/3T a b SCR1 SCR5 SCR1 30°

12/3T 12/5T a c SCR1 SCR6 SCR6 90°

12/5T 12/7T b c SCR2 SCR6 SCR2 150°

12/7T 12/9T b a SCR2 SCR4 SCR4 210°

12/9T 12/11T c a SCR3 SCR4 SCR3 270°

12/11T 12/T c b SCR3 SCR5 SCR5 330°

89

T

/12

(b) If each SCR is triggered halfway through the half-cycle during which it is forward biased, the

resulting phase a, b, and c voltages will be zero before the first half of each half-cycle, and the full

sinusoidal value for the second half of each half-cycle. These waveforms are shown below. (These plots

were created by the MATLAB program that appears later in this answer.)

and the resulting output voltage will be:

90

A MATLAB program to generate these waveforms and to calculate the ripple on the output waveform is

shown below. The first function biphase_controller.m generates a switched ac waveform. The

inputs to this function are the current phase angle in degrees, the offset angle of the waveform in degrees,

and the firing angle in degrees.

function volts = biphase_controller(wt,theta0,fire)

% Function to simulate the output of an ac phase

% angle controller that operates symmetrically on

% positive and negative half cycles. Assume a peak

% voltage VM = 120 * SQRT(2) = 170 V for convenience.

%

% wt = Current phase in degrees

% theta0 = Starting phase angle in degrees

% fire = Firing angle in degrees

% Degrees to radians conversion factor

deg2rad = pi / 180;

% Remove phase ambiguities: 0 <= wt < 360 deg

ang = wt + theta0;

while ang >= 360

ang = ang - 360;

end

while ang < 0

ang = ang + 360;

end

% Simulate the output of the phase angle controller.

if (ang >= fire & ang <= 180)

volts = 170 * sin(ang * deg2rad);

elseif (ang >= (fire+180) & ang <= 360)

volts = 170 * sin(ang * deg2rad);

else

91

volts = 0;

end

The main program below creates and plots the three-phase waveforms, calculates and plots the output

waveform, and determines the ripple in the output waveform.

% M-file: prob3_14b.m

% M-file to calculate and plot the three phase voltages

% when each SCR in a three-phase full-wave rectifier

% triggers at a phase angle of 90 degrees.

% Calculate the waveforms for times from 0 to 1/30 s

t = (0:1/21600:1/30);

deg = zeros(size(t));

rms = zeros(size(t));

va = zeros(size(t));

vb = zeros(size(t));

vc = zeros(size(t));

out = zeros(size(t));

for ii = 1:length(t)

% Get equivalent angle in degrees. Note that

% 1/60 s = 360 degrees for a 60 Hz waveform!

theta = 21600 * t(ii);

% Calculate the voltage in each phase at each

% angle.

va(ii) = biphase_controller(theta,0,90);

vb(ii) = biphase_controller(theta,-120,90);

vc(ii) = biphase_controller(theta,120,90);

end

% Calculate the output voltage of the rectifier

for ii = 1:length(t)

vals = [ va(ii) vb(ii) vc(ii) ];

out(ii) = max( vals ) - min( vals );

end

% Calculate and display the ripple

disp( ['The ripple is ' num2str(ripple(out))] );

% Plot the voltages versus time

figure(1)

plot(t,va,'b','Linewidth',2.0);

hold on;

plot(t,vb,'r:','Linewidth',2.0);

plot(t,vc,'k--','Linewidth',2.0);

title('\bfPhase Voltages');

xlabel('\bfTime (s)');

ylabel('\bfVoltage (V)');

grid on;

legend('Phase a','Phase b','Phase c');

hold off;

92

% Plot the output voltages versus time

figure(2)

plot(t,out,'b','Linewidth',2.0);

title('\bfOutput Voltage');

xlabel('\bfTime (s)');

ylabel('\bfVoltage (V)');

axis( [0 1/30 0 260]);

grid on;

hold off;

When this program is executed, the results are:

» prob_3_14b

The ripple is 30.9547

3-15. Write a MATLAB program that imitates the operation of the Pulse-Width Modulation circuit shown in

Figure 3-55, and answer the following questions.

(a) Assume that the comparison voltages vt

x()

and vt

y()

have peak amplitudes of 10 V and a frequency

of 500 Hz. Plot the output voltage when the input voltage is vt ft

in () sin=10 2π V, and f = 50 Hz.

(b) What does the spectrum of the output voltage look like? What could be done to reduce the harmonic

content of the output voltage?

(c) Now assume that the frequency of the comparison voltages is increased to 1000 Hz. Plot the output

voltage when the input voltage is vt ft

in () sin=10 2π V, and f = 50 Hz.

(d) What does the spectrum of the output voltage in (c) look like?

(e) What is the advantage of using a higher comparison frequency and more rapid switching in a PWM

modulator?

S

OLUTION The PWM circuit is shown below:

93

(a) To write a MATLAB simulator of this circuit, note that if in

v > x

v, then u

v = DC

V, and if in

v < x

v,

then u

v = 0. Similarly, if in

v > y

v, then v

v = 0, and if in

v < y

v, then v

v = DC

V. The output voltage is

then uv vvv −=

out . A MATLAB function that performs these calculations is shown below. (Note that

this function arbitrarily assumes that DC

V = 100 V. It would be easy to modify the function to use any

arbitrary dc voltage, if desired.)

function [vout,vu,vv] = vout(vin, vx, vy)

% Function to calculate the output voltage of a

% PWM modulator from the values of vin and the

% reference voltages vx and vy. This function

% arbitrarily assumes that VDC = 100 V.

%

% vin = Input voltage

% vx = x reference

% vy = y reference

% vout = Ouput voltage

% vu, vv = Components of output voltage

94

% fire = Firing angle in degrees

% vu

if ( vin > vx )

vu = 100;

else

vu = 0;

end

% vv

if ( vin >= vy )

vv = 0;

else

vv = 100;

end

% Caclulate vout

vout = vv - vu;

Now we need a MATLAB program to generate the input voltage

()

tvin and the reference voltages

()

tvx

and

()

tvy. After the voltages are generated, function vout will be used to calculate

()

tvout and the

frequency spectrum of

()

tvout . Finally, the program will plot

()

tvin ,

()

tvx and

()

tv y,

()

tvout , and the

spectrum of

()

tvout . (Note that in order to have a valid spectrum, we need to create several cycles of the

60 Hz output waveform, and we need to sample the data at a fairly high frequency. This problem creates 4

cycles of

()

tvout and samples all data at a 20,000 Hz rate.)

% M-file: prob3_15a.m

% M-file to calculate the output voltage from a PWM

% modulator with a 500 Hz reference frequency. Note

% that the only change between this program and that

% of part b is the frequency of the reference "fr".

% Sample the data at 20000 Hz to get enough information

% for spectral analysis. Declare arrays.

fs = 20000; % Sampling frequency (Hz)

t = (0:1/fs:4/15); % Time in seconds

vx = zeros(size(t)); % vx

vy = zeros(size(t)); % vy

vin = zeros(size(t)); % Driving signal

vu = zeros(size(t)); % vx

vv = zeros(size(t)); % vy

vout = zeros(size(t)); % Output signal

fr = 500; % Frequency of reference signal

T = 1/fr; % Period of refernce signal

% Calculate vx at fr = 500 Hz.

for ii = 1:length(t)

vx(ii) = vref(t(ii),T);

vy(ii) = - vx(ii);

end

% Calculate vin as a 50 Hz sine wave with a peak voltage of

95

% 10 V.

for ii = 1:length(t)

vin(ii) = 10 * sin(2*pi*50*t(ii));

end

% Now calculate vout

for ii = 1:length(t)

[vout(ii) vu(ii) vv(ii)] = vout(vin(ii), vx(ii), vy(ii));

end

% Plot the reference voltages vs time

figure(1)

plot(t,vx,'b','Linewidth',1.0);

hold on;

plot(t,vy,'k--','Linewidth',1.0);

title('\bfReference Voltages for fr = 500 Hz');

xlabel('\bfTime (s)');

ylabel('\bfVoltage (V)');

legend('vx','vy');

axis( [0 1/30 -10 10]);

hold off;

% Plot the input voltage vs time

figure(2)

plot(t,vin,'b','Linewidth',1.0);

title('\bfInput Voltage');

xlabel('\bfTime (s)');

ylabel('\bfVoltage (V)');

axis( [0 1/30 -10 10]);

% Plot the output voltages versus time

figure(3)

plot(t,vout,'b','Linewidth',1.0);

title('\bfOutput Voltage for fr = 500 Hz');

xlabel('\bfTime (s)');

ylabel('\bfVoltage (V)');

axis( [0 1/30 -120 120]);

% Now calculate the spectrum of the output voltage

spec = fft(vout);

% Calculate sampling frequency labels

len = length(t);

df = fs / len;

fstep = zeros(size(t));

for ii = 2:len/2

fstep(ii) = df * (ii-1);

fstep(len-ii+2) = -fstep(ii);

end

% Plot the spectrum

figure(4);

plot(fftshift(fstep),fftshift(abs(spec)));

title('\bfSpectrum of Output Voltage for fr = 500 Hz');

xlabel('\bfFrequency (Hz)');

ylabel('\bfAmplitude');

96

% Plot a closeup of the near spectrum

% (positive side only)

figure(5);

plot(fftshift(fstep),fftshift(abs(spec)));

title('\bfSpectrum of Output Voltage for fr = 500 Hz');

xlabel('\bfFrequency (Hz)');

ylabel('\bfAmplitude');

set(gca,'Xlim',[0 1000]);

When this program is executed, the input, reference, and output voltages are:

97

(b) The output spectrum of this PWM modulator is shown below. There are two plots here, one showing

the entire spectrum, and the other one showing the close-in frequencies (those under 1000 Hz), which will

have the most effect on machinery. Note that there is a sharp peak at 50 Hz, which is there desired

frequency, but there are also strong contaminating signals at about 850 Hz and 950 Hz. If necessary, these

components could be filtered out using a low-pass filter.

98

(c) A version of the program with 1000 Hz reference functions is shown below:

% M-file: prob3_15b.m

% M-file to calculate the output voltage from a PWM

% modulator with a 1000 Hz reference frequency. Note

% that the only change between this program and that

% of part a is the frequency of the reference "fr".

% Sample the data at 20000 Hz to get enough information

% for spectral analysis. Declare arrays.

fs = 20000; % Sampling frequency (Hz)

t = (0:1/fs:4/15); % Time in seconds

vx = zeros(size(t)); % vx

vy = zeros(size(t)); % vy

vin = zeros(size(t)); % Driving signal

vu = zeros(size(t)); % vx

vv = zeros(size(t)); % vy

vout = zeros(size(t)); % Output signal

fr = 1000; % Frequency of reference signal

T = 1/fr; % Period of refernce signal

% Calculate vx at 1000 Hz.

for ii = 1:length(t)

vx(ii) = vref(t(ii),T);

vy(ii) = - vx(ii);

end

% Calculate vin as a 50 Hz sine wave with a peak voltage of

% 10 V.

for ii = 1:length(t)

vin(ii) = 10 * sin(2*pi*50*t(ii));

end

99

% Now calculate vout

for ii = 1:length(t)

[vout(ii) vu(ii) vv(ii)] = vout(vin(ii), vx(ii), vy(ii));

end

% Plot the reference voltages vs time

figure(1)

plot(t,vx,'b','Linewidth',1.0);

hold on;

plot(t,vy,'k--','Linewidth',1.0);

title('\bfReference Voltages for fr = 1000 Hz');

xlabel('\bfTime (s)');

ylabel('\bfVoltage (V)');

legend('vx','vy');

axis( [0 1/30 -10 10]);

hold off;

% Plot the input voltage vs time

figure(2)

plot(t,vin,'b','Linewidth',1.0);

title('\bfInput Voltage');

xlabel('\bfTime (s)');

ylabel('\bfVoltage (V)');

axis( [0 1/30 -10 10]);

% Plot the output voltages versus time

figure(3)

plot(t,vout,'b','Linewidth',1.0);

title('\bfOutput Voltage for fr = 1000 Hz');

xlabel('\bfTime (s)');

ylabel('\bfVoltage (V)');

axis( [0 1/30 -120 120]);

% Now calculate the spectrum of the output voltage

spec = fft(vout);

% Calculate sampling frequency labels

len = length(t);

df = fs / len;

fstep = zeros(size(t));

for ii = 2:len/2

fstep(ii) = df * (ii-1);

fstep(len-ii+2) = -fstep(ii);

end

% Plot the spectrum

figure(4);

plot(fftshift(fstep),fftshift(abs(spec)));

title('\bfSpectrum of Output Voltage for fr = 1000 Hz');

xlabel('\bfFrequency (Hz)');

ylabel('\bfAmplitude');

% Plot a closeup of the near spectrum

% (positive side only)

figure(5);

plot(fftshift(fstep),fftshift(abs(spec)));

100

title('\bfSpectrum of Output Voltage for fr = 1000 Hz');

xlabel('\bfFrequency (Hz)');

ylabel('\bfAmplitude');

set(gca,'Xlim',[0 1000]);

When this program is executed, the input, reference, and output voltages are:

101

(d) The output spectrum of this PWM modulator is shown below.

102

(e) Comparing the spectra in (b) and (d), we can see that the frequencies of the first large sidelobes

doubled from about 900 Hz to about 1800 Hz when the reference frequency was doubled. This increase in

sidelobe frequency has two major advantages: it makes the harmonics easier to filter, and it also makes it

less necessary to filter them at all. Since large machines have their own internal inductances, they form

natural low-pass filters. If the contaminating sidelobes are at high enough frequencies, they will never

affect the operation of the machine. Thus, it is a good idea to design PWM modulators with a high

frequency reference signal and rapid switching.

103

Chapter 4: AC Machinery Fundamentals

4-1. The simple loop is rotating in a uniform magnetic field shown in Figure 4-1 has the following

characteristics:

B=05. T to the right

r

=01.m

l=05. m

ω

= 103 rad/s

(a) Calculate the voltage et

tot ()induced in this rotating loop.

(b) Suppose that a 5 Ω resistor is connected as a load across the terminals of the loop. Calculate the

current that would flow through the resistor.

(c) Calculate the magnitude and direction of the induced torque on the loop for the conditions in (b).

(d) Calculate the electric power being generated by the loop for the conditions in (b).

(e) Calculate the mechanical power being consumed by the loop for the conditions in (b). How does

this number compare to the amount of electric power being generated by the loop?

ω

m

r

v

ab

v

cd

B

NS

B

is a uniform magnetic

field, aligned as shown.

a

b

c

d

S

OLUTION

(a) The induced voltage on a simple rotating loop is given by

(

)

ind 2 sin et rBl t

ω

=ω (4-8)

() ( )( )( )( )

ind 2 0.1 m 103 rad/s 0.5 T 0.5 m sin103et t=

()

ind 5.15 sin103 Vet t=

(b) If a 5 Ω resistor is connected as a load across the terminals of the loop, the current flow would be:

()

ind 5.15 sin 103 V 1.03 sin 103 A

5

et

it t

R

== =

Ω

(c) The induced torque would be:

()

ind 2 sin trilΒ

τθ

= (4-17)

() ( )( )( )( )

ind 2 0.1 m 1.03 sin A 0.5 m 0.5 T sin tt t

τωω

=

()

2

ind 0.0515 sin N m, counterclockwisett

τω

=⋅

(d) The instantaneous power generated by the loop is:

104

() ( )( )

2

ind 5.15 sin V 1.03 sin A 5.30 sin WPt e i t t t

ωω ω

== =

The average power generated by the loop is

2

ave

15.30 sin 2.65 W

T

Ptdt

T

ω

==

(e) The mechanical power being consumed by the loop is:

()

22

ind 0.0515 sin V 103 rad/s 5.30 sin WPt t

τω ω ω

== =

Note that the amount of mechanical power consumed by the loop is equal to the amount of electrical power

created by the loop. This machine is acting as a generator, converting mechanical power into electrical

power.

4-2. Develop a table showing the speed of magnetic field rotation in ac machines of 2, 4, 6, 8, 10, 12, and 14

poles operating at frequencies of 50, 60, and 400 Hz.

S

OLUTION The equation relating the speed of magnetic field rotation to the number of poles and electrical

frequency is

120 e

m

f

nP

=

The resulting table is

Number of Poles e

f = 50 Hz e

f = 60 Hz e

f = 400 Hz

2 3000 r/min 3600 r/min 24000 r/min

4 1500 r/min 1800 r/min 12000 r/min

6 1000 r/min 1200 r/min 8000 r/min

8 750 r/min 900 r/min 6000 r/min

10 600 r/min 720 r/min 4800 r/min

12 500 r/min 600 r/min 4000 r/min

14 428.6 r/min 514.3 r/min 3429 r/min

4-3. A three-phase four-pole winding is installed in 12 slots on a stator. There are 40 turns of wire in each slot

of the windings. All coils in each phase are connected in series, and the three phases are connected in ∆.

The flux per pole in the machine is 0.060 Wb, and the speed of rotation of the magnetic field is 1800 r/min.

(a) What is the frequency of the voltage produced in this winding?

(b) What are the resulting phase and terminal voltages of this stator?

S

OLUTION

(a) The frequency of the voltage produced in this winding is

()()

1800 r/min 4 poles 60 Hz

120 120

m

e

nP

f== =

(b) There are 12 slots on this stator, with 40 turns of wire per slot. Since this is a four-pole machine,

there are two sets of coils (4 slots) associated with each phase. The voltage in the coils in one pair of slots

is

()( )( )

2 2 40 t 0.060 Wb 60 Hz 640 V

AC

ENf

πφ π

== =

There are two sets of coils per phase, since this is a four-pole machine, and they are connected in series, so

the total phase voltage is

105

(

)

2 640 V 1280 VV

φ

==

Since the machine is ∆-connected, 1280 V

L

VV

φ

== .

4-4. A three-phase Y-connected 50-Hz two-pole synchronous machine has a stator with 2000 turns of wire per

phase. What rotor flux would be required to produce a terminal (line-to-line) voltage of 6 kV?

S

OLUTION The phase voltage of this machine should be

/

33464 V

L

VV

φ

== . The induced voltage per

phase in this machine (which is equal to

φ

V at no-load conditions) is given by the equation

2

AC

ENf

πφ

=

so

()()

3464 V 0.0078 Wb

2 2 2000 t 50 Hz

A

C

E

Nf

φππ

== =

4-5. Modify the MATLAB program in Example 4-1 by swapping the currents flowing in any two phases. What

happens to the resulting net magnetic field?

S

OLUTION This modification is very simple—just swap the currents supplied to two of the three phases.

% M-file: mag_field2.m

% M-file to calculate the net magetic field produced

% by a three-phase stator.

% Set up the basic conditions

bmax = 1; % Normalize bmax to 1

freq = 60; % 60 Hz

w = 2*pi*freq; % angluar velocity (rad/s)

% First, generate the three component magnetic fields

t = 0:1/6000:1/60;

Baa = sin(w*t) .* (cos(0) + j*sin(0));

Bbb = sin(w*t+2*pi/3) .* (cos(2*pi/3) + j*sin(2*pi/3));

Bcc = sin(w*t-2*pi/3) .* (cos(-2*pi/3) + j*sin(-2*pi/3));

% Calculate Bnet

Bnet = Baa + Bbb + Bcc;

% Calculate a circle representing the expected maximum

% value of Bnet

circle = 1.5 * (cos(w*t) + j*sin(w*t));

% Plot the magnitude and direction of the resulting magnetic

% fields. Note that Baa is black, Bbb is blue, Bcc is

% magneta, and Bnet is red.

for ii = 1:length(t)

% Plot the reference circle

plot(circle,'k');

hold on;

% Plot the four magnetic fields

plot([0 real(Baa(ii))],[0 imag(Baa(ii))],'k','LineWidth',2);

plot([0 real(Bbb(ii))],[0 imag(Bbb(ii))],'b','LineWidth',2);

106

plot([0 real(Bcc(ii))],[0 imag(Bcc(ii))],'m','LineWidth',2);

plot([0 real(Bnet(ii))],[0 imag(Bnet(ii))],'r','LineWidth',3);

axis square;

axis([-2 2 -2 2]);

drawnow;

hold off;

end

When this program executes, the net magnetic field rotates clockwise, instead of counterclockwise.

4-6. If an ac machine has the rotor and stator magnetic fields shown in Figure P4-1, what is the direction of the

induced torque in the machine? Is the machine acting as a motor or generator?

S

OLUTION Since ind netR

k=×τBB

, the induced torque is clockwise, opposite the direction of motion. The

machine is acting as a generator.

4-7. The flux density distribution over the surface of a two-pole stator of radius r and length l is given by

()

cos

Mm

BB t=ω−α (4-37b)

Prove that the total flux under each pole face is

2M

rlB

φ

=

107

S

OLUTION The total flux under a pole face is given by the equation

d

φ

=⋅

BA

Under a pole face, the flux density B is always parallel to the vector dA, since the flux density is always

perpendicular to the surface of the rotor and stator in the air gap. Therefore,

BdA

φ

=

A differential area on the surface of a cylinder is given by the differential length along the cylinder (dl)

times the differential width around the radius of the cylinder (

θ

rd ).

()( )

dA dl rd

θ

= where r is the radius of the cylinder

Therefore, the flux under the pole face is

Bdl rd

φ

θ

=

Since r is constant and B is constant with respect to l, this equation reduces to

rl B d

φ

θ

=

Now,

()

cos cos

MM

BB t B

ωα θ

=−= (when we substitute t

θω α

=−), so

rl B d

φ

θ

=

[]

()

/2

/2 /2

cos sin 1 1

MM M

rl B d rlB rlB

π

ππ

φθθθ

−−

===

2

M

rlB

φ

=

108

4-8. In the early days of ac motor development, machine designers had great difficulty controlling the core losses

(hysteresis and eddy currents) in machines. They had not yet developed steels with low hysteresis, and

were not making laminations as thin as the ones used today. To help control these losses, early ac motors

in the USA were run from a 25 Hz ac power supply, while lighting systems were run from a separate 60 Hz

ac power supply.

(a) Develop a table showing the speed of magnetic field rotation in ac machines of 2, 4, 6, 8, 10, 12, and

14 poles operating at 25 Hz. What was the fastest rotational speed available to these early motors?

(b) For a given motor operating at a constant flux density B, how would the core losses of the motor

running at 25 Hz compare to the core losses of the motor running at 60 Hz?

(c) Why did the early engineers provide a separate 60 Hz power system for lighting?

S

OLUTION

(a) The equation relating the speed of magnetic field rotation to the number of poles and electrical

frequency is

120 e

m

f

nP

=

The resulting table is

Number of Poles e

f = 25 Hz

2 1500 r/min

4 750 r/min

6 500 r/min

8 375 r/min

10 300 r/min

12 250 r/min

14 214.3 r/min

The highest possible rotational speed was 1500 r/min.

(b) Core losses scale according to the 1.5th power of the speed of rotation, so the ratio of the core losses

at 25 Hz to the core losses at 60 Hz (for a given machine) would be:

1.5

1500

ratio 0.269

3600

==

or 26.9%

(c) At 25 Hz, the light from incandescent lamps would visibly flicker in a very annoying way.

109

Chapter 5: Synchronous Generators

5-1. At a location in Europe, it is necessary to supply 300 kW of 60-Hz power. The only power sources

available operate at 50 Hz. It is decided to generate the power by means of a motor-generator set

consisting of a synchronous motor driving a synchronous generator. How many poles should each of the

two machines have in order to convert 50-Hz power to 60-Hz power?

S

OLUTION The speed of a synchronous machine is related to its frequency by the equation

120 e

m

f

nP

=

To make a 50 Hz and a 60 Hz machine have the same mechanical speed so that they can be coupled

together, we see that

()()

sync

12

120 50 Hz 120 60 Hz

nPP

==

2

1

612

510

P

P==

Therefore, a 10-pole synchronous motor must be coupled to a 12-pole synchronous generator to accomplish

this frequency conversion.

5-2. A 2300-V 1000-kVA 0.8-PF-lagging 60-Hz two-pole Y-connected synchronous generator has a

synchronous reactance of 1.1 Ω and an armature resistance of 0.15 Ω. At 60 Hz, its friction and windage

losses are 24 kW, and its core losses are 18 kW. The field circuit has a dc voltage of 200 V, and the

maximum IF is 10 A. The resistance of the field circuit is adjustable over the range from 20 to 200 Ω.

The OCC of this generator is shown in Figure P5-1.

(a) How much field current is required to make VT equal to 2300 V when the generator is running at no

load?

(b) What is the internal generated voltage of this machine at rated conditions?

(c) How much field current is required to make VT equal to 2300 V when the generator is running at rated

conditions?

(d) How much power and torque must the generator’s prime mover be capable of supplying?

(e) Construct a capability curve for this generator.

Note: An electronic version of this open circuit characteristic can be found in file

p51_occ.dat, which can be used with MATLAB programs. Column 1

contains field current in amps, and column 2 contains open-circuit terminal

voltage in volts.

110

S

OLUTION

(a) If the no-load terminal voltage is 2300 V, the required field current can be read directly from the

open-circuit characteristic. It is 4.25 A.

(b) This generator is Y-connected, so AL II =. At rated conditions, the line and phase current in this

generator is

()

1000 kVA 251 A

3 3 2300 V

AL

L

P

II V

== = = at an angle of –36.87°

The phase voltage of this machine is

/

31328 V

T

VV

φ

==. The internal generated voltage of the machine

is

AAASA

RjX

φ

=+ +EV I I

()()()()

1328 0 0.15 251 36.87 A 1.1 251 36.87 A

Aj=∠°+ Ω∠− °+ Ω∠− °E

1537 7.4 V

A=∠°E

(c) The equivalent open-circuit terminal voltage corresponding to an A

E of 1537 volts is

()

,oc 3 1527 V 2662 V

T

V==

From the OCC, the required field current is 5.9 A.

(d) The input power to this generator is equal to the output power plus losses. The rated output power is

()()

OUT 1000 kVA 0.8 800 kWP==

(

)

(

)

2

CU 3 3 251 A 0.15 28.4 kW

AA

PIR== Ω=

F&W 24 kWP=

111

core 18 kWP=

stray (assumed 0)P=

IN OUT CU F&W core stray 870.4 kWPP PP P P=++++=

Therefore the prime mover must be capable of supplying 175 kW. Since the generator is a two-pole 60 Hz

machine, to must be turning at 3600 r/min. The required torque is

()

mN 465

r 1

rad 2

s 60

min 1

r/min 3600

kW 2.175

IN

APP ⋅=

























==

π

ω

τ

m

P

(e) The rotor current limit of the capability curve would be drawn from an origin of

()

2

33 1328 V 4810 kVAR

1.1

S

V

QX

φ

=− =− =−

Ω

The radius of the rotor current limit is

()()

33 1328 V 1537 V 5567 kVA

1.1

A

E

S

VE

DX

φ

== =

Ω

The stator current limit is a circle at the origin of radius

(

)

(

)

3 3 1328 V 251 A 1000 kVA

A

SVI

φ

== =

A MATLAB program that plots this capability diagram is shown below:

% M-file: prob5_2.m

% M-file to display a capability curve for a

% synchronous generator.

% Calculate the waveforms for times from 0 to 1/30 s

Q = -4810;

DE = 5567;

S = 1000;

% Get points for stator current limit

theta = -95:1:95; % Angle in degrees

rad = theta * pi / 180; % Angle in radians

s_curve = S .* ( cos(rad) + j*sin(rad) );

% Get points for rotor current limit

orig = j*Q;

theta = 75:1:105; % Angle in degrees

rad = theta * pi / 180; % Angle in radians

r_curve = orig + DE .* ( cos(rad) + j*sin(rad) );

% Plot the capability diagram

figure(1);

plot(real(s_curve),imag(s_curve),'b','LineWidth',2.0);

hold on;

plot(real(r_curve),imag(r_curve),'r--','LineWidth',2.0);

% Add x and y axes

112

plot( [-1500 1500],[0 0],'k');

plot( [0,0],[-1500 1500],'k');

% Set titles and axes

title ('\bfSynchronous Generator Capability Diagram');

xlabel('\bfPower (kW)');

ylabel('\bfReactive Power (kVAR)');

axis( [ -1500 1500 -1500 1500] );

axis square;

hold off;

The resulting capability diagram is shown below:

5-3. Assume that the field current of the generator in Problem 5-2 has been adjusted to a value of 4.5 A.

(a) What will the terminal voltage of this generator be if it is connected to a ∆-connected load with an

impedance of 20 30 ∠°Ω

?

(b) Sketch the phasor diagram of this generator.

(c) What is the efficiency of the generator at these conditions?

(d) Now assume that another identical ∆-connected load is to be paralleled with the first one. What

happens to the phasor diagram for the generator?

(e) What is the new terminal voltage after the load has been added?

(f) What must be done to restore the terminal voltage to its original value?

S

OLUTION

(a) If the field current is 4.5 A, the open-circuit terminal voltage will be about 2385 V, and the phase

voltage in the generator will be 2385 / 3 1377 V=.

The load is ∆-connected with three impedances of 20 30 ∠°Ω. From the Y-∆ transform, this load is

equivalent to a Y-connected load with three impedances of 6.667 30 ∠°Ω

. The resulting per-phase

equivalent circuit is shown below:

113

+

-

EA

0.15 Ωj1.1 Ω

6.667∠30°Z

+

-

V

φ

IA

The magnitude of the phase current flowing in this generator is

1377 V 1377 V 186 A

0.15 1.1 6.667 30 1.829

A

AS

E

IRjXZ j

== ==

++ ++ ∠° Ω

Therefore, the magnitude of the phase voltage is

()( )

186 A 6.667 1240 V

A

VIZ

φ

== Ω=

and the terminal voltage is

()

3 3 1240 V 2148 V

T

VV

φ

== =

(b) Armature current is 186 30 A

A=∠−°I, and the phase voltage is 1240 0 V

φ

=∠°V. Therefore, the

internal generated voltage is

AAASA

RjX

φ

=+ +EV I I

(

)

(

)

(

)

(

)

1240 0 0.15 186 30 A 1.1 186 30 A

Aj=∠°+ Ω∠−°+ Ω∠−°E

1377 6.8 V

A=∠°E

The resulting phasor diagram is shown below (not to scale):

I

= 186

∠

-30°

A

V

= 1240

∠

0° V

φ

E

= 1377

∠

6.8° V

A

θ

(c) The efficiency of the generator under these conditions 3can be found as follows:

(

)

(

)

(

)

OUT 3 cos 3 1240 V 186 A 0.8 554 kW

A

PVI

φ

θ

== =

()( )

2

CU 3 3 186 A 0.15 15.6 kW

AA

PIR== Ω=

F&W 24 kWP=

core 18 kWP=

stray (assumed 0)P=

IN OUT CU F&W core stray 612 kWPP PP P P=++++=

114

OUT

IN

554 kW

100% 100% 90.5%

612 kW

P

η

=× = × =

(d) When the new load is added, the total current flow increases at the same phase angle. Therefore,

SS

jX I increases in length at the same angle, while the magnitude of A

E must remain constant. Therefore,

A

E “swings” out along the arc of constant magnitude until the new SS

jX I fits exactly between

φ

V and

A

E.

I

= 186

∠

-30°

A

V

= 1240

∠

0° V

φ

θ

E

′

A

V

′

φ

I

′

A

E

= 1377

∠

6.8° V

A

(e) The new impedance per phase will be half of the old value, so 3.333 30 Z=∠°Ω. The magnitude of

the phase current flowing in this generator is

1377 V 1377 V 335 A

0.15 1.1 3.333 30 1.829

A

AS

E

IRjXZ j

== ==

++ ++ ∠°Ω

Therefore, the magnitude of the phase voltage is

()( )

335 A 3.333 1117 V

A

VIZ

φ

== Ω=

and the terminal voltage is

()

3 3 1117 V 1934 V

T

VV

φ

== =

(f) To restore the terminal voltage to its original value, increase the field current F

I.

5-4. Assume that the field current of the generator in Problem 5-2 is adjusted to achieve rated voltage (2300 V)

at full load conditions in each of the questions below.

(a) What is the efficiency of the generator at rated load?

(b) What is the voltage regulation of the generator if it is loaded to rated kilovoltamperes with 0.8-PF-

lagging loads?

(c) What is the voltage regulation of the generator if it is loaded to rated kilovoltamperes with 0.8-PF-

leading loads?

(d) What is the voltage regulation of the generator if it is loaded to rated kilovoltamperes with unity-power-

factor loads?

(e) Use MATLAB to plot the terminal voltage of the generator as a function of load for all three power

factors.

S

OLUTION

115

(a) This generator is Y-connected, so LA

II=. At rated conditions, the line and phase current in this

generator is

()

1000 kVA 251 A

3 3 2300 V

AL

L

P

II V

== = = at an angle of –36.87°

The phase voltage of this machine is

/

31328 V

T

VV

φ

==. The internal generated voltage of the machine

is

AAASA

RjX

φ

=+ +EV I I

(

)

(

)

(

)

(

)

1328 0 0.15 251 36.87 A 1.1 251 36.87 A

Aj=∠°+ Ω∠− °+ Ω∠− °E

1537 7.4 V

A=∠°E

The input power to this generator is equal to the output power plus losses. The rated output power is

()()

OUT 1000 kVA 0.8 800 kWP==

()( )

2

CU 3 3 251 A 0.15 28.4 kW

AA

PIR== Ω=

F&W 24 kWP=

core 18 kWP=

stray (assumed 0)P=

IN OUT CU F&W core stray 870.4 kWPP PP P P=++++=

OUT

IN

800 kW

100% 100% 91.9%

870.4 kW

P

η=× = × =

(b) If the generator is loaded to rated kVA with lagging loads, the phase voltage is 1328 0 V

φ=∠°Vand

the internal generated voltage is 1537 7.4 V

A

E=∠°. Therefore, the phase voltage at no-load would be

1537 0 VV

φ

=∠°. The voltage regulation would be:

1537 1328

VR 100% 15.7%

1328

−

=×=

(c) If the generator is loaded to rated kVA with leading loads, the phase voltage is 1328 0 V

φ=∠°Vand

the internal generated voltage is

AAASA

RjX

φ

=+ +EV I I

()( )()( )

1328 0 0.15 251 36.87 A 1.1 251 36.87 A

AjE=∠°+ Ω∠°+ Ω∠°

1217 11.5 V

A

E=∠°

The voltage regulation would be:

1217 1328

VR 100% 8.4%

1328

−

=×=−

(d) If the generator is loaded to rated kVA at unity power factor, the phase voltage is

1328 0 V

φ=∠°Vand the internal generated voltage is

AAASA

RjX

φ

=+ +EV I I

116

()()()()

1328 0 0.15 251 0 A 1.1 251 0 A

AjE=∠°+ Ω∠°+ Ω∠°

1393 11.4 V

A

E=∠°

The voltage regulation would be:

1393 1328

VR 100% 4.9%

1328

−

=×=

(e) For this problem, we will assume that the terminal voltage is adjusted to 2300 V at no load

conditions, and see what happens to the voltage as load increases at 0.8 lagging, unity, and 0.8 leading

power factors. Note that the maximum current will be 251 A in any case. A phasor diagram representing

the situation at lagging power factor is shown below:

I

A

V

φ

E

A

θ

δθ

θ

I

A

R

A

jX

S

I

A

By the Pythagorean Theorem,

()

22

2cos sin cos sin

AAASA SAAS

E V RI XI XI RI

φ

θθ θθ

=+ + + −

()

2

2cos sin cos sin

ASA AS AA SA

VEXI RI RI XI

φ

θθ θ θ

=− − − −

A phasor diagram representing the situation at leading power factor is shown below:

I

A

V

φ

E

A

θ

δθ

θ

I

A

R

A

jX

S

I

A

By the Pythagorean Theorem,

()

22

2cos sin cos sin

AAASA SAAS

E V RI XI XI RI

φ

θθ θθ

=+ − + +

()

2

2cos sin cos sin

ASA AS AA SA

VEXI RI RI XI

φ

θθ θ θ

=− + − +

A phasor diagram representing the situation at unity power factor is shown below:

I

A

V

φ

E

A

δ

I

A

R

A

jX

S

I

A

117

By the Pythagorean Theorem,

(

)

2

22

ASA

EV XI

φ

=+

()

2

ASA

VEXI

φ

=−

The MATLAB program is shown below takes advantage of this fact.

% M-file: prob5_4e.m

% M-file to calculate and plot the terminal voltage

% of a synchronous generator as a function of load

% for power factors of 0.8 lagging, 1.0, and 0.8 leading.

% Define values for this generator

EA = 1328; % Internal gen voltage

I = 0:2.51:251; % Current values (A)

R = 0.15; % R (ohms)

X = 1.10; % XS (ohms)

% Calculate the voltage for the lagging PF case

VP_lag = sqrt( EA^2 - (X.*I.*0.8 - R.*I.*0.6).^2 ) ...

- R.*I.*0.8 - X.*I.*0.6;

VT_lag = VP_lag .* sqrt(3);

% Calculate the voltage for the leading PF case

VP_lead = sqrt( EA^2 - (X.*I.*0.8 + R.*I.*0.6).^2 ) ...

- R.*I.*0.8 + X.*I.*0.6;

VT_lead = VP_lead .* sqrt(3);

% Calculate the voltage for the unity PF case

VP_unity = sqrt( EA^2 - (X.*I).^2 );

VT_unity = VP_unity .* sqrt(3);

% Plot the terminal voltage versus load

plot(I,abs(VT_lag),'b-','LineWidth',2.0);

hold on;

plot(I,abs(VT_unity),'k--','LineWidth',2.0);

plot(I,abs(VT_lead),'r-.','LineWidth',2.0);

title ('\bfTerminal Voltage Versus Load');

xlabel ('\bfLoad (A)');

ylabel ('\bfTerminal Voltage (V)');

legend('0.8 PF lagging','1.0 PF','0.8 PF leading');

axis([0 260 1500 2500]);

grid on;

hold off;

The resulting plot is shown below:

118

5-5. Assume that the field current of the generator in Problem 5-2 has been adjusted so that it supplies rated

voltage when loaded with rated current at unity power factor. (You may ignore the effects of A

R when

answering these questions.)

(a) What is the torque angle

δ

of the generator when supplying rated current at unity power factor?

(b) When this generator is running at full load with unity power factor, how close is it to the static stability

limit of the machine?

S

OLUTION

(a) The torque

δ

angle can be found by calculating A

E:

AAASA

RjX

φ

=+ +EV I I

()()()()

1328 0 0.15 251 0 A 1.1 251 0 A

AjE=∠°+ Ω∠°+ Ω∠°

1393 11.4 V

A

E=∠°

Thus the torque angle

δ

= 11.4°.

(b) The static stability limit occurs at °= 90

δ

. This generator is a very long way from that limit. If we

ignore the internal resistance of the generator, the output power will be given by

3sin

A

S

VE

PX

φ

δ

=

and the output power is proportional to sin

δ

. Since sin 11.4 0.198°= , and sin 90 1.00°= , the static

stability limit is about 5 times the current output power of the generator.

5-6. A 480-V 400-kVA 0.85-PF-lagging 50-Hz four-pole ∆-connected generator is driven by a 500-hp diesel

engine and is used as a standby or emergency generator. This machine can also be paralleled with the

normal power supply (a very large power system) if desired.

(a) What are the conditions required for paralleling the emergency generator with the existing power

system? What is the generator’s rate of shaft rotation after paralleling occurs?

119

(b) If the generator is connected to the power system and is initially floating on the line, sketch the resulting

magnetic fields and phasor diagram.

(c) The governor setting on the diesel is now increased. Show both by means of house diagrams and by

means of phasor diagrams what happens to the generator. How much reactive power does the generator

supply now?

(d) With the diesel generator now supplying real power to the power system, what happens to the generator

as its field current is increased and decreased? Show this behavior both with phasor diagrams and with

house diagrams.

S

OLUTION

(a) To parallel this generator to the large power system, the required conditions are:

1. The generator must have the same voltage as the power system.

2. The

phase sequence of the oncoming generator must be the same as the phase sequence of the

power system.

3. The

frequency of the oncoming generator should be slightly higher than the frequency of the

running system.

4. The circuit breaker connecting the two systems together should be shut when the above conditions

are met and the generator is in phase with the power system.

After paralleling, the generator’s shaft will be rotating at

()

120 50 Hz

120 1500 r/min

4

e

m

f

nP

== =

(b) The magnetic field and phasor diagrams immediately after paralleling are shown below:

I

A

V

φ

E

A

jX

S

I

A

B

R

B

S

B

net

(c) When the governor setpoints on the generator are increased, the emergency generator begins to supply

more power to the loads, as shown below:

I

A

V

φ

E

A

jX

S

I

A

f

e

P

1

P

2

P

sys

P

G

Note that as the load increased with A

E constant, the generator began to consume a small amount of

reactive power.

(d) With the generator now supplying power to the system, an increase in field current increases the

reactive power supplied to the loads, and a decrease in field current decreases the reactive power supplied

to the loads.

120

V

φ

E

A

1

jX

S

I

A

Q

sys

Q

G

Q

2

Q

1

Q

3

E

A

2

E

A

3

I

A

3

I

A

2

I

A

1

V

φ

E

A

1

jX

S

I

A

Q

sys

Q

G

Q

2

Q

1

E

A

2

I

A

2

I

A

1

V

T

V

T

5-7. A 13.8-kV 10-MVA 0.8-PF-lagging 60-Hz two-pole Y-connected steam-turbine generator has a

synchronous reactance of 12 Ω per phase and an armature resistance of 1.5 Ω per phase. This generator is

operating in parallel with a large power system (infinite bus).

(a) What is the magnitude of EA at rated conditions?

(b) What is the torque angle of the generator at rated conditions?

(c) If the field current is constant, what is the maximum power possible out of this generator? How much

reserve power or torque does this generator have at full load?

(d) At the absolute maximum power possible, how much reactive power will this generator be supplying or

consuming? Sketch the corresponding phasor diagram. (Assume IF is still unchanged.)

S

OLUTION

(a) The phase voltage of this generator at rated conditions is

13,800 V 7967 V

3

V

φ

==

The armature current per phase at rated conditions is

()

10,000,000 VA 418 A

3 3 13,800 V

A

T

S

IV

== =

Therefore, the internal generated voltage at rated conditions is

AAASA

RjXEV I I

φ

=+ +

()( )( )( )

7967 0 1.5 418 36.87 A 12.0 418 36.87 A

AjE=∠°+Ω∠− °+ Ω∠− °

12,040 17.6 V

A

E=∠°

121

The magnitude of A

E is 12,040 V.

(b) The torque angle of the generator at rated conditions is

δ

= 17.6°.

(c) Ignoring A

R, the maximum output power of the generator is given by

()( )

MAX

3 3 7967 V 12,040 V 24.0 MW

12

A

S

VE

PX

φ

== =

Ω

The power at maximum load is 8 MW, so the maximum output power is three times the full load output

power.

(d) The phasor diagram at these conditions is shown below:

jX

S

I

A

V

φ

I

A

E

A

I

A

R

A

Under these conditions, the armature current is

12,040 90 V - 7967 0 V 1194 40.6 A

1.5 12.0

A

AS

RjX j

EV

I

φ

−∠° ∠°

== =∠°

++Ω

The reactive power produced by the generator at this point is

()()( )

3 sin 3 7967 V 1194 A sin 0 40.6 18.6 MVAR

A

QVI

φ

θ

== °−°=−

The generator is actually consuming reactive power at this time.

5-8. A 480-V, 100-kW, two-pole, three-phase, 60-Hz synchronous generator’s prime mover has a no-load speed

of 3630 r/min and a full-load speed of 3570 r/min. It is operating in parallel with a 480-V, 75-kW, four-

pole, 60-Hz synchronous generator whose prime mover has a no-load speed of 1800 r/min and a full-load

speed of 1785 r/min. The loads supplied by the two generators consist of 100 kW at 0.85 PF lagging.

(a) Calculate the speed droops of generator 1 and generator 2.

(b) Find the operating frequency of the power system.

(c) Find the power being supplied by each of the generators in this system.

(d) If VT is 460 V, what must the generator’s operators do to correct for the low terminal voltage?

S

OLUTION The no-load frequency of generator 1 corresponds to a frequency of

()()

nl1

3630 r/min 2

60.5 Hz

120 120

m

nP

f== =

The full-load frequency of generator 1 corresponds to a frequency of

122

()()

fl1

3570 r/min 2

59.5 Hz

120 120

m

nP

f== =

The no-load frequency of generator 2 corresponds to a frequency of

()()

nl2

1800 r/min 4

60.00 Hz

120 120

m

nP

f== =

The full-load frequency of generator 2 corresponds to a frequency of

()()

fl2

1785 r/min 4

59.50 Hz

120 120

m

nP

f== =

(a) The speed droop of generator 1 is given by

nl fl

1

fl

3630 r/min 3570 r/min

SD 100% 100% 1.68%

3570 r/min

nn

n

−−

=×= ×=

The speed droop of generator 2 is given by

nl fl

2

fl

1800 r/min 1785 r/min

SD 100% 100% 0.84%

1785 r/min

nn

n

−−

=×= ×=

(b) The power supplied by generator 1 is given by

()

1 1 nl1 sysP

Ps f f=−

and the power supplied by generator 1 is given by

()

2 2 nl2 sysP

Ps f f=−

The power curve’s slope for generator 1 is

1

nl fl

0.1 MW 0.1 MW/Hz

60.5 Hz 59.5 Hz

P

sff

== =

−−

The power curve’s slope for generator 1 is

2

nl fl

0.075 MW 0.150 MW/Hz

60.00 Hz 59.50 Hz

P

sff

== =

−−

The no-load frequency of generator 1 is 60.5 Hz and the no-load frequency of generator 2 is 60 Hz. The

total power that they must supply is 100 kW, so the system frequency can be found from the equations

LOAD 1 2

PPP=+

()()

LOAD 1 nl1 sys 2 nl2 sysPP

Psffsff=−+−

()

sys sys

100 kW 0.1 MW/Hz 60.5 Hz 0.15 MW/Hz 60.0 Hzff=−+ −

() ()

sys sys

100 kW 6050 kW 0.10 MW/Hz 9000 kW 0.15 MW/Hzff=− +−

()

sys

0.25 MW/Hz 6050 kW 9000 kW 100 kWf=+−

sys

14,950 kW 59.8 Hz

0.25 MW/Hz

f==

(c) The power supplied by generator 1 is

123

()

()( )

11nl1sys0.1 MW/Hz 60.5 Hz 59.8 Hz 70 kW

P

Ps f f=−= − =

The power supplied by generator 2 is

()

()( )

22nl2sys

0.15 MW/Hz 60.0 Hz 59.8 Hz 30 kW

P

Ps f f=−= − =

(d) If the terminal voltage is 460 V, the operators of the generators must increase the field currents on

both generators simultaneously. That action will increase the terminal voltages of the system without

changing the power sharing between the generators.

5-9. Three physically identical synchronous generators are operating in parallel. They are all rated for a full

load of 3 MW at 0.8 PF lagging. The no-load frequency of generator A is 61 Hz, and its speed droop is 3.4

percent. The no-load frequency of generator B is 61.5 Hz, and its speed droop is 3 percent. The no-load

frequency of generator C is 60.5 Hz, and its speed droop is 2.6 percent.

(a) If a total load consisting of 7 MW is being supplied by this power system, what will the system

frequency be and how will the power be shared among the three generators?

(b) Create a plot showing the power supplied by each generator as a function of the total power supplied to

all loads (you may use MATLAB to create this plot). At what load does one of the generators exceed

its ratings? Which generator exceeds its ratings first?

(c) Is this power sharing in (a) acceptable? Why or why not?

(d) What actions could an operator take to improve the real power sharing among these generators?

S

OLUTION

(a) Speed droop is defined as

nl fl nl fl

fl fl

SD 100% 100%

nn f f

nf

−−

=×=×

so nl

fl SD 1

100

f

f=

+

Thus, the full-load frequencies of generators A, B, and C are

nl,A

fl,A

A

61 Hz 59.0 Hz

SD 3.4

11

100 100

f

f===

++

nl,B

fl,B

B

61.5 Hz 59.71 Hz

SD 3.0

11

100 100

f

f===

++

nl,C

fl,C

C

60.5 Hz 58.97 Hz

SD 2.6

11

100 100

f

f===

++

and the slopes of the power-frequency curves are:

3 MW 1.5 MW/Hz

2 Hz

PA

S==

3 MW 1.676 MW/Hz

1.79 Hz

PB

S==

3 MW 1.961 MW/Hz

1.53 Hz

PC

S==

124

(a) The total load is 7 MW, so the system frequency is

()()()

LOAD nlA sys nlB sys nlC sysPA PB PC

P sffsffsff=−+−+−

(

)

()

(

)

()

(

)

()

sys sys sys

7 MW 1.5 61.0 1.676 61.5 1.961 60.5fff=−+ −+ −

sys sys sys

7 MW 91.5 1.5 103.07 1.676 118.64 1.961fff=−+− +−

sys

5.137 306.2f=

sys 59.61 Hzf=

The power supplied by each generator will be

()

()( )

nlA sys 1.5 MW/Hz 61.0 Hz 59.61 Hz 2.09 MW

APA

Psf f=−= − =

()

()( )

nlB sys 1.676 MW/Hz 61.5 Hz 59.61 Hz 3.17 MW

BPB

Psf f=−= − =

()

()( )

nlC sys 1.961 MW/Hz 60.5 Hz 59.61 Hz 1.74 MW

CPC

Ps f f=−= − =

(b) The equation in part (a) can be re-written slightly to express system frequency as a function of load.

()

LOAD sys sys sys

1.5 61.0 1.676 61.5 1.961 60.5Pf f f=−+ −+ −

LOAD sys sys sys

91.5 1.5 103.07 1.676 118.64 1.961Pf f f=− + − + −

sys LOAD

5.137 313.2fP=−

LOAD

sys

313.2

5.137

P

f−

=

A MATLAB program that uses this equation to determine the power sharing among the generators as a

function of load is shown below:

% M-file: prob5_9b.m

% M-file to calculate and plot the power sharing among

% three generators as a function of load.

% Define values for this generator

fnlA = 61.0; % No-load freq of Gen A

fnlB = 61.5; % No-load freq of Gen B

fnlC = 60.5; % No-load freq of Gen C

spA = 1.5; % Slope of Gen A (MW/Hz)

spB = 1.676; % Slope of Gen B (MW/Hz)

spC = 1.961; % Slope of Gen C (MW/Hz)

Pload = 0:0.05:10; % Load in MW

% Calculate the system frequency

fsys = (313.2 - Pload) ./ 5.137;

% Calculate the power of each generator

PA = spA .* ( fnlA - fsys);

PB = spB .* ( fnlB - fsys);

PC = spC .* ( fnlC - fsys);

% Plot the power sharing versus load

plot(Pload,PA,'b-','LineWidth',2.0);

125

hold on;

plot(Pload,PB,'k--','LineWidth',2.0);

plot(Pload,PC,'r-.','LineWidth',2.0);

plot([0 10],[3 3],'k','LineWidth',1.0);

plot([0 10],[0 0],'k:');

title ('\bfPower Sharing Versus Total Load');

xlabel ('\bfTotal Load (MW)');

ylabel ('\bfGenerator Power (MW)');

legend('Generator A','Generator B','Generator C','Power Limit');

grid on;

hold off;

The resulting plot is shown below:

This plot reveals that there are power sharing problems both for high loads and for low loads. Generator B

is the first to exceed its ratings as load increases. Its rated power is reached at a total load of 6.45 MW.

On the other hand, Generator C gets into trouble as the total load is reduced. When the total load drops to

2.4 MW, the direction of power flow reverses in Generator C.

(c) The power sharing in (a) is not acceptable, because Generator 2 has exceeded its power limits.

(d) To improve the power sharing among the three generators in (a) without affecting the operating

frequency of the system, the operator should decrease the governor setpoints on Generator B while

simultaneously increasing them in Generators A and C.

5-10. A paper mill has installed three steam generators (boilers) to provide process steam and also to use some its

waste products as an energy source. Since there is extra capacity, the mill has installed three 5-MW

turbine generators to take advantage of the situation. Each generator is a 4160-V 6250-kVA 0.85-PF-

lagging two-pole Y-connected synchronous generator with a synchronous reactance of 0.75 Ω and an

armature resistance of 0.04 Ω. Generators 1 and 2 have a characteristic power-frequency slope sP of 2.5

MW/Hz, and generators 2 and 3 have a slope of 3 MW/Hz.

(a) If the no-load frequency of each of the three generators is adjusted to 61 Hz, how much power will the

three machines be supplying when actual system frequency is 60 Hz?

126

(b) What is the maximum power the three generators can supply in this condition without the ratings of one

of them being exceeded? At what frequency does this limit occur? How much power does each

generator supply at that point?

(c) What would have to be done to get all three generators to supply their rated real and reactive powers at

an overall operating frequency of 60 Hz?

(d) What would the internal generated voltages of the three generators be under this condition?

S

OLUTION

(a) If the system frequency is 60 Hz and the no-load frequencies of the generators are 61 Hz, then the

power supplied by the generators will be

()

()( )

11nl1sys2.5 MW/Hz 61 Hz 60 Hz 2.5 MW

P

Ps f f=−= −=

()

(

)

(

)

2 2 nl2 sys 2.5 MW/Hz 61 Hz 60 Hz 2.5 MW

P

Ps f f=−= −=

()

(

)

(

)

3 3 nl3 sys 3.0 MW/Hz 61 Hz 60 Hz 3.0 MW

P

Ps f f=−= −=

Therefore the total power supplied by the generators is 8 MW.

(b) The maximum power supplied by any one generator is (6250 kVA)(0.85) = 5.31 MW. Generator 3

will be the first machine to reach that limit. Generator 3 will supply this power at a frequency of

()

sys

5.31 MW 3.0 MW/Hz 61 Hz f=−

Hz23.59

sys =f

At this point the power supplied by Generators 1 and 2 is

()

()( )

12 1nl1sys 2.5 MW/Hz 61 Hz 59.23 Hz 4.425 MW

P

PPs f f== − = − =

The total power supplied by the generators at this condition is 14.16 MW.

(c) To get each of the generators to supply 5.31 MW at 60 Hz, the no-load frequencies of Generator 1

and Generator 2 would have to be adjusted to 62.12 Hz, and the no-load frequency of Generator 3 would

have to be adjusted to 61.77 Hz. The field currents of the three generators must then be adjusted to get

them supplying a power factor of 0.85 lagging. At that point, each generator will be supplying its rated

real and reactive power.

(d) Under the conditions of part (c), which are the rated conditions of the generators, the internal

generated voltage would be given by

AAASA

RjXEV I I

φ

=+ +

The phase voltage of the generators is 4160 V / 3 = 2402 V, and since the generators are Y-connected,

their rated current is

()

6250 kVA 867 A

3 3 4160 V

AL

T

S

II V

== = =

The power factor is 0.85 lagging, so 867 31.8 A

A

I=∠− °. Therefore,

AAASA

RjXEV I I

φ

=+ +

()()()()

2402 0 0.04 867 31.8 A 0.75 867 31.8 A

AjE=∠°+ Ω∠−°+ Ω∠−°

127

2825 10.9 V

A

E=∠°

Problems 5-11 to 5-21 refer to a two-pole Y-connected synchronous generator rated at 470 kVA, 480 V, 60 Hz,

and 0.85 PF lagging. Its armature resistance RA is 0.016 Ω. The core losses of this generator at rated conditions

are 7 kW, and the friction and windage losses are 8 kW. The open-circuit and short-circuit characteristics are

shown in Figure P5-2.

128

Note: An electronic version of the saturated open circuit characteristic can be found

in file p52_occ.dat, and an electronic version of the air-gap characteristic

can be found in file p52_ag_occ.dat. These files can be used with

MATLAB programs. Column 1 contains field current in amps, and column 2

contains open-circuit terminal voltage in volts. An electronic version of the

short circuit characteristic can be found in file p52_scc.dat. Column 1

contains field current in amps, and column 2 contains short-circuit terminal

current in amps.

5-11. (a) What is the saturated synchronous reactance of this generator at the rated conditions? (b) What is the

unsaturated synchronous reactance of this generator? (c) Plot the saturated synchronous reactance of this

generator as a function of load.

S

OLUTION

(a) The rated armature current for this generator is

()

470 kVA 565 A

3 3 480 V

AL

T

S

II V

== = =

The field current required to produce this much short-circuit current may be read from the SCC. It is 0.534

A3. The open circuit voltage at 0.532 A is 880 V4, so the open-circuit phase voltage (= A

E) is 880/ 3 =

508 V. The approximate saturated synchronous reactance S

X is

3 If you have MATLAB available, you can use the file p52_scc.dat and the interp1 function to look up this

value as shown below. Note that column 1 of p52_scc contains field current, and column 2 contains short-circuit

terminal current.

load p52_scc.dat

if = interp1(p52_scc(:,2),p52_scc(:,1),565)

if =

129

508 V 0.899

565 A

S

X==Ω

(b) The unsaturated synchronous reactance Su

X is the ratio of the air-gap line to the SCC. This is a

straight line, so we can determine its value by comparing the ratio of the air-gap voltage to the short-circuit

current at any given field current. For example, at F

I = 0.50 A, the air-gap line voltage is 1040 V, and the

SCC is 532 A.

()

1040 V / 3 1.13

532 A

Su

X==Ω

(c) This task can best be performed with MATLAB. The open-circuit characteristic is available in a file

called p52_occ.dat, and the short-circuit characteristic is available in a file called p52_scc.dat.

Each of these files are organized in two columns, where the first column is field current and the second

column is either open-circuit terminal voltage or short-circuit current. A program to read these files and

calculate and plot S

X is shown below.

% M-file: prob5_11c.m

% M-file to calculate and plot the saturated

% synchronous reactance of a synchronous

% generator.

% Load the open-circuit characteristic. It is in

% two columns, with the first column being field

% current and the second column being terminal

% voltage.

load p52.occ;

if_occ = p52(:,1);

vt_occ = p52(:,2);

% Load the short-circuit characteristic. It is in

% two columns, with the first column being field

% current and the second column being line current

% (= armature current)

load p52.scc;

if_scc = p52(:,1);

ia_scc = p52(:,2);

% Calculate Xs

if = 0.001:0.02:1; % Current steps

vt = interp1(if_occ,vt_occ,If); % Terminal voltage

ia = interp1(if_scc,ia_scc,If); % Current

Xs = (vt ./ sqrt(3)) ./ ia;

0.534

4 If you have MATLAB available, you can use the file p52_occ.dat and the interp1 function to look up this

value as shown below. Note that column 1 of p52_occ contains field current, and column 2 contains open-circuit

terminal voltage.

load p52_occ.dat

vt = interp1(p52_occ(:,1),p52_occ(:,2),0.534)

vt =

880.400

130

% Plot the synchronous reactance

figure(1)

plot(If,Xs,'LineWidth',2.0);

title ('\bfSaturated Synchronous Reactance \itX_{s} \rm');

xlabel ('\bfField Current (A)');

ylabel ('\bf\itX_{s} \rm\bf(\Omega)');

grid on;

The resulting plot is:

5-12. (a) What are the rated current and internal generated voltage of this generator? (b) What field current

does this generator require to operate at the rated voltage, current, and power factor?

S

OLUTION

(a) The rated line and armature current for this generator is

()

470 kVA 565 A

3 3 480 V

AL

T

S

II V

== = =

The power factor is 0.85 lagging, so 565.3 31.8 A

A

I=∠−°

. The rated phase voltage is V

φ

= 480 V / 3

= 277 V. The saturated synchronous reactance at rated conditions was found to be 0.450 Ω in the previous

problem. Therefore, the internal generated voltage is

AAASA

RjXEV I I

φ

=+ +

()( )()( )

277 0 0.016 565.3 31.8 A 0.899 565.3 31.8 A

Aj=∠°+ Ω ∠− °+ Ω ∠− °E

509 30.5 V

A

E=∠°

(b) This internal generated voltage corresponds to a no-load terminal voltage of

()

3509 = 881 V.

From the open-circuit characteristic, the required field current would be 0.535 A.

5-13. What is the voltage regulation of this generator at the rated current and power factor?

S

OLUTION The voltage regulation is

131

,nl ,fl

,fl

881 480

VR 100% 100% 83.5%

480

TT

T

VV

V

−−

=×=×=

5-14. If this generator is operating at the rated conditions and the load is suddenly removed, what will the

terminal voltage be?

S

OLUTION From the above calculations, T

V will be 881 V.

5-15. What are the electrical losses in this generator at rated conditions?

S

OLUTION The electrical losses are

()( )

2

CU 3 3 565 A 0.016 15.3 kW

AA

PIR== Ω=

5-16. If this machine is operating at rated conditions, what input torque must be applied to the shaft of this

generator? Express your answer both in newton-meters and in pound-feet.

S

OLUTION To get the applied torque, we must know the input power. The input power to this generator is

equal to the output power plus losses. The rated output power and the losses are

()()

OUT 470 kVA 0.85 400 kWP==

()( )

2

CU 3 3 565 A 0.016 15.3 kW

AA

PIR== Ω=

F&W 8 kWP=

core 7 kWP=

stray (assumed 0)P=

IN OUT CU F&W core stray 430.3 kWPP PP P P=++++=

Therefore, the applied torque is

()

IN

APP

430.3 kW 2280 N m

2 rad 1 min

1800 r/min 1 r 60 s

m

P

τπ

ω

== = ⋅

or

(

)

APP

7.04 430.3 kW

7.04 1680 lb ft

1800 r/min

m

P

n

τ

== =⋅

5-17. What is the torque angle

δ

of this generator at rated conditions?

S

OLUTION From the calculations in Problem 5-12,

δ

= 30.5°.

5-18. Assume that the generator field current is adjusted to supply 480 V under rated conditions. What is the

static stability limit of this generator? (Note: You may ignore A

R to make this calculation easier.) How

close is the full-load condition of this generator to the static stability limit?

S

OLUTION At rated conditions, 509 30.5 V

A=∠°E. Therefore, the static stability limit is

()()

MAX

3 3277 V 509 V 471 kW

0.899

A

S

VE

PX

φ

== =

Ω

The full-load rated power of this generator is reasonably close to the static stability limit. Normal

generators would have more margin than this.

132

5-19. Assume that the generator field current is adjusted to supply 480 V under rated conditions. Plot the power

supplied by the generator as a function of the torque angle

δ

. (Note: You may ignore A

R to make this

calculation easier.)

S

OLUTION We will again ignore A

R to make this calculation easier. The power supplied by the generator is

()() ()

3 3 277 V 509 V

sin sin 471 kW sin

0.899

A

G

S

VE

PX

φ

δδδ

== =

Ω

The power supplied as a function of the torque angle

δ

may be plotted using a simple MATLAB program:

% M-file: prob5_19.m

% M-file to plot the power output of a

% synchronous generator as a function of

% the torque angle.

% Calculate Xs

delta = (0:1:90); % Torque angle (deg)

Pout = 561 .* sin(delta * pi/180); % Pout

% Plot the output power

figure(1)

plot(delta,Pout,'LineWidth',2.0);

title ('\bfOutput power vs torque angle \delta');

xlabel ('\bfTorque angle \delta (deg)');

ylabel ('\bf\itP_{OUT} \rm\bf(kW)');

grid on;

The resulting plot is:

5-20. Assume that the generator’s field current is adjusted so that the generator supplies rated voltage at the rated

load current and power factor. If the field current and the magnitude of the load current are held constant,

how will the terminal voltage change as the load power factor varies from 0.85 PF lagging to 0.85 PF

133

leading? Make a plot of the terminal voltage versus the impedance angle of the load being supplied by this

generator.

S

OLUTION If the field current is held constant, then the magnitude of A

E will be constant, although its

angle

δ

will vary. Also, the magnitude of the armature current is constant. Since we also know A

R, S

X,

and the current angle

θ

, we know enough to find the phase voltage

φ

V, and therefore the terminal voltage

T

V. At lagging power factors,

φ

V can be found from the following relationships:

I

A

V

φ

E

A

θ

δθ

θ

I

A

R

A

jX

S

I

A

By the Pythagorean Theorem,

()

222 sincossincos

θθθθ

φ

SAASASAAA IRIXIXIRVE −+++=

()

θθθθ

φ

sincossincos 22

ASAASAASA IXIRIRIXEV −−−−=

At unity power factor,

φ

V can be found from the following relationships:

I

A

V

φ

E

A

δ

I

A

R

A

jX

S

I

A

By the Pythagorean Theorem,

(

)

2

22

ASA

EV XI

φ

=+

()

22

ASA IXEV −=

φ

At leading power factors,

φ

V can be found from the following relationships:

I

A

V

φ

E

A

θ

δθ

θ

I

A

R

A

jX

S

I

A

By the Pythagorean Theorem,

134

()

222 sincossincos

θθθθ

φ

SAASASAAA IRIXIXIRVE ++−+=

()

θθθθ

φ

sincossincos 22

ASAASAASA IXIRIRIXEV +−+−=

If we examine these three cases, we can see that the only difference among them is the sign of the term

θ

sin . If

θ

is taken as positive for lagging power factors and negative for leading power factors (in other

words, if

θ

is the impedance angle), then all three cases can be represented by the single equation:

()

θθθθ

φ

sincossincos 22

ASAASAASA IXIRIRIXEV −−−−=

A MATLAB program that calculates terminal voltage as function of impedance angle is shown below:

% M-file: prob5_20.m

% M-file to calculate and plot the terminal voltage

% of a synchronous generator as a function of impedance

% angle as PF changes from 0.85 lagging to 0.85

% leading.

% Define values for this generator

EA = 509; % Internal gen voltage

I = 361; % Current (A)

R = 0.04; % R (ohms)

X = 0.695; % XS (ohms)

% Calculate impedance angle theta in degrees

theta = -31.8:0.318:31.8;

th = theta * pi/180; % In radians

% Calculate the phase voltage and terminal voltage

VP = sqrt( EA^2 - (X.*I.*cos(th) - R.*I.*sin(th)).^2 ) ...

- R.*I.*cos(th) - X.*I.*sin(th);

VT = VP .* sqrt(3);

% Plot the terminal voltage versus power factor

figure(1);

plot(theta,abs(VT),'b-','LineWidth',2.0);

title ('\bfTerminal Voltage Versus Impedance Angle');

xlabel ('\bfImpedance Angle (deg)');

ylabel ('\bfTerminal Voltage (V)');

%axis([0 260 300 540]);

grid on;

hold off;

The resulting plot of terminal voltage versus impedance angle (with field and armature currents held

constant) is shown below:

135

5-21. Assume that the generator is connected to a 480-V infinite bus, and that its field current has been adjusted

so that it is supplying rated power and power factor to the bus. You may ignore the armature resistance

A

R when answering the following questions.

(a) What would happen to the real and reactive power supplied by this generator if the field flux (and

therefore A

E) is reduced by 5%.

(b) Plot the real power supplied by this generator as a function of the flux

φ

as the flux is varied from 75%

to 100% of the flux at rated conditions.

(c) Plot the reactive power supplied by this generator as a function of the flux

φ

as the flux is varied from

75% to 100% of the flux at rated conditions.

(d) Plot the line current supplied by this generator as a function of the flux

φ

as the flux is varied from 75%

to 100% of the flux at rated conditions.

S

OLUTION

(a) If the field flux in increase by 5%, nothing would happen to the real power. The reactive power

supplied would increase as shown below.

V

φ

E

A

1

jX

S

I

A

Q

sys

Q

G

Q

2

Q

1

E

A

2

I

A

2

I

A

1

V

T

Q

∝

I

sin

θ

A

The reactive power

136

(b) If armature resistance is ignored, the power supplied to the bus will not change as flux is varied.

Therefore, the plot of real power versus flux is

(c) If armature resistance is ignored, the internal generated voltage A

E will increase as flux increases,

but the quantity

δ

sin

A

E will remain constant. Therefore, the voltage for any flux can be found from the

expression

Ar

r

AEE













=

φ

and the angle

δ

for any A

E can be found from the expression













=−

r

A

Ar

E

δδ

sinsin 1

where

φ

is the flux in the machine, r

φ

is the flux at rated conditions, Ar

E is the magnitude of the internal

generated voltage at rated conditions, and r

δ

is the angle of the internal generated voltage at rated

conditions. From this information, we can calculate A

I for any given load from equation

S

A

AjX

φ

VE

I−

=

and the resulting reactive power from the equation

θ

φ

sin 3 A

IVQ =

where

θ

is the impedance angle, which is the negative of the current angle. Ignoring A

R, the internal

generated voltage at rated conditions is

ASA jX IVE +=

φ

()( )

277 0 0.899 565.3 31.8 A

Aj=∠°+ Ω ∠− °E

137

695 38.4 V

A=∠°E

so V 461=

Ar

E and °= 5.27

r

δ

. A MATLAB program that calculates the reactive power supplied

voltage as a function of flux is shown below:

% M-file: prob5_21c.m

% M-file to calculate and plot the reactive power

% supplied to an infinite bus as flux is varied from

% 75% to 100% of the flux at rated conditions.

% Define values for this generator

flux_ratio = 0.90:0.01:1.00; % Flux ratio

Ear = 695; % Ea at full flux

dr = 38.4 * pi/180; % Torque ang at full flux

Vp = 277; % Phase voltage

Xs = 0.899; % Xs (ohms)

% Calculate Ea for each flux

Ea = flux_ratio * Ear;

% Calculate delta for each flux

d = asin( Ear ./ Ea .* sin(dr));

% Calculate Ia for each flux

Ea = Ea .* ( cos(d) + j.*sin(d) );

Ia = ( Ea - Vp ) ./ (j*Xs);

% Calculate reactive power for each flux

theta = -atan2(imag(Ia),real(Ia));

Q = 3 .* Vp .* abs(Ia) .* sin(theta);

% Plot the power supplied versus flux

figure(1);

plot(flux_ratio,Q/1000,'b-','LineWidth',2.0);

title ('\bfReactive power versus flux');

xlabel ('\bfFlux (% of full-load flux)');

ylabel ('\bf\itQ\rm\bf (kVAR)');

grid on;

hold off;

138

When this program is executed, the plot of reactive power versus flux is

(d) The program in part (c) of this program calculated A

I as a function of flux. A MATLAB program

that plots the magnitude of this current as a function of flux is shown below:

% M-file: prob5_21d.m

% M-file to calculate and plot the armature current

% supplied to an infinite bus as flux is varied from

% 75% to 100% of the flux at rated conditions.

% Define values for this generator

flux_ratio = 0.75:0.01:1.00; % Flux ratio

Ear = 695; % Ea at full flux

dr = 38.4 * pi/180; % Torque ang at full flux

Vp = 277; % Phase voltage

Xs = 0.899; % Xs (ohms)

% Calculate Ea for each flux

Ea = flux_ratio * Ear;

% Calculate delta for each flux

d = asin( Ear ./ Ea .* sin(dr));

% Calculate Ia for each flux

Ea = Ea .* ( cos(d) + j.*sin(d) );

Ia = ( Ea - Vp ) ./ (j*Xs);

% Plot the armature current versus flux

figure(1);

plot(flux_ratio,abs(Ia),'b-','LineWidth',2.0);

title ('\bfArmature current versus flux');

xlabel ('\bfFlux (% of full-load flux)');

ylabel ('\bf\itI_{A}\rm\bf (A)');

grid on;

139

hold off;

When this program is executed, the plot of armature current versus flux is

5-22. A 100-MVA 12.5-kV 0.85-PF-lagging 50-Hz two-pole Y-connected synchronous generator has a per-unit

synchronous reactance of 1.1 and a per-unit armature resistance of 0.012.

(a) What are its synchronous reactance and armature resistance in ohms?

(b) What is the magnitude of the internal generated voltage EA at the rated conditions? What is its torque

angle

δ

at these conditions?

(c) Ignoring losses in this generator, what torque must be applied to its shaft by the prime mover at full

load?

S

OLUTION The base phase voltage of this generator is ,base 12,500/ 3 7217 VV

φ

==. Therefore, the base

impedance of the generator is

()

2

,base

base

3 37217 V 1.56

100,000,000 VA

V

ZS

φ

== =Ω

(a) The generator impedance in ohms are:

()( )

0.012 1.56 0.0187

A

R=Ω=Ω

()( )

1.1 1.56 1.716

S

X=Ω=Ω

(b) The rated armature current is

()

100 MVA 4619 A

3 3 12.5 kV

AL

T

S

II V

== = =

The power factor is 0.8 lagging, so 4619 36.87 A

A=∠− °I. Therefore, the internal generated voltage is

AAASA

RjX

φ

=+ +EV I I

140

()()()()

7217 0 0.0187 4619 36.87 A 1.716 4619 36.87 A

Aj=∠°+ Ω ∠− °+ Ω ∠− °E

13,590 27.6 V

A=∠°E

Therefore, the magnitude of the internal generated voltage A

E = 13,590 V, and the torque angle

δ

= 23°.

(c) Ignoring losses, the input power would equal the output power. Since

()( )

OUT 0.85 100 MVA 85 MWP==

and

()

sync

120 50 Hz

120 3000 r/min

2

e

f

nP

== =

the applied torque would be

()()()

app ind

85,000,000 W 270,000 N m

3000 r/min 2 rad/r 1 min/60 s

ττ π

== = ⋅

5-23. A three-phase Y-connected synchronous generator is rated 120 MVA, 13.2 kV, 0.8 PF lagging, and 60 Hz.

Its synchronous reactance is 0.9 Ω, and its resistance may be ignored.

(a) What is its voltage regulation?

(b) What would the voltage and apparent power rating of this generator be if it were operated at 50 Hz

with the same armature and field losses as it had at 60 Hz?

(c) What would the voltage regulation of the generator be at 50 Hz?

S

OLUTION

(a) The rated armature current is

()

120 MVA 5249 A

3 3 13.2 kV

AL

T

S

II V

== = =

The power factor is 0.8 lagging, so 5249 36.87 A

A=∠− °I. The phase voltage is 13.2 kV / 3 = 7621

V. Therefore, the internal generated voltage is

AAASA

RjX

φ

=+ +EV I I

()( )

7621 0 0.9 5249 36.87 A

Aj=∠°+ Ω ∠− °E

11,120 19.9 V

A=∠°E

The resulting voltage regulation is

11,120 7621

VR 100% 45.9%

7621

−

=×=

(b) If the generator is to be operated at 50 Hz with the same armature and field losses as at 60 Hz (so

that the windings do not overheat), then its armature and field currents must not change. Since the voltage

of the generator is directly proportional to the speed of the generator, the voltage rating (and hence the

apparent power rating) of the generator will be reduced by a factor of 5/6.

()

,rated

513.2 kV 11.0 kV

6

T

V==

()

rated

5120 MVA 100 MVA

6

S==

141

Also, the synchronous reactance will be reduced by a factor of 5/6.

()

50.9 0.75

6

S

X=Ω=Ω

(c) At 50 Hz rated conditions, the armature current would be

()

100 MVA 5247 A

3 3 11.0 kV

AL

T

S

II V

== = =

The power factor is 0.8 lagging, so 5247 36.87 A

A=∠− °I. The phase voltage is 11.0 kV / 3 = 6351

V. Therefore, the internal generated voltage is

AAASA

RjX

φ

=+ +EV I I

()( )

6351 0 0.75 5247 36.87 A

Aj=∠°+ Ω ∠− °E

9264 19.9 V

A=∠°E

The resulting voltage regulation is

9264 6351

VR 100% 45.9%

6351

−

=×=

Because voltage, apparent power, and synchronous reactance all scale linearly with frequency, the voltage

regulation at 50 Hz is the same as that at 60 Hz. Note that this is not quite true, if the armature resistance

A

R is included, since A

R does not scale with frequency in the same fashion as the other terms.

5-24. Two identical 600-kVA 480-V synchronous generators are connected in parallel to supply a load. The

prime movers of the two generators happen to have different speed droop characteristics. When the field

currents of the two generators are equal, one delivers 400 A at 0.9 PF lagging, while the other delivers 300

A at 0.72 PF lagging.

(a) What are the real power and the reactive power supplied by each generator to the load?

(b) What is the overall power factor of the load?

(c) In what direction must the field current on each generator be adjusted in order for them to operate at the

same power factor?

S

OLUTION

(a) The real and reactive powers are

()()()

13 cos 3 480 V 400 A 0.9 299 kW

TL

PVI

θ

== =

(

)

(

)

(

)

1

13 sin 3 480 V 400 A sin cos 0.9 145 kVAR

TL

QVI

θ

−

== =

(

)

(

)

(

)

23 cos 3 480 V 200 A 0.72 120 kW

TL

PVI

θ

== =

(

)

(

)

(

)

1

23 sin 3 480 V 200 A sin cos 0.72 115 kVAR

TL

QVI

θ

−

== =

(b) The overall power factor can be found from the total real and reactive power supplied to the load.

TOT 1 2 299 kW 120 kW 419 kWPPP=+= + =

TOT 1 2 145 kVAR 115 kVAR 260 kVARQQQ=+= + =

The overall power factor is

142

1TOT

TOT

PF cos tan 0.850 lagging

Q

P

−

==

(c) The field current of generator 1 should be increased, and the field current of generator 2 should be

simultaneously decreased.

5-25. A generating station for a power system consists of four 120-MVA 15-kV 0.85-PF-lagging synchronous

generators with identical speed droop characteristics operating in parallel. The governors on the

generators’ prime movers are adjusted to produce a 3-Hz drop from no load to full load. Three of these

generators are each supplying a steady 75 MW at a frequency of 60 Hz, while the fourth generator (called

the swing generator) handles all incremental load changes on the system while maintaining the system's

frequency at 60 Hz.

(a) At a given instant, the total system loads are 260 MW at a frequency of 60 Hz. What are the no-load

frequencies of each of the system’s generators?

(b) If the system load rises to 290 MW and the generator’s governor set points do not change, what will the

new system frequency be?

(c) To what frequency must the no-load frequency of the swing generator be adjusted in order to restore the

system frequency to 60 Hz?

(d) If the system is operating at the conditions described in part (c), what would happen if the swing

generator were tripped off the line (disconnected from the power line)?

S

OLUTION

(a) The full-load power of these generators is

()()

MW10285.0 MVA120 = and the droop from no-

load to full-load is 3 Hz. Therefore, the slope of the power-frequency curve for these four generators is

102 MW 34 MW/Hz

3 Hz

P

s==

If generators 1, 2, and 3 are supplying 75 MW each, then generator 4 must be supplying 35 MW. The no-

load frequency of the first three generators is

()

1 1 nl1 sysP

Ps f f=−

(

)

()

nl1

75 MW 34 MW/Hz 60 Hzf=−

nl1 62.21 Hzf=

The no-load frequency of the fourth generator is

()

4 4 nl4 sysP

Ps f f=−

()

nl1

35 MW 34 MW/Hz 60 Hzf=−

Hz03.61

nl1 =f

(b) The setpoints of generators 1, 2, 3, and 4 do not change, so the new system frequency will be

()()()()

LOAD 1 nl1 sys 2 nl2 sys 3 nl3 sys 4 nl4 sysPP P P

Psffsffsffsff=−+−+−+−

()

sys sys sys sys

290 MW 34 62.21 34 62.21 34 62.21 34 61.03ffff=−+−+−+−

sys

8.529 247.66 4 f=−

143

sys 59.78 Hzf=

(c) The governor setpoints of the swing generator must be increased until the system frequency rises back

to 60 Hz. At 60 Hz, the other three generators will be supplying 75 MW each, so the swing generator must

supply 290 MW – 3(75 MW) = 65 MW at 60 Hz. Therefore, the swing generator’s setpoints must be set

to

()

4 4 nl4 sysP

Ps f f=−

(

)

()

nl1

65 MW 34 MW/Hz 60 Hzf=−

nl1 61.91 Hzf=

(d) If the swing generator trips off the line, the other three generators would have to supply all 290 MW

of the load. Therefore, the system frequency will become

()()()

LOAD 1 nl1 sys 2 nl2 sys 3 nl3 sysPP P

P sff sff sff=−+−+−

(

)

()

(

)

()

(

)

()

sys sys sys

290 MW 34 62.21 34 62.21 34 62.21fff=−+−+−

sys

8.529 186.63 3 f=−

sys 59.37 Hzf=

Each generator will supply 96.7 MW to the loads.

5-26. Suppose that you were an engineer planning a new electric co-generation facility for a plant with excess

process steam. You have a choice of either two 10 MW turbine-generators or a single 20 MW turbine

generator. What would be the advantages and disadvantages of each choice?

S

OLUTION A single 20 MW generator will probably be cheaper and more efficient than two 10 MW

generators, but if the 20 MW generator goes down all 20 MW of generation would be lost at once. If two

10 MW generators are chosen, one of them could go down for maintenance and some power could still be

generated.

5-27. A 25-MVA three-phase 13.8-kV two-pole 60-Hz synchronous generator was tested by the open-circuit test,

and its air-gap voltage was extrapolated with the following results:

Open-circuit test

Field current, A 320 365 380 475 570

Line voltage, kV 13.0 13.8 14.1 15.2 16.0

Extrapolated air-gap voltage, kV 15.4 17.5 18.3 22.8 27.4

The short-circuit test was then performed with the following results:

Short-circuit test

Field current, A 320 365 380 475 570

Armature current, A 1040 1190 1240 1550 1885

The armature resistance is 0.24 Ω per phase.

(a) Find the unsaturated synchronous reactance of this generator in ohms per phase and in per-unit.

(b) Find the approximate saturated synchronous reactance XS at a field current of 380 A. Express the

answer both in ohms per phase and in per-unit.

144

(c) Find the approximate saturated synchronous reactance at a field current of 475 A. Express the answer

both in ohms per phase and in per-unit.

(d) Find the short-circuit ratio for this generator.

S

OLUTION

(a) The unsaturated synchronous reactance of this generator is the same at any field current, so we will

look at it at a field current of 380 A. The extrapolated air-gap voltage at this point is 18.3 kV, and the

short-circuit current is 1240 A. Since this generator is Y-connected, the phase voltage is

18.3 kV/ 3 10,566 V V

φ

==

and the armature current is 1240 A

A

I=. Therefore, the unsaturated

synchronous reactance is

10,566 V 8.52

1240 A

Su

X==Ω

The base impedance of this generator is

()

2

,base

base

3 3 7967 V 7.62

25,000,000 VA

V

ZS

φ

== =Ω

Therefore, the per-unit unsaturated synchronous reactance is

,pu

8.52 1.12

7.62

Su

XΩ

==

Ω

(b) The saturated synchronous reactance at a field current of 380 A can be found from the OCC and the

SCC. The OCC voltage at F

I = 380 A is 14.1 kV, and the short-circuit current is 1240 A. Since this

generator is Y-connected, the corresponding phase voltage is 14.1 kV/ 3 8141 V V

φ

== and the armature

current is 1240 A

A

I=. Therefore, the saturated synchronous reactance is

8141 V 6.57

1240 A

Su

X==Ω

and the per-unit unsaturated synchronous reactance is

,pu

6.57 0.862

7.62

Su

XΩ

==

Ω

(c) The saturated synchronous reactance at a field current of 475 A can be found from the OCC and the

SCC. The OCC voltage at F

I = 475 A is 15.2 kV, and the short-circuit current is 1550 A. Since this

generator is Y-connected, the corresponding phase voltage is 15.2 kV/ 3 8776 V V

φ

==

and the armature

current is 1550 A

A

I=. Therefore, the saturated synchronous reactance is

8776 V 5.66

1550 A

Su

X==Ω

and the per-unit unsaturated synchronous reactance is

,pu

5.66 0.743

7.62

Su

XΩ

==

Ω

(d) The rated voltage of this generator is 13.8 kV, which requires a field current of 365 A. The rated line

and armature current of this generator is

145

()

25 MVA 1046 A

3 13.8 kV

L

I==

The field current required to produce a short-circuit current of 10465 A is about 320 A. Therefore, the

short-circuit ratio of this generator is

365 A

SCR 1.14

320 A

==

5-28. A 20-MVA 12.2-kV 0.8-PF-lagging Y-connected synchronous generator has a negligible armature

resistance and a synchronous reactance of 1.1 per-unit. The generator is connected in parallel with a 60-Hz

12.2-kV infinite bus that is capable of supplying or consuming any amount of real or reactive power with

no change in frequency or terminal voltage.

(a) What is the synchronous reactance of the generator in ohms?

(b) What is the internal generated voltage EA of this generator under rated conditions?

(c) What is the armature current IA in this machine at rated conditions?

(d) Suppose that the generator is initially operating at rated conditions. If the internal generated voltage

EA is decreased by 5 percent, what will the new armature current IA be?

(e) Repeat part (d) for 10, 15, 20, and 25 percent reductions in EA.

(f) Plot the magnitude of the armature current IA as a function of EA. (You may wish to use MATLAB

to create this plot.)

S

OLUTION

(a) The rated phase voltage of this generator is 12.2 kV / 3 = 7044 V. The base impedance of this

generator is

()

2

,base

base

3 37044 V 7.44

20,000,000 VA

V

ZS

φ

== =Ω

Therefore,

0 (negligible)

A

R≈Ω

()( )

1.1 7.44 8.18

S

X=Ω=Ω

(b) The rated armature current is

()

20 MVA 946 A

3 3 12.2 kV

AL

T

S

II V

== = =

The power factor is 0.8 lagging, so 946 36.87 A

A=∠− °I. Therefore, the internal generated voltage is

AAASA

RjX

φ

=+ +EV I I

()( )

7044 0 8.18 946 36.87 A

Aj=∠°+ Ω∠− °E

13,230 27.9 V

A=∠°E

(c) From the above calculations, 946 36.87 A

A=∠− °I.

146

(d) If A

E is decreased by 5%, the armature current will change as shown below. Note that the infinite

bus will keep

φ

V and m

ω

constant. Also, since the prime mover hasn’t changed, the power supplied by the

generator will be constant.

V

φ

E

A

1

jX

S

I

A

E

A

2

I

A

2

I

A

1

Q

∝

I

sin

θ

A

3sin constant

A

S

VE

PX

φ

δ

==, so 11 2 2

sin sin

AA

EE

δδ

=

With a 5% decrease, 212,570 V

A

E=, and

11

1

22

2

13, 230 V

sin sin sin sin 27.9 29.5

12,570 V

A

E

δδ

−−

== °=°

Therefore, the new armature current is

212,570 29.5 7044 0 894 32.2 A

8.18

A

S

jX j

φ

−∠°− ∠°

== =∠−°

EV

I

(e) Repeating part (d):

With a

10% decrease, 211,907 V

A

E=, and

11

1

22

2

13, 230 V

sin sin sin sin 27.9 31.3

11,907 V

A

E

δδ

−−

== °=°

Therefore, the new armature current is

211, 907 31.3 7044 0 848 26.8 A

8.18

A

S

jX j

φ

−∠°− ∠°

== =∠−°

EV

I

With a

15% decrease, 211,246 V

A

E=, and

11

1

22

2

13, 230 V

sin sin sin sin 27.9 33.4

11,246 V

A

E

δδ

−−

== °=°

Therefore, the new armature current is

211,246 33.4 7044 0 809 20.7 A

8.18

A

S

jX j

φ

−∠°− ∠°

== =∠−°

EV

I

With a

20% decrease, 210,584 V

A

E=, and

11

1

22

2

13, 230 V

sin sin sin sin 27.9 35.8

10,584 V

A

E

δδ

−−

== °=°

Therefore, the new armature current is

147

210,584 35.8 7044 0 780 14.0 A

8.18

A

S

jX j

φ

−∠°− ∠°

== =∠−°

EV

I

With a

25% decrease, 29, 923 V

A

E=, and

11

1

22

2

13, 230 V

sin sin sin sin 27.9 38.6

9,923 V

A

E

δδ

−−

== °=°

Therefore, the new armature current is

29,923 38.6 7044 0 762 6.6 A

8.18

A

S

jX j

φ

−∠°− ∠°

== =∠−°

EV

I

(f) A MATLAB program to plot the magnitude of the armature current A

I as a function of A

E is shown

below.

% M-file: prob5_28f.m

% M-file to calculate and plot the armature current

% supplied to an infinite bus as Ea is varied.

% Define values for this generator

Ea = (0.65:0.01:1.00)*13230; % Ea

Vp = 7044; % Phase voltage

d1 = 27.9*pi/180; % torque angle at full Ea

Xs = 8.18; % Xs (ohms)

% Calculate delta for each Ea

d = asin( 13230 ./ Ea .* sin(d1));

% Calculate Ia for each flux

Ea = Ea .* ( cos(d) + j.*sin(d) );

Ia = ( Ea - Vp ) ./ (j*Xs);

% Plot the armature current versus Ea

figure(1);

plot(abs(Ea)/1000,abs(Ia),'b-','LineWidth',2.0);

title ('\bfArmature current versus \itE_{A}\rm');

xlabel ('\bf\itE_{A}\rm\bf (kV)');

ylabel ('\bf\itI_{A}\rm\bf (A)');

grid on;

hold off;

148

The resulting plot is shown below:

149

Chapter 6: Synchronous Motors

6-1. A 480-V, 60 Hz, four-pole synchronous motor draws 50 A from the line at unity power factor and full

load. Assuming that the motor is lossless, answer the following questions:

(a) What is the output torque of this motor? Express the answer both in newton-meters and in pound-feet.

(b) What must be done to change the power factor to 0.8 leading? Explain your answer, using phasor

diagrams.

(c) What will the magnitude of the line current be if the power factor is adjusted to 0.8 leading?

S

OLUTION

(a) If this motor is assumed lossless, then the input power is equal to the output power. The input power

to this motor is

()()()

IN 3 cos 3 480 V 50 A 1.0 41.6 kW

TL

PVI

θ

== =

The output torque would be

()

OUT

LOAD

41.6 kW 221 N m

1 min 2 rad

1800 r/min 60 s 1 r

m

P

τπ

ω

== = ⋅

In English units,

(

)

(

)

()

OUT

LOAD

7.04 41.6 kW

7.04 163 lb ft

1800 r/min

m

P

n

τ

== =⋅

(b) To change the motor’s power factor to 0.8 leading, its field current must be increased. Since the

power supplied to the load is independent of the field current level, an increase in field current increases

A

E while keeping the distance

δ

sin

A

E constant. This increase in A

E changes the angle of the current

A

I, eventually causing it to reach a power factor of 0.8 leading.

V

φ

E

A

1

jX

S

I

A

E

A

2

I

A

2

I

A

1

Q

∝

I

sin

θ

A

}

∝

P

}

∝

P

(c) The magnitude of the line current will be

()()

41.6 kW 62.5 A

3 PF 3 480 V 0.8

L

T

P

IV

== =

6-2. A 480-V, 60 Hz, 400-hp 0.8-PF-leading six-pole ∆-connected synchronous motor has a synchronous

reactance of 1.1 Ω and negligible armature resistance. Ignore its friction, windage, and core losses for the

purposes of this problem.

150

(a) If this motor is initially supplying 400 hp at 0.8 PF lagging, what are the magnitudes and angles of EA

and IA?

(b) How much torque is this motor producing? What is the torque angle

δ

? How near is this value to the

maximum possible induced torque of the motor for this field current setting?

(c) If EA is increased by 15 percent, what is the new magnitude of the armature current? What is the

motor’s new power factor?

(d) Calculate and plot the motor’s V-curve for this load condition.

S

OLUTION

(a) If losses are being ignored, the output power is equal to the input power, so the input power will be

()( )

IN 400 hp 746 W/hp 298.4 kWP==

This situation is shown in the phasor diagram below:

V

φ

E

A

jX

S

I

A

I

A

The line current flow under these circumstances is

()()

298.4 kW 449 A

3 PF 3 480 V 0.8

L

T

P

IV

== =

Because the motor is ∆-connected, the corresponding phase current is 449 / 3 259 A

A

I==

. The angle of

the current is

()

1

cos 0.80 36.87

−

−=−°, so 259 36.87 A

A=∠− °I. The internal generated voltage A

E is

ASA

jX

φ

=−EV I

()()( )

480 0 V 1.1 259 36.87 A 384 36.4 V

Aj=∠°− Ω ∠− °=∠−°E

(b) This motor has 6 poles and an electrical frequency of 60 Hz, so its rotation speed is m

n = 1200 r/min.

The induced torque is

()

OUT

ind

298.4 kW 2375 N m

1 min 2 rad

1200 r/min 60 s 1 r

m

P

τπ

ω

== = ⋅

The maximum possible induced torque for the motor at this field setting is

()()

() ()

ind,max

3 3 480 V 384 V 4000 N m

1 min 2 rad

1200 r/min 1.1

60 s 1 r

A

mS

VE

X

φ

τπ

ω

== =⋅

Ω

(c) If the magnitude of the internal generated voltage A

E is increased by 15%, the new torque angle can

be found from the fact that constantsin =∝ PEA

δ

.

()

21

1.15 1.15 384 V 441.6 V

AA

EE== =

151

()

11

1

21

2

384 V

sin sin sin sin 36.4 31.1

441.6 V

A

E

δδ

−−

== −°=−°

The new armature current is

2

480 0 V 441.6 31.1 V 227 24.1 A

1.1

A

S

jX j

φ

−∠° − ∠− °

== =∠−°

Ω

VE

I

The magnitude of the armature current is 227 A, and the power factor is cos (-24.1°) = 0.913 lagging.

(d) A MATLAB program to calculate and plot the motor’s V-curve is shown below:

% M-file: prob6_2d.m

% M-file create a plot of armature current versus Ea

% for the synchronous motor of Problem 6-2.

% Initialize values

Ea = (1:0.01:1.70)*384; % Magnitude of Ea volts

Ear = 384; % Reference Ea

deltar = -36.4 * pi/180; % Reference torque angle

Xs = 1.1; % Synchronous reactance

Vp = 480; % Phase voltage at 0 degrees

Ear = Ear * (cos(deltar) + j * sin(deltar));

% Calculate delta2

delta2 = asin ( abs(Ear) ./ abs(Ea) .* sin(deltar) );

% Calculate the phasor Ea

Ea = Ea .* (cos(delta2) + j .* sin(delta2));

% Calculate Ia

Ia = ( Vp - Ea ) / ( j * Xs);

% Plot the v-curve

figure(1);

plot(abs(Ea),abs(Ia),'b','Linewidth',2.0);

xlabel('\bf\itE_{A}\rm\bf (V)');

ylabel('\bf\itI_{A}\rm\bf (A)');

title ('\bfSynchronous Motor V-Curve');

grid on;

152

The resulting plot is shown below

350 400 450 500 550 600 650 700

200

210

220

230

240

250

260

E

A

(V)

I

A

(A)

Synchronous Motor V-Curve

6-3. A 2300-V 1000-hp 0.8-PF leading 60-Hz two-pole Y-connected synchronous motor has a synchronous

reactance of 2.8 Ω and an armature resistance of 0.4 Ω. At 60 Hz, its friction and windage losses are 24

kW, and its core losses are 18 kW. The field circuit has a dc voltage of 200 V, and the maximum IF is 10

A. The open-circuit characteristic of this motor is shown in Figure P6-1. Answer the following questions

about the motor, assuming that it is being supplied by an infinite bus.

(a) How much field current would be required to make this machine operate at unity power factor when

supplying full load?

(b) What is the motor’s efficiency at full load and unity power factor?

(c) If the field current were increased by 5 percent, what would the new value of the armature current be?

What would the new power factor be? How much reactive power is being consumed or supplied by the

motor?

(d) What is the maximum torque this machine is theoretically capable of supplying at unity power factor?

At 0.8 PF leading?

Note: An electronic version of this open circuit characteristic can be found in file

p61_occ.dat, which can be used with MATLAB programs. Column 1

contains field current in amps, and column 2 contains open-circuit terminal

voltage in volts.

153

S

OLUTION

(a) At full load, the input power to the motor is

CUcoremechOUTIN PPPPP +++=

We can’t know the copper losses until the armature current is known, so we will find the input power and

armature current ignoring that term, and then correct the input power after we know it.

()( )

IN 1000 hp 746 W/hp 24 kW 18 kW 788 kWP=++=

Therefore, the line and phase current at unity power factor is

()()

788 kW 198 A

3 PF 3 2300 V 1.0

AL

T

P

II V

== = =

The copper losses due to a current of 198 A are

()()

2

CU 3 3 198 A 0.4 47.0 kW

AA

PIR== Ω=

Therefore, a better estimate of the input power at full load is

()( )

IN 1000 hp 746 W/hp 24 kW 18 kW 47 kW 835 kWP=+++=

and a better estimate of the line and phase current at unity power factor is

154

()()

835 kW 210 A

3 PF 3 2300 V 1.0

AL

T

P

II V

== = =

The phasor diagram of this motor operating a unity power factor is shown below:

jX

S

I

A

V

φ

I

A

E

A

I

A

R

A

The phase voltage of this motor is 2300 / 3 = 1328 V. The required internal generated voltage is

AAASA

RjX

φ

=− −EV I I

()( )()( )

1328 0 V 0.4 210 0 A 2.8 210 0 A

Aj=∠°−Ω∠°− Ω∠°E

1376 25.3 V

A=∠−°E

This internal generated voltage corresponds to a terminal voltage of

()

3 1376 2383 V=. This voltage

would require a field current of 4.6 A.

(b) The motor’s efficiency at full load and unity power factor is

OUT

IN

746 kW

100% 100% 89.3%

835 kW

P

η

=× = × =

(c) To solve this problem, we will temporarily ignore the effects of the armature resistance A

R. If A

R is

ignored, then

δ

sin

A

E is directly proportional to the power supplied by the motor. Since the power

supplied by the motor does not change when F

I is changed, this quantity will be a constant.

If the field current is increased by 5%, then the new field current will be 4.83 A, and the new value of

the open-circuit terminal voltage will be 2450 V. The new value of A

E will be 2450 V / 3 = 1415 V.

Therefore, the new torque angle

δ

will be

()

11

1

21

2

1376 V

sin sin sin sin 25.3 24.6

1415 V

A

E

δδ

−−

== −°=−°

Therefore, the new armature current will be

1328 0 V 1415 -25.3 V 214.5 3.5 A

0.4 2.8

A

AS

RjX j

φ

−∠° − ∠ °

== =∠°

++Ω

VE

I

The new current is about the same as before, but the phase angle has become positive. The new power

factor is cos 3.5° = 0.998 leading, and the reactive power supplied by the motor is

(

)

(

)

(

)

3 sin 3 2300 V 214.5 A sin 3.5 52.2 kVAR

TL

QVI

θ

== °=

(d) The maximum torque possible at unity power factor (ignoring the effects of A

R) is:

(

)

(

)

() ()

ind,max

3 3 1328 V 1376 V 5193 N m

1 min 2 rad

3600 r/min 2.8

60 s 1 r

A

mS

VE

X

φ

τπ

ω

== =⋅

Ω

155

If we are ignoring the resistance of the motor, then the input power would be 788 kW (note that copper

losses are ignored!). At a power factor of 0.8 leading, the current flow will be

()()

788 kW 247 A

3 PF 3 2300 V 0.8

AL

T

P

II V

== = =

so

247 36.87 A

A=∠ °I. The internal generated voltage at 0.8 PF leading (ignoring copper losses) is

AAASA

RjX

φ

=− −EV I I

()( )

1328 0 V 2.8 247 36.87 A

Aj=∠°− Ω ∠ °E

1829 17.6 V

A=∠−°E

Therefore, the maximum torque at a power factor of 0.8 leading is

(

)

(

)

() ()

ind,max

3 3 1328 V 1829 V 6093 N m

1 min 2 rad

3600 r/min 2.8

60 s 1 r

A

mS

VE

X

φ

τπ

ω

== =⋅

Ω

6-4. Plot the V-curves ( IA versus IF) for the synchronous motor of Problem 6-3 at no-load, half-load, and full-

load conditions. (Note that an electronic version of the open-circuit characteristics in Figure P6-1 is

available at the book’s Web site. It may simplify the calculations required by this problem. Also, you may

assume that A

R is negligible for this calculation.)

S

OLUTION The input power at no-load, half-load and full-load conditions is given below. Note that we are

assuming that A

R is negligible in each case.

IN,nl 24 kW 18 kW 42 kWP=+ =

()( )

IN,half 500 hp 746 W/hp 24 kW 18 kW 373 kWP=++=

()( )

IN,full 1000 hp 746 W/hp 24 kW 18 kW 788 kWP=++=

If the power factor is adjusted to unity, then armature currents will be

()()

,nl

42 kW 10.5 A

3 PF 3 2300 V 1.0

A

T

P

IV

== =

()()

,fl

373 kW 93.6 A

3 PF 3 2300 V 1.0

A

T

P

IV

== =

()()

,fl

788 kW 198 A

3 PF 3 2300 V 1.0

A

T

P

IV

== =

The corresponding internal generated voltages at unity power factor are:

ASA

jX

φ

=−EV I

(

)

(

)

,nl 1328 0 V 2.8 10.5 0 A 1328.3 1.27 V

Aj=∠°− Ω ∠°= ∠−°E

()( )

,half 1328 0 V 1.5 93.6 0 A 1354 11.2 V

Aj=∠°− Ω ∠°=∠−°E

(

)

(

)

,full 1328 0 V 2.8 198 0 A 1439 22.7 V

Aj=∠°− Ω∠°=∠−°E

These values of

A

E and

δ

at unity power factor can serve as reference points in calculating the

synchronous motor V-curves. The MATLAB program to solve this problem is shown below:

156

% M-file: prob6_4.m

% M-file create a plot of armature current versus field

% current for the synchronous motor of Problem 6-4 at

% no-load, half-load, and full-load.

% First, initialize the field current values (21 values

% in the range 3.8-5.8 A)

If = 2.5:0.1:8;

% Get the OCC

load p61_occ.dat;

if_values = p61_occ(:,1);

vt_values = p61_occ(:,2);

% Now initialize all other values

Xs = 1.5; % Synchronous reactance

Vp = 1328; % Phase voltage

% The following values of Ea and delta are for unity

% power factor. They will serve as reference values

% when calculating the V-curves.

d_nl = -1.27 * pi/180; % delta at no-load

d_half = -11.2 * pi/180; % delta at half-load

d_full = -22.7 * pi/180; % delta at full-load

Ea_nl = 1328.3; % Ea at no-load

Ea_half = 1354; % Ea at half-load

Ea_full = 1439; % Ea at full-load

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% Calculate the actual Ea corresponding to each level

% of field current

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

Ea = interp1(if_values,vt_values,If) / sqrt(3);

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% Calculate the armature currents associated with

% each value of Ea for the no-load case.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% First, calculate delta.

delta = asin ( Ea_nl ./ Ea .* sin(d_nl) );

% Calculate the phasor Ea

Ea2 = Ea .* (cos(delta) + j .* sin(delta));

% Now calculate Ia

Ia_nl = ( Vp - Ea2 ) / (j * Xs);

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% Calculate the armature currents associated with

% each value of Ea for the half-load case.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% First, calculate delta.

delta = asin ( Ea_half ./ Ea .* sin(d_half) );

% Calculate the phasor Ea

Ea2 = Ea .* (cos(delta) + j .* sin(delta));

157

% Now calculate Ia

Ia_half = ( Vp - Ea2 ) / (j * Xs);

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% Calculate the armature currents associated with

% each value of Ea for the full-load case.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% First, calculate delta.

delta = asin ( Ea_full ./ Ea .* sin(d_full) );

% Calculate the phasor Ea

Ea2 = Ea .* (cos(delta) + j .* sin(delta));

% Now calculate Ia

Ia_full = ( Vp - Ea2 ) / (j * Xs);

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% Plot the v-curves

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

plot(If,abs(Ia_nl),'k-','Linewidth',2.0);

hold on;

plot(If,abs(Ia_half),'b--','Linewidth',2.0);

plot(If,abs(Ia_full),'r:','Linewidth',2.0);

xlabel('\bfField Current (A)');

ylabel('\bfArmature Current (A)');

title ('\bfSynchronous Motor V-Curve');

grid on;

The resulting plot is shown below. The flattening visible to the right of the V-curves is due to magnetic

saturation in the machine.

6-5. If a 60-Hz synchronous motor is to be operated at 50 Hz, will its synchronous reactance be the same as at

60 Hz, or will it change? (Hint: Think about the derivation of XS.)

158

S

OLUTION The synchronous reactance represents the effects of the armature reaction voltage stat

E and the

armature self-inductance. The armature reaction voltage is caused by the armature magnetic field S

B, and

the amount of voltage is directly proportional to the speed with which the magnetic field sweeps over the

stator surface. The higher the frequency, the faster S

B sweeps over the stator, and the higher the armature

reaction voltage stat

E is. Therefore, the armature reaction voltage is directly proportional to frequency.

Similarly, the reactance of the armature self-inductance is directly proportional to frequency, so the total

synchronous reactance XS is directly proportional to frequency. If the frequency is changed from 60 Hz to

50 Hz, the synchronous reactance will be decreased by a factor of 5/6.

6-6. A 480-V 100-kW 0.85-PF leading 50-Hz six-pole Y-connected synchronous motor has a synchronous

reactance of 1.5 Ω and a negligible armature resistance. The rotational losses are also to be ignored. This

motor is to be operated over a continuous range of speeds from 300 to 1000 r/min, where the speed changes

are to be accomplished by controlling the system frequency with a solid-state drive.

(a) Over what range must the input frequency be varied to provide this speed control range?

(b) How large is EA at the motor’s rated conditions?

(c) What is the maximum power the motor can produce at the rated conditions?

(d) What is the largest EA could be at 300 r/min?

(e) Assuming that the applied voltage Vφ is derated by the same amount as EA, what is the maximum

power the motor could supply at 300 r/min?

(f) How does the power capability of a synchronous motor relate to its speed?

S

OLUTION

(a) A speed of 300 r/min corresponds to a frequency of

(

)

(

)

300 r/min 6 15 Hz

120 120

m

e

nP

f== =

A speed of 1000 r/min corresponds to a frequency of

()()

1000 r/min 6 50 Hz

120 120

m

e

nP

f== =

The frequency must be controlled in the range 15 to 50 Hz.

(b) The armature current at rated conditions is

()()

100 kW 141.5 A

3 PF 3 480 V 0.85

AL

T

P

II V

== = =

so 141.5 31.8 A

A=∠°I. This machine is Y-connected, so the phase voltage is V

φ

= 480 / 3 = 277 V.

The internal generated voltage is

AAASA

RjX

φ

=− −EV I I

()( )

277 0 V 1.5 141.5 31.8 A

Aj=∠°− Ω ∠°E

429 24.9 V

A=∠− °E

So

A

E = 429 V at rated conditions.

(c) The maximum power that the motor can produce at rated speed with the value of A

E from part (b) is

159

()()

max

3 3 277 V 429 V 238 kW

1.5

A

S

VE

PX

φ

== =

Ω

(d) Since A

E must be decreased linearly with frequency, the maximum value at 300 r/min would be

()

,300

15 Hz 429 V 129 V

50 Hz

A

E

==

(e) If the applied voltage

φ

V is derated by the same amount as A

E, then V

φ

= (15/50)(277) = 83.1 V.

Also, note that S

X = (15/50)(1.5 Ω) = 0.45 Ω. The maximum power that the motor could supply would

be

()()

max

3 3 83.1 V 129 V 71.5 kW

0.45

A

S

VE

PX

φ

== =

Ω

(f) As we can see by comparing the results of (c) and (e), the power-handling capability of the

synchronous motor varies linearly with the speed of the motor.

6-7. A 208-V Y-connected synchronous motor is drawing 40 A at unity power factor from a 208-V power

system. The field current flowing under these conditions is 2.7 A. Its synchronous reactance is 0.8 Ω.

Assume a linear open-circuit characteristic.

(a) Find the torque angle

δ

.

(b) How much field current would be required to make the motor operate at 0.8 PF leading?

(c) What is the new torque angle in part (b)?

S

OLUTION

(a) The phase voltage of this motor is V

φ

= 120 V, and the armature current is 40 0 A

A=∠°I.

Therefore, the internal generated voltage is

AAASA

RjX

φ

=− −EV I I

()( )

120 0 V 0.8 40 0 A

Aj=∠°− Ω ∠°E

124 14.9 V

A=∠− °E

The torque angle

δ

of this machine is –14.9°.

(b) A phasor diagram of the motor operating at a power factor of 0.78 leading is shown below.

V

φ

E

A

1

jX

S

I

A

E

A

2

I

A

2

I

A

1

}

∝

P

}

∝

P

Since the power supplied by the motor is constant, the quantity

θ

cos

A

I, which is directly proportional to

power, must be constant. Therefore,

()( )( )

20.8 40 A 1.00

A

I=

160

250 36.87 A

A=∠ °I

The internal generated voltage required to produce this current would be

222AAASA

RjX

φ

=− −EV I I

()( )

2120 0 V 0.8 50 36.87 A

Aj=∠°− Ω ∠ °E

2147.5 12.5 V

A=∠−°E

The internal generated voltage A

E is directly proportional to the field flux, and we have assumed in this

problem that the flux is directly proportional to the field current. Therefore, the required field current is

()

2

21

1

147 V 2.7 A 3.20 A

124 V

A

FF

A

E

II

E

== =

(c) The new torque angle

δ

of this machine is –12.5°.

6-8. A synchronous machine has a synchronous reactance of 2.0 Ω per phase and an armature resistance of 0.4

Ω per phase. If EA =460∠-8° V and

φ

V = 480∠0° V, is this machine a motor or a generator? How

much power P is this machine consuming from or supplying to the electrical system? How much reactive

power Q is this machine consuming from or supplying to the electrical system?

S

OLUTION This machine is a motor, consuming power from the power system, because A

E is lagging

φ

V.

It is also consuming reactive power, because cos

A

EV

φ

δ

<. The current flowing in this machine is

480 0 V 460 8 V 33.6 9.6 A

0.4 2.0

A

AS

RjX j

φ

−∠° − ∠−°

== =∠−°

++Ω

VE

I

Therefore the real power consumed by this motor is

()( )()

3 cos 3 480 V 33.6 A cos 9.6 47.7 kW

A

PVI

φ

θ

== °=

and the reactive power consumed by this motor is

()( )()

3 sin 3 480 V 33.6 A sin 9.6 8.07 kVAR

A

QVI

φ

θ

== °=

6-9. Figure P6-2 shows a synchronous motor phasor diagram for a motor operating at a leading power factor

with no RA. For this motor, the torque angle is given by

cos

tan = sin

SA

XI

VXI

φ

θ

δθ

+

-1 cos

=tan sin

SA

XI

VXI

φ

θ

δθ

+

Derive an equation for the torque angle of the synchronous motor if the armature resistance is included.

161

S

OLUTION The phasor diagram with the armature resistance considered is shown below.

jX

S

I

A

V

φ

I

A

E

A

I

A

R

A

I

A

R

A

cos

θ

X

S

I

A

sin

θ

}

X

S

I

A

cos

θ

}

δ

Therefore,

cos sin

tan sin cos

SA AA

XI RI

VXI RI

φ

θθ

δθθ

+

=+−

1cos sin

tan sin cos

SA AA

XI RI

VXI RI

φ

θθ

δθθ

−

+

=

+−

6-10. A 480-V 375-kVA 0.8-PF-lagging Y-connected synchronous generator has a synchronous reactance of 0.4

Ω and a negligible armature resistance. This generator is supplying power to a 480-V 80-kW 0.8-PF-

leading Y-connected synchronous motor with a synchronous reactance of 1.1 Ω and a negligible armature

resistance. The synchronous generator is adjusted to have a terminal voltage of 480 V when the motor is

drawing the rated power at unity power factor.

(a) Calculate the magnitudes and angles of A

E for both machines.

(b) If the flux of the motor is increased by 10 percent, what happens to the terminal voltage of the power

system? What is its new value?

(c) What is the power factor of the motor after the increase in motor flux?

S

OLUTION

(a) The motor is operating at rated power and unity power factor, so the current flowing in the motor is

()()

,m ,m

80 kW 96.2 A

3 PF 3 480 V 1.0

AL

T

P

II V

== = =

so

,m 96.2 0 A

A=∠°I. This machine is Y-connected, so the phase voltage is V

φ

= 480 / 3 = 277 V. The

internal generated voltage of the motor is

162

,m ,m ,mASA

jX

φ

=−EV I

()( )

,m 277 0 V 1.1 96.2 0 A

Aj=∠°− Ω ∠°E

,m 297 20.9 V

A=∠− °E

This same current comes from the generator, so the internal generated voltage of the generator is

,g ,g ,gASA

jX

φ

=+EV I

()( )

,g 277 0 V 0.4 96.2 0 A

Aj=∠°+ Ω ∠°E

,g 280 7.9 V

A=∠°E

+

-

E

A,g

j

0.4

Ω

+

-

V

φ

,m

I

A,g

+

-

I

A,m

E

A,m

j

1.1

Ω

+

-

V

φ

,g

I

A

jX

S,m

V

φ

I

A

E

A,m

jX

S,g

I

A

V

φ

I

A

E

A,g

Generator Motor

(b) The power supplied by the generator to the motor will be constant as the field current of the motor is

varied. The 10% increase in flux will raise the internal generated voltage of the motor to (1.1)(297 V) =

327 V.

To make finding the new conditions easier, we will make the angle of the phasor ,Ag

E the reference during

the following calculations. The resulting phasor diagram is shown below.

I

A

jX

S,m

V

φ

I

A

E

A

,

m

jX

S,g

I

A

E

A,g

δ

m

δ

g

Then by Kirchhoff’s Voltage Law,

,, ,,

()

Ag Am S g Sm A

jX X=+ +EE I

163

or ,,

,,

()

Ag Am

A

Sg Sm

jX X

−

=+

EE

I

Note that this combined phasor diagram looks just like the diagram of a synchronous motor, so we can

apply the power equation for synchronous motors to this system.

,,

3sin

Ag Am

Sg Sm

EE

PXX

γ

=+

where

gm

γ

δδ

=+. From this equation,

()

()( )

,,

11

,,

1.5 80 kW

sin sin 25.9

3 3 280V 327 V

Sg Sm

Ag Am

XXP

EE

γ

−−

+Ω

== =°

Therefore,

,,

280 0 V 327 25.9 V 95.7 5.7 A

() 1.5

Ag Am

A

Sg Sm

jX X j

−∠° − ∠− °

== =∠°

+Ω

EE

I

The phase voltage of the system would be

(

)

(

)

,,

280 0 V 0.4 95.7 5.7 A 286 7.6 V

Ag Sg A

jX j

φ

=− =∠°− Ω ∠°=∠−°VE I

If we make

φ

V the reference (as we usually do), these voltages and currents become:

,280 7.6 V

Ag =∠°E

286 0 V

φ

=∠°V

,327 18.3 V

Am =∠−°E

95.7 13.3 A

A=∠°I

The new terminal voltage is

()

3286 V 495 V

T

V==

, so the system voltage has increased.

(c) The power factor of the motor is now

()

PF cos 13.3 0.973 leading=−°= , since a current angle of

-18.3° implies an impedance angle of 18.3°.

Note: The reactive power in the motor is now

()( )( )

motor 3 sin 3 286 V 95.7 A sin 13.3 18.9 kVAR

A

QVI

φ

θ

== −°=−

The motor is now supplying 18.9 kVAR to the system. Note that an increase in machine flux has

increased the reactive power supplied by the motor and also raised the terminal voltage of the system.

This is consistent with what we learned about reactive power sharing in Chapter 5.

6-11. A 480-V, 100-kW, 50-Hz, four-pole, Y-connected synchronous motor has a rated power factor of 0.85

leading. At full load, the efficiency is 91 percent. The armature resistance is 0.08 Ω, and the synchronous

reactance is 1.0 Ω. Find the following quantities for this machine when it is operating at full load:

(a) Output torque

(b) Input power

(c) m

n

(d) A

E

164

(e) A

I

(f) conv

P

(g) mech core stray

PPP++

S

OLUTION

(a) Since this machine has 8 poles, it rotates at a speed of

(

)

120 50 Hz

120 1500 r/min

4

e

m

f

nP

== =

If the output power is 100 kW, the output torque is

()

out

load

m

100,000 W 637 N m

2 rad 1 min

1500 r/min 1 r 60 s

P

τπ

ω

== = ⋅

(b) The input power is

OUT

IN

100 kW 110 kW

0.91

P

η

== =

(c) The mechanical speed is

1500 r/min

m

n=

(d) The armature current is

()()

110 kW 156 A

3 PF 3 480 V 0.85

AL

T

P

II V

== = =

156 31.8 A

A=∠°I

Therefore,

A

E is

AAASA

RjX

φ

=− −EV I I

()()()()()

277 0 V 0.08 156 31.8 A 1.0 156 31.8 A

Aj=∠°− Ω∠°− Ω∠°E

375 21.8 V

A=∠− °E

(e) The magnitude of the armature current is 375 A.

(f) The power converted from electrical to mechanical form is given by the equation conv IN CU

PPP=−

()( )

2

CU 3 3 156 A 0.08 5.8 kW

AA

PIR== Ω=

conv IN CU 110 kW 5.8 kW 104.2 kWPPP=− = − =

(g) The mechanical, core, and stray losses are given by the equation

mech core stray conv OUT 104.2 kW 100 kW 4.2 kWPPPPP++=−= − =

6-12. The Y-connected synchronous motor whose nameplate is shown in Figure 6-21 has a per-unit synchronous

reactance of 0.90 and a per-unit resistance of 0.02.

(a) What is the rated input power of this motor?

(b) What is the magnitude of A

E at rated conditions?

165

(c) If the input power of this motor is 10 MW, what is the maximum reactive power the motor can

simultaneously supply? Is it the armature current or the field current that limits the reactive power

output?

(d) How much power does the field circuit consume at the rated conditions?

(e) What is the efficiency of this motor at full load?

(f) What is the output torque of the motor at the rated conditions? Express the answer both in newton-

meters and in pound-feet.

S

OLUTION The base quantities for this motor are:

,base 6600 V

T

V=

,base

6600 V 3811 V

3

V

φ

==

,base ,base 1404 A

AL

II==

()()()

base rated 3 PF 3 6600 V 1404 A 1.0 16.05 MW

TL

SP VI== = =

(a) The rated input power of this motor is

()()()

IN 3 PF 3 6600 V 1404 A 1.0 16.05 MW

TL

PVI== =

(b) At rated conditions, 1.0 0 pu

φ

=∠°V and 1.0 0 pu

φ

=∠°I, so A

E is given in per-unit quantities as

AAASA

RjX

φ

=− −EV I I

()()( )()()

1 0 0.02 1.0 0 0.90 1 0

Aj=∠°− ∠°− ∠°E

1.33 42.6 pu

A=∠−°E

The base phase voltage of this motor is 6600 / 3= 3810 V, so A

E is

()()

1.33 42.6 3810 V 5067 42.6 V

A=∠−° =∠−°E

(c) From the capability diagram, we know that there are two possible constraints on the maximum

reactive power—the maximum stator current and the maximum rotor current. We will have to check each

one separately, and limit the reactive power to the lesser of the two limits.

The stator apparent power limit defines a maximum safe stator current. This limit is the same as the rated

input power for this motor, since the motor is rated at unity power factor. Therefore, the stator apparent

166

power limit is 16.05 MVA. If the input power is 10 MW, then the maximum reactive power that still

protects the stator current is

()()

22

22 16.05 MVA 10 MW 12.6 MVARQSP=−= − =

Now we must determine the rotor current limit. The per-unit power supplied to the motor is 10 MW /

16.05 MW = 0.623. The maximum A

E is 5067 V or 1.33 pu, so with A

E set to maximum and the motor

consuming 10 MW, the torque angle (ignoring armature resistance) is

()( )

11

0.90 0.623

sin sin 24.9

3 1.0 1.33

S

A

XP

VE

φ

δ

−−

== =°

At rated voltage and 10 MW of power supplied, the armature current will be

1 0 1.33 24.9 0.663 20.2 pu

0.90

A

AS

RjX j

φ

−∠°− ∠− °

== =∠°

+

VE

I

In actual amps, this current is

()( )

1404 A 0.663 20.2 931 20.2 A

A=∠°=∠°I

The reactive power supplied at the conditions of maximum A

E and 10 MW power is

()()()

3 sin 3 3811 V 931 A sin 20.2 3.68 MVAR

A

QVI

φ

θ

== °=

Therefore, the field current limit occurs before the stator current limit for these conditions, and the

maximum reactive power that the motor can supply is 3.68 MVAR under these conditions.

(d) At rated conditions, the field circuit consumes

()()

field 125 V 5.2 A 650 W

FF

PVI== =

(e) The efficiency of this motor at full load is

(

)

(

)

OUT

IN

21000 hp 746 W/hp

100% 100% 97.6%

16.05 MW

P

η

=× = × =

(f) The output torque in SI and English units is

()( )

()

OUT

load

21000 hp 746 W/hp 124,700 N m

1 min 2 rad

1200 r/min 60 s 1 r

m

P

τπ

ω

== = ⋅

()

load

5252 21000 hp

5252 91,910 lb ft

1200 r/min

m

P

n

τ

== = ⋅

6-13. A 440-V three-phase Y-connected synchronous motor has a synchronous reactance of 1.5 Ω per phase.

The field current has been adjusted so that the torque angle

δ

is 28° when the power supplied by the

generator is 90 kW.

(a) What is the magnitude of the internal generated voltage EA in this machine?

(b) What are the magnitude and angle of the armature current in the machine? What is the motor’s power

factor?

(c) If the field current remains constant, what is the absolute maximum power this motor could supply?

167

S

OLUTION

(a) The power supplied to the motor is 90 kW. This power is give by the equation

3sin

A

S

VE

PX

φ

δ

=

so the magnitude of A

E is

()( )

()

1.5 90 kW

377 V

3 sin 3 254 V sin 28

S

A

XP

EV

φ

δ

Ω

== =

°

(b) The armature current in this machine is given by

254 0 V 377 28 129 24 A

1.5

A

S

jX j

φ

−∠° − ∠− °

== =∠°

VE

I

The power factor of the motor is PF = cos 24º = 0.914 leading.

(c) The maximum power that the motor could supply at this field current

()()

max

33 254 V 377 V 191.5 kW

1.5

A

S

VE

PX

φ

== =

Ω

6-14. A 460-V, 200-kVA, 0.80-PF-leading, 400-Hz, six-pole, Y-connected synchronous motor has negligible

armature resistance and a synchronous reactance of 0.50 per unit. Ignore all losses.

(a) What is the speed of rotation of this motor?

(b) What is the output torque of this motor at the rated conditions?

(c) What is the internal generated voltage of this motor at the rated conditions?

(d) With the field current remaining at the value present in the motor in part (c), what is the maximum

possible output power from the machine?

S

OLUTION

(a) The speed of rotation of this motor is

()

sync

120 400 Hz

120 8000 r/min

6

e

f

nP

== =

(b) Since all losses are ignored,

()()

,rated ,rated rated PF 200 kVA 0.8 160 kW

IN OUT

PP S==×= =. The output

torque of this motor is

()

OUT

load

160 kW 191 N m

1 min 2 rad

8000 r/min 60 s 1 r

m

P

τπ

ω

== = ⋅

(c) The phase voltage of this motor is 460 V / 3 = 266 V. The rated armature current of this motor is

()()

160 kW 251 A

3 PF 3 460 V 0.80

AL

T

P

II V

== = =

Therefore, 251 36.87 A

A=∠ °I. The base impedance of this motor is

()

2

,base

base

33 266 V 1.06

200,000 VA

V

ZS

φ

== =Ω

168

so the actual synchronous reactance is

()()

0.50 pu 1.06 0.53

S

X=Ω=Ω. The internal generated voltage

of this machine at rated conditions is given by

ASA

jX

φ

=−EV I

()( )

266 0 V 0.53 251 36.87 A 362 17.1 V

Aj=∠°− Ω ∠ °=∠−°E

(d) The maximum power that the motor could supply at these conditions is

(

)

(

)

MAX

33 266 V 362 V 545 kW

0.53

A

S

VE

PX

φ

== =

Ω

6-15. A 100-hp 440-V 0.8-PF-leading ∆-connected synchronous motor has an armature resistance of 0.22 Ω and

a synchronous reactance of 3.0 Ω. Its efficiency at full load is 89 percent.

(a) What is the input power to the motor at rated conditions?

(b) What is the line current of the motor at rated conditions? What is the phase current of the motor at

rated conditions?

(c) What is the reactive power consumed by or supplied by the motor at rated conditions?

(d) What is the internal generated voltage EA of this motor at rated conditions?

(e) What are the stator copper losses in the motor at rated conditions?

(f) What is

P

conv at rated conditions?

(g) If EA is decreased by 10 percent, how much reactive power will be consumed by or supplied by the

motor?

S

OLUTION

(a) The input power to the motor at rated conditions is

()( )

OUT

IN

100 hp 746 W/hp 83.8 kW

0.89

P

η

== =

(b) The line current to the motor at rated conditions is

()()

83.8 kW 137 A

3 PF 3 440 V 0.8

L

T

P

IV

== =

The phase current to the motor at rated conditions is

137 A 79.4 A

33

L

I

φ

== =

(c) The reactive power supplied by this motor to the power system at rated conditions is

()( )

rated 3 sin 3 440 V 79.4 A sin36.87 62.9 kVAR

A

QVI

φ

θ

== °=

(d) The internal generated voltage at rated conditions is

AAASA

RjX

φ

=− −EV I I

()()()()

440 0 V 0.22 79.4 36.87 A 3.0 79.4 36.87 A

Aj=∠°−Ω∠°−Ω∠°E

603 19.5 V

A=∠−°E

(e) The stator copper losses at rated conditions are

169

()()

2

CU 3 3 79.4 A 0.22 4.16 kW

AA

PIR== Ω=

(f) conv

P at rated conditions is

conv IN CU 83.8 kW 4.16 kW 79.6 kWPPP=− = − =

(g) If A

E is decreased by 10%, the new value if A

E = (0.9)(603 V) = 543 V. To simplify this part of the

problem, we will ignore A

R. Then the quantity sin

A

E

δ

will be constant as A

E changes. Therefore,

()

11

1

21

2

603 V

sin sin sin sin 19.5 21.8

543 V

A

E

δδ

−−

==−°=−°

Therefore,

440 0 V 543 21.8 70.5 17.7 A

3.0

A

S

jX j

φ

−∠° − ∠− °

== =∠°

VE

I

and the reactive power supplied by the motor to the power system will be

()( )()

3 sin 3 440 V 70.5 A sin 17.7 28.3 kVAR

A

QVI

φ

θ

== °=

6-16. Answer the following questions about the machine of Problem 6-15.

(a) If A

E = 430∠13.5° V and φ

V = 440∠0° V, is this machine consuming real power from or supplying

real power to the power system? Is it consuming reactive power from or supplying reactive power to

the power system?

(b) Calculate the real power P and reactive power Q supplied or consumed by the machine under the

conditions in part (a). Is the machine operating within its ratings under these circumstances?

(c) If A

E = 470∠-12° V and φ

V = 440∠0° V, is this machine consuming real power from or supplying

real power to the power system? Is it consuming reactive power from or supplying reactive power to

the power system?

(d) Calculate the real power P and reactive power Q supplied or consumed by the machine under the

conditions in part (c). Is the machine operating within its ratings under these circumstances?

SOLUTION

(a) This machine is a generator supplying real power to the power system, because A

E is ahead of

φ

V.

It is consuming reactive power because cos

A

EV

φ

δ

<.

(b) This machine is acting as a generator, and the current flow in these conditions is

430 13.5 440 0 V 34.2 16.5 A

0.22 3.0

A

AS

RjX j

φ

−∠°−∠°

== =∠°

++

EV

I

The real power supplied by this machine is

()( )( )

3 cos 3 440 V 34.2 A cos 16.5 43.3 kW

A

PVI

φ

θ

== −°=

The reactive power supplied by this machine is

()( )( )

3 sin 3 440 V 34.2 A sin 16.5 12.8 kVAR

A

QVI

φ

θ

== −°=−

170

(c) This machine is a motor consuming real power from the power system, because A

E is behind

φ

V. It

is supplying reactive power because cos

A

EV

φ

δ

>.

(d) This machine is acting as a motor, and the current flow in these conditions is

440 0 V 470 12 33.1 15.6 A

0.22 3.0

A

AS

RjX j

φ

−∠° − ∠− °

== =∠°

++

VE

I

The real power consumed by this machine is

()()()

3 cos 3 440 V 33.1 A cos 15.6 42.1 kW

A

PVI

φ

θ

== °=

The reactive power supplied by this machine is

()()()

3 sin 3 440 V 33.1 A sin 15.6 11.7 kVAR

A

QVI

φ

θ

== °=+

171

Chapter 7: Induction Motors

7-1. A dc test is performed on a 460-V ∆-connected 100-hp induction motor. If DC

V = 24 V and DC

I = 80 A,

what is the stator resistance 1

R? Why is this so?

S

OLUTION If this motor’s armature is connected in delta, then there will be two phases in parallel with one

phase between the lines tested.

R

1

R

1

R

1

V

DC

Therefore, the stator resistance 1

R will be

()

11 1

DC

1

DC 1 1 1

2

3

RR R

VR

IRRR

+

==

++

DC

1

DC

3324 V

0.45

2280 A

V

RI

== =Ω

7-2. A 220-V, three-phase, two-pole, 50-Hz induction motor is running at a slip of 5 percent. Find:

(a) The speed of the magnetic fields in revolutions per minute

(b) The speed of the rotor in revolutions per minute

(c) The slip speed of the rotor

(d) The rotor frequency in hertz

S

OLUTION

(a) The speed of the magnetic fields is

()

sync

120 50 Hz

120 3000 r/min

2

e

f

nP

== =

(b) The speed of the rotor is

() ( )( )

sync

1 1 0.05 3000 r/min 2850 r/min

m

nsn=− =− =

(c) The slip speed of the rotor is

()( )

slip sync 0.05 3000 r/min 150 r/minnsn== =

(d) The rotor frequency is

()()

slip 150 r/min 2 2.5 Hz

120 120

r

nP

f== =

7-3. Answer the questions in Problem 7-2 for a 480-V, three-phase, four-pole, 60-Hz induction motor running at

a slip of 0.035.

S

OLUTION

(a) The speed of the magnetic fields is

172

(

)

sync

120 60 Hz

120 1800 r/min

4

e

f

nP

== =

(b) The speed of the rotor is

() ( )( )

sync

1 1 0.035 1800 r/min 1737 r/min

m

nsn=− =− =

(c) The slip speed of the rotor is

()( )

slip sync 0.035 1800 r/min 63 r/minnsn== =

(d) The rotor frequency is

()()

slip 63 r/min 4 2.1 Hz

120 120

r

nP

f== =

7-4. A three-phase, 60-Hz induction motor runs at 890 r/min at no load and at 840 r/min at full load.

(a) How many poles does this motor have?

(b) What is the slip at rated load?

(c) What is the speed at one-quarter of the rated load?

(d) What is the rotor’s electrical frequency at one-quarter of the rated load?

SOLUTION

(a) This machine has 8 poles, which produces a synchronous speed of

()

sync

120 60 Hz

120 900 r/min

8

e

f

nP

== =

(b) The slip at rated load is

sync

900 840

100% 100% 6.67%

900

m

nn

sn

−−

=×= ×=

(c) The motor is operating in the linear region of its torque-speed curve, so the slip at ¼ load will be

0.25(0.0667) 0.0167s==

The resulting speed is

() ( )( )

sync

1 1 0.0167 900 r/min 885 r/min

m

nsn=− =− =

(d) The electrical frequency at ¼ load is

()()

0.0167 60 Hz 1.00 Hz

re

fsf== =

7-5. A 50-kW, 440-V, 50-Hz, six-pole induction motor has a slip of 6 percent when operating at full-load

conditions. At full-load conditions, the friction and windage losses are 300 W, and the core losses are 600

W. Find the following values for full-load conditions:

(a) The shaft speed nm

(b) The output power in watts

(c) The load torque load

τ

in newton-meters

(d) The induced torque ind

τ

in newton-meters

173

(e) The rotor frequency in hertz

S

OLUTION

(a) The synchronous speed of this machine is

(

)

sync

120 50 Hz

120 1000 r/min

6

e

f

nP

== =

Therefore, the shaft speed is

(

)

(

)

(

)

sync

1 1 0.06 1000 r/min 940 r/min

m

nsn=− =− =

(b) The output power in watts is 50 kW (stated in the problem).

(c) The load torque is

()

OUT

load

50 kW 508 N m

2 rad 1 min

940 r/min 1 r 60 s

m

P

τπ

ω

== = ⋅

(d) The induced torque can be found as follows:

conv OUT F&W core misc 50 kW 300 W 600 W 0 W 50.9 kWPPPPP=+++= + + +=

()

conv

ind

50.9 kW 517 N m

2 rad 1 min

940 r/min 1 r 60 s

m

P

τπ

ω

== = ⋅

(e) The rotor frequency is

()( )

0.06 50 Hz 3.00 Hz

re

fsf== =

7-6. A three-phase, 60-Hz, four-pole induction motor runs at a no-load speed of 1790 r/min and a full-load

speed of 1720 r/min. Calculate the slip and the electrical frequency of the rotor at no-load and full-load

conditions. What is the speed regulation of this motor [Equation (4-68)]?

S

OLUTION The synchronous speed of this machine is 1800 r/min. The slip and electrical frequency at no-

load conditions is

sync nl

nl

sync

1800 1790

100% 100% 0.56%

1800

nn

sn

−−

=×= ×=

()()

,nl 0.0056 60 Hz 0.33 Hz

re

fsf== =

The slip and electrical frequency at full load conditions is

sync nl

fl

sync

1800 1720

100% 100% 4.44%

1800

nn

sn

−−

=×= ×=

()()

,fl 0.0444 60 Hz 2.67 Hz

re

fsf== =

The speed regulation is

nl fl

fl

1790 1720

SR 100% 100% 4.1%

1720

nn

n

−−

=×= ×=

7-7. A 208-V, two-pole, 60-Hz Y-connected wound-rotor induction motor is rated at 15 hp. Its equivalent

circuit components are

174

1

R = 0.200 Ω 2

R = 0.120 Ω M

X = 15.0 Ω

1

X = 0.410 Ω 2

X = 0.410 Ω

mech

P = 250 W misc

P ≈ 0 core

P = 180 W

For a slip of 0.05, find

(a) The line current L

I

(b) The stator copper losses SCL

P

(c) The air-gap power AG

P

(d) The power converted from electrical to mechanical form conv

P

(e) The induced torque ind

τ

(f) The load torque load

τ

(g) The overall machine efficiency

(h) The motor speed in revolutions per minute and radians per second

S

OLUTION The equivalent circuit of this induction motor is shown below:

0.20 Ωj0.41 Ω

+

-

V

φ

IAR1jX1R2











−

s

R1

2

jX2

jXM

0.120 Ωj0.41 Ω

j15 Ω

2.28 Ω

(a) The easiest way to find the line current (or armature current) is to get the equivalent impedance F

Z

of the rotor circuit in parallel with M

jX , and then calculate the current as the phase voltage divided by the

sum of the series impedances, as shown below.

0.20 Ωj0.41 Ω

+

-

V

φ

IAR1jX1RF

jXF

The equivalent impedance of the rotor circuit in parallel with M

jX is:

2

11

2.220 0.745 2.34 18.5

11 1 1

15 2.40 0.41

F

M

Zj

jX Z j j

== =+=∠°Ω

++

Ω+

The phase voltage is 208/ 3 = 120 V, so line current L

I is

175

11

120 0 V

0.20 0.41 2.22 0.745

LA

FF

V

IIRjXR jX j j

φ

∠°

== =

+++ Ω+ Ω+ Ω+ Ω

44.8 25.5 A

LA

II== ∠− °

(b) The stator copper losses are

()()

2

SCL 1

3 3 44.8 A 0.20 1205 W

A

PIR== Ω=

(c) The air gap power is 22

2

AG 2

33

AF

R

PI IR

s

==

(Note that

2

3AF

IR

is equal to 22

2

3R

Is, since the only resistance in the original rotor circuit was 2

/

Rs, and

the resistance in the Thevenin equivalent circuit is F

R. The power consumed by the Thevenin equivalent

circuit must be the same as the power consumed by the original circuit.)

()( )

2

22

2

AG 2

3 3 3 44.8 A 2.220 13.4 kW

AF

R

PI IR

s

=== Ω=

(d) The power converted from electrical to mechanical form is

() ( )( )

conv AG

1 1 0.05 13.4 kW 12.73 kWPsP=− =− =

(e) The induced torque in the motor is

()

AG

ind

sync

13.4 kW 35.5 N m

2 rad 1 min

3600 r/min 1 r 60 s

P

τπ

ω

== = ⋅

(f) The output power of this motor is

OUT conv mech core misc 12.73 kW 250 W 180 W 0 W 12.3 kWPPPPP=−−−= − − − =

The output speed is

() ( )( )

sync

1 1 0.05 3600 r/min 3420 r/min

m

nsn=− =− =

Therefore the load torque is

()

OUT

load

12.3 kW 34.3 N m

2 rad 1 min

3420 r/min 1 r 60 s

m

P

τπ

ω

== = ⋅

(g) The overall efficiency is

OUT OUT

IN

100% 100%

3cos

A

PP

PVI

φ

ηθ

=× = ×

()( )

12.3 kW 100% 84.5%

3 120 V 44.8 A cos25.5

η

=×=

°

(h) The motor speed in revolutions per minute is 3420 r/min. The motor speed in radians per second is

()

2 rad 1 min

3420 r/min 358 rad/s

1 r 60 s

m

π

ω

==

7-8. For the motor in Problem 7-7, what is the slip at the pullout torque? What is the pullout torque of this

motor?

176

S

OLUTION The slip at pullout torque is found by calculating the Thevenin equivalent of the input circuit

from the rotor back to the power supply, and then using that with the rotor circuit model.

()

(

)

(

)

()

11

TH

11

15 0.20 0.41 0.1895 0.4016 0.444 64.7

0.20 0.41 15

M

jX R jX j j

Zj

RjXX j

+ΩΩ+Ω

== =+Ω=∠°Ω

++ Ω+ Ω+Ω

()

(

)

()

TH

11

15 120 0 V 116.8 0.7 V

0.22 0.43 15

M

j

jX

RjXX j

φ

Ω

== ∠°=∠°

++ Ω+ Ω+Ω

VV

The slip at pullout torque is

()

2

max 2

2

TH TH 2

R

s

RXX

=++

()( )

max 22

0.120 0.144

0.1895 0.4016 0.410

sΩ

==

Ω+ Ω+ Ω

The pullout torque of the motor is

()

2

TH

max 2

2

sync TH TH TH 2

3V

RRXX

τ

ω

=

2+++

()

() ()( )

2

max 22

3 116.8 V

377 rad/s 0.1895 0.1895 0.4016 0.410

τ

=

2Ω+Ω+Ω+Ω

177

max 53.1 N m

τ

=⋅

7-9. (a) Calculate and plot the torque-speed characteristic of the motor in Problem 7-7. (b) Calculate and plot

the output power versus speed curve of the motor in Problem 7-7.

S

OLUTION

(a) A MATLAB program to calculate the torque-speed characteristic is shown below.

% M-file: prob7_9a.m

% M-file create a plot of the torque-speed curve of the

% induction motor of Problem 7-7.

% First, initialize the values needed in this program.

r1 = 0.200; % Stator resistance

x1 = 0.410; % Stator reactance

r2 = 0.120; % Rotor resistance

x2 = 0.410; % Rotor reactance

xm = 15.0; % Magnetization branch reactance

v_phase = 208 / sqrt(3); % Phase voltage

n_sync = 3600; % Synchronous speed (r/min)

w_sync = 377; % Synchronous speed (rad/s)

% Calculate the Thevenin voltage and impedance from Equations

% 7-41a and 7-43.

v_th = v_phase * ( xm / sqrt(r1^2 + (x1 + xm)^2) );

z_th = ((j*xm) * (r1 + j*x1)) / (r1 + j*(x1 + xm));

r_th = real(z_th);

x_th = imag(z_th);

% Now calculate the torque-speed characteristic for many

% slips between 0 and 1. Note that the first slip value

% is set to 0.001 instead of exactly 0 to avoid divide-

% by-zero problems.

s = (0:1:50) / 50; % Slip

s(1) = 0.001;

nm = (1 - s) * n_sync; % Mechanical speed

% Calculate torque versus speed

for ii = 1:51

t_ind(ii) = (3 * v_th^2 * r2 / s(ii)) / ...

(w_sync * ((r_th + r2/s(ii))^2 + (x_th + x2)^2) );

end

% Plot the torque-speed curve

figure(1);

plot(nm,t_ind,'k-','LineWidth',2.0);

xlabel('\bf\itn_{m}');

ylabel('\bf\tau_{ind}');

title ('\bfInduction Motor Torque-Speed Characteristic');

grid on;

The resulting plot is shown below:

178

0500 1000 1500 2000 2500 3000 3500 4000

0

10

20

30

40

50

60

n

m

τ

ind

Induction Motor Torque-Speed Characteristic

(b) A MATLAB program to calculate the output-power versus speed curve is shown below.

% M-file: prob7_9b.m

% M-file create a plot of the output pwer versus speed

% curve of the induction motor of Problem 7-7.

% First, initialize the values needed in this program.

r1 = 0.200; % Stator resistance

x1 = 0.410; % Stator reactance

r2 = 0.120; % Rotor resistance

x2 = 0.410; % Rotor reactance

xm = 15.0; % Magnetization branch reactance

v_phase = 208 / sqrt(3); % Phase voltage

n_sync = 3600; % Synchronous speed (r/min)

w_sync = 377; % Synchronous speed (rad/s)

% Calculate the Thevenin voltage and impedance from Equations

% 7-41a and 7-43.

v_th = v_phase * ( xm / sqrt(r1^2 + (x1 + xm)^2) );

z_th = ((j*xm) * (r1 + j*x1)) / (r1 + j*(x1 + xm));

r_th = real(z_th);

x_th = imag(z_th);

% Now calculate the torque-speed characteristic for many

% slips between 0 and 1. Note that the first slip value

% is set to 0.001 instead of exactly 0 to avoid divide-

% by-zero problems.

s = (0:1:50) / 50; % Slip

s(1) = 0.001;

nm = (1 - s) * n_sync; % Mechanical speed (r/min)

wm = (1 - s) * w_sync; % Mechanical speed (rad/s)

% Calculate torque and output power versus speed

for ii = 1:51

179

t_ind(ii) = (3 * v_th^2 * r2 / s(ii)) / ...

(w_sync * ((r_th + r2/s(ii))^2 + (x_th + x2)^2) );

p_out(ii) = t_ind(ii) * wm(ii);

end

% Plot the torque-speed curve

figure(1);

plot(nm,p_out/1000,'k-','LineWidth',2.0);

xlabel('\bf\itn_{m} \rm\bf(r/min)');

ylabel('\bf\itP_{OUT} \rm\bf(kW)');

title ('\bfInduction Motor Ouput Power versus Speed');

grid on;

The resulting plot is shown below:

7-10. For the motor of Problem 7-7, how much additional resistance (referred to the stator circuit) would it be

necessary to add to the rotor circuit to make the maximum torque occur at starting conditions (when the

shaft is not moving)? Plot the torque-speed characteristic of this motor with the additional resistance

inserted.

S

OLUTION To get the maximum torque at starting, the max

s must be 1.00. Therefore,

()

2

max 2

2

TH TH 2

R

s

RXX

=++

()( )

2

22

1.00

0.1895 0.4016 0.410

R

=Ω+ Ω+ Ω

20.833 R=Ω

Since the existing resistance is 0.120 Ω, an additional 0.713 Ω must be added to the rotor circuit. The

resulting torque-speed characteristic is:

180

7-11. If the motor in Problem 7-7 is to be operated on a 50-Hz power system, what must be done to its supply

voltage? Why? What will the equivalent circuit component values be at 50 Hz? Answer the questions in

Problem 7-7 for operation at 50 Hz with a slip of 0.05 and the proper voltage for this machine.

S

OLUTION If the input frequency is decreased to 50 Hz, then the applied voltage must be decreased by 5/6

also. If this were not done, the flux in the motor would go into saturation, since

∫

=Tdtv

N

1

φ

and the period T would be increased. At 50 Hz, the resistances will be unchanged, but the reactances will

be reduced to 5/6 of their previous values. The equivalent circuit of the induction motor at 50 Hz is shown

below:

0.20 Ωj0.342 Ω

+

-

V

φ

IAR1jX1R2











−

s

R1

2

jX2

jXM

0.120 Ωj0.342 Ω

j12.5 Ω

2.28 Ω

(a) The easiest way to find the line current (or armature current) is to get the equivalent impedance F

Z

of the rotor circuit in parallel with M

jX , and then calculate the current as the phase voltage divided by the

sum of the series impedances, as shown below.

181

0.20 Ωj0.342 Ω

+

-

V

φ

IAR1jX1RF

jXF

The equivalent impedance of the rotor circuit in parallel with M

jX is:

2

11

2.197 0.744 2.32 18.7

11 1 1

12.5 2.40 0.342

F

M

Zj

jX Z j j

== =+=∠°Ω

++

Ω+

The line voltage must be derated by 5/6, so the new line voltage is 173.3 V

T

V=. The phase voltage is

173.3 / 3 = 100 V, so line current L

I is

11

100 0 V

0.20 0.342 2.197 0.744

LA

FF

V

IIRjXR jX j j

φ

∠°

== =

+++ Ω+Ω+Ω+Ω

38.0 24.4 A

LA

II== ∠− °

(b) The stator copper losses are

()( )

2

SCL 1

3 3 38 A 0.20 866 W

A

PIR== Ω=

(c) The air gap power is 22

2

AG 2

33

AF

R

PI IR

s

==

(Note that

2

3AF

IR

is equal to 22

2

3R

Is, since the only resistance in the original rotor circuit was 2

/

Rs, and

the resistance in the Thevenin equivalent circuit is F

R. The power consumed by the Thevenin equivalent

circuit must be the same as the power consumed by the original circuit.)

()( )

2

22

2

AG 2

3 3 3 38 A 2.197 9.52 kW

AF

R

PI IR

s

=== Ω=

(d) The power converted from electrical to mechanical form is

() ( )( )

conv AG

1 1 0.05 9.52 kW 9.04 kWPsP=− =− =

(e) The induced torque in the motor is

()

AG

ind

sync

9.52 kW 30.3 N m

2 rad 1 min

3000 r/min 1 r 60 s

P

τπ

ω

== = ⋅

(f) In the absence of better information, we will treat the mechanical and core losses as constant despite

the change in speed. This is not true, but we don’t have reason for a better guess. Therefore, the output

power of this motor is

OUT conv mech core misc 9.04 kW 250 W 180 W 0 W 8.61 kWPPPPP=−−−= − − − =

The output speed is

() ( )( )

sync

1 1 0.05 3000 r/min 2850 r/min

m

nsn=− =− =

182

Therefore the load torque is

()

OUT

load

8.61 kW 28.8 N m

2 rad 1 min

2850 r/min 1 r 60 s

m

P

τπ

ω

== = ⋅

(g) The overall efficiency is

OUT OUT

IN

100% 100%

3cos

A

PP

PVI

φ

ηθ

=× = ×

()( )

8.61 kW 100% 82.9%

3 100 V 38.0 A cos 24.4

η

=×=

°

(h) The motor speed in revolutions per minute is 2850 r/min. The motor speed in radians per second is

()

2 rad 1 min

2850 r/min 298.5 rad/s

1 r 60 s

m

π

ω

==

7-12. Figure 7-18a shows a simple circuit consisting of a voltage source, a resistor, and two reactances. Find the

Thevenin equivalent voltage and impedance of this circuit at the terminals. Then derive the expressions for

the magnitude of VTH and for RTH given in Equations (7-41b) and (7-44).

S

OLUTION The Thevenin voltage of this circuit is

()

TH

11

M

jX

RjXX

φ

=++

VV

The magnitude of this voltage is

()

TH 2

2

11

M

X

VV

RXX

φ

=++

If 1M

XX>> , then

()()

22

2

11 1

MM

RXX XX++ ≈+ , so

TH

1

M

X

VV

XX

φ

≈+

The Thevenin impedance of this circuit is

()

11

TH

11

M

jX R jX

ZRjXX

+

=++

183

()( )

() ()

111 1

TH

11 11

MM

jX R jX R j X X

ZRjXX RjXX

+−+

= ++ −+

()

222 2

11 11 1 1 1 1

TH 2

2

11

MMM MMM

M

RX X RX X RX j R X X X X X

ZRXX

−+++ ++

=++

() ()

2222

1 111

TH TH TH 22

22

11 11

MMMM

MM

RX R X X X XX

ZRjX j

RXX RXX

++

=+ = +

++ ++

The Thevenin resistance is

()

2

1

TH 2

2

11

M

RX

R

RXX

=++ . If 1M

XR>> , then

()()

22

2

11 1

MM

RXX XX++ ≈+ ,

so

2

TH 1

1

M

X

RR

XX

≈

+

The Thevenin reactance is

()

22 2

111

TH 2

2

11

MMM

M

RX X X XX

X

RXX

++

=++ .

If 1M

XR>> and 1M

XX>> then 22 2

111MMM

XX R X X X>> + and

()

222

11

MM

XX X R+≈>>

, so

2

1

TH 1

2

M

XX

X

≈=

7-13. Figure P7-1 shows a simple circuit consisting of a voltage source, two resistors, and two reactances in

parallel with each other. If the resistor RL is allowed to vary but all the other components are constant, at

what value of RL will the maximum possible power be supplied to it? Prove your answer. (Hint: Derive

an expression for load power in terms of V, RS, XS, RL and XL and take the partial derivative of that

expression with respect to RL.) Use this result to derive the expression for the pullout torque [Equation (7-

54)].

S

OLUTION The current flowing in this circuit is given by the equation

L

SSLL

RjXRjX

=+++

V

I

()( )

22

L

SL S L

V

I

RR X X

=+++

The power supplied to the load is

184

()( )

2

22

L

LL

SL S L

VR

PIR

RR X X

==

+++

()( ) ()

()( )

22

2

22

2

SL S L L SL

LSL S L

RR X X VVR RR

P

RRR X X

+++ − +

∂

=

∂

+++

To find the point of maximum power supplied to the load, set / L

PR∂∂ = 0 and solve for L

R.

()( ) ()

22

20

SL S L L SL

RR X X VVR RR

+++ − +=

()( ) ()

22

2

SL S L LSL

RR X X RRR

+++ = +

()

2

22 2

222

SSLLSL SLL

RRRRXX RRR++++=+

()

2

22 2

2

SL SL L

RR XX R++ + =

()

2

22

SSLL

RXX R++ =

Therefore, for maximum power transfer, the load resistor should be

()

2

LS SL

RRXX=++

7-14. A 440-V 50-Hz two-pole Y-connected induction motor is rated at 75 kW. The equivalent circuit

parameters are

1

R = 0.075 Ω 2

R = 0.065 Ω M

X = 7.2 Ω

1

X = 0.17 Ω 2

X = 0.17 Ω

F&W

P = 1.0 kW misc

P = 150 W core

P = 1.1 kW

For a slip of 0.04, find

(a) The line current L

I

(b) The stator power factor

(c) The rotor power factor

(d) The stator copper losses SCL

P

(e) The air-gap power AG

P

(f) The power converted from electrical to mechanical form conv

P

(g) The induced torque ind

τ

(h) The load torque load

τ

(i) The overall machine efficiency

η

(j) The motor speed in revolutions per minute and radians per second

S

OLUTION The equivalent circuit of this induction motor is shown below:

185

0.075 Ωj0.17 Ω

+

-

V

φ

IAR1jX1R2











−

s

R1

2

jX2

jXM

0.065 Ωj0.17 Ω

j7.2 Ω

1.56 Ω

(a) The easiest way to find the line current (or armature current) is to get the equivalent impedance F

Z

of the rotor circuit in parallel with M

jX , and then calculate the current as the phase voltage divided by the

sum of the series impedances, as shown below.

0.075 Ωj0.17 Ω

+

-

V

φ

IAR1jX1RF

jXF

The equivalent impedance of the rotor circuit in parallel with M

jX is:

2

11

1.539 0.364 1.58 13.2

11 1 1

7.2 1.625 0.17

F

M

Zj

jX Z j j

== =+=∠°Ω

++

Ω+

The phase voltage is 440/ 3 = 254 V, so line current L

I is

11

254 0 V

0.075 0.17 1.539 0.364

LA

FF

V

IIRjXR jX j j

φ

∠°

== =

+++ Ω+ Ω+ Ω+ Ω

149.4 18.3 A

LA

II== ∠− °

(b) The stator power factor is

()

PF cos 18.3 0.949 lagging=°=

(c) To find the rotor power factor, we must find the impedance angle of the rotor

11

2

0.17

tan tan 5.97

/1.625

R

X

Rs

θ

−−

===°

Therefore the rotor power factor is

PF cos5.97 0.995 lagging

R=°=

(d) The stator copper losses are

()()

2

SCL 1

3 3 149.4 A 0.075 1675 W

A

PIR== Ω=

(e) The air gap power is 22

2

AG 2

33

AF

R

PI IR

s

==

186

(Note that

2

3AF

IR

is equal to 22

2

3R

Is, since the only resistance in the original rotor circuit was 2

/

Rs

, and

the resistance in the Thevenin equivalent circuit is F

R. The power consumed by the Thevenin equivalent

circuit must be the same as the power consumed by the original circuit.)

()()

2

22

2

AG 2

3 3 3 149.4 A 1.539 103 kW

AF

R

PI IR

s

=== Ω=

(f) The power converted from electrical to mechanical form is

() ( )( )

conv AG

1 1 0.04 103 kW 98.9 kWPsP=− =− =

(g) The synchronous speed of this motor is

()

sync

120 50 Hz

120 3000 r/min

2

e

f

nP

== =

()

sync

2 rad 1 min

3000 r/min 314 rad/s

1 r 60 s

π

ω

==

Therefore the induced torque in the motor is

()

AG

ind

sync

103 kW 327.9 N m

2 rad 1 min

3000 r/min 1 r 60 s

P

τπ

ω

== = ⋅

(h) The output power of this motor is

OUT conv mech core misc 98.8 kW 1.0 kW 1.1 kW 150 W 96.6 kWPPPPP=−−−= − − − =

The output speed is

() ( )( )

sync

1 1 0.04 3000 r/min 2880 r/min

m

nsn=− =− =

Therefore the load torque is

()

OUT

load

98.8 kW 327.6 N m

2 rad 1 min

2880 r/min 1 r 60 s

m

P

τπ

ω

== = ⋅

(i) The overall efficiency is

OUT OUT

IN

100% 100%

3cos

A

PP

PVI

φ

ηθ

=× = ×

()( )()

96.6 kW 100% 89.4%

3 254 V 149.4 A cos 18.3

η

=×=

°

(j) The motor speed in revolutions per minute is 2880 r/min. The motor speed in radians per second is

()

2 rad 1 min

2880 r/min 301.6 rad/s

1 r 60 s

m

π

ω

==

7-15. For the motor in Problem 7-14, what is the pullout torque? What is the slip at the pullout torque? What is

the rotor speed at the pullout torque?

S

OLUTION The slip at pullout torque is found by calculating the Thevenin equivalent of the input circuit

from the rotor back to the power supply, and then using that with the rotor circuit model.

187

()

()( )

()

11

TH

11

7.2 0.075 0.17 0.0731 0.1662 0.182 66.3

0.075 0.17 7.2

M

jX R jX jj

Zj

RjXX j

+ΩΩ+Ω

== =+Ω=∠°Ω

++ Ω+ Ω+Ω

()

TH

11

7.2 254 0 V 248 0.06 V

0.075 0.17 7.2

M

j

jX

RjXX j

φ

Ω

== ∠°=∠°

++ Ω+ Ω+Ω

VV

The slip at pullout torque is

()

2

max 2

2

TH TH 2

R

s

RXX

=++

()( )

max 22

0.065 0.189

0.0731 0.1662 0.17

sΩ

==

Ω+ Ω+ Ω

The pullout torque of the motor is

()











+++2

=2

2TH

2

THTHsync

2

TH

max

3

XXRR

V

ω

τ

()

() ()( )

2

max 22

3248 V

314.2 rad/s 0.0731 0.0731 0.1662 0.17

τ

=

2Ω+Ω+Ω+Ω

max 704 N m

τ

=⋅

7-16. If the motor in Problem 7-14 is to be driven from a 440-V 60-Hz power supply, what will the pullout

torque be? What will the slip be at pullout?

S

OLUTION If this motor is driven from a 60 Hz source, the resistances will be unchanged and the reactances

will be increased by a ratio of 6/5. The resulting equivalent circuit is shown below.

0.075 Ωj0.204 Ω

+

-

V

φ

IAR1jX1R2











−

s

R1

2

jX2

jXM

0.065 Ωj0.204 Ω

j8.64 Ω

1.56 Ω

The slip at pullout torque is found by calculating the Thevenin equivalent of the input circuit from the rotor

back to the power supply, and then using that with the rotor circuit model.

()

()( )

()

11

TH

11

8.64 0.075 0.204 0.0731 0.1994 0.212 69.9

0.075 0.204 8.64

M

jX R jX j j

Zj

RjXX j

+ΩΩ+Ω

== =+Ω=∠°Ω

++ Ω+ Ω+Ω

()

TH

11

8.64 254 0 V 248 0.05 V

0.075 0.204 8.64

M

j

jX

RjXX j

φ

Ω

== ∠°=∠°

++ Ω+ Ω+Ω

VV

The slip at pullout torque is

()

2

max 2

2

TH TH 2

R

s

RXX

=++

188

()( )

max 22

0.065 0.159

0.0731 0.1994 0.204

sΩ

==

Ω+ Ω+ Ω

The synchronous speed of this motor is

()

sync

120 60 Hz

120 3600 r/min

2

e

f

nP

== =

()

sync

2 rad 1 min

3600 r/min 377 rad/s

1 r 60 s

π

ω

==

Therefore the pullout torque of the motor is

()

2

TH

max 2

2

sync TH TH TH 2

3V

RRXX

τ

ω

=

2+++

()

() ()( )

2

max 22

3248 V

377 rad/s 0.0731 0.0731 0.1994 0.204

τ

=

2Ω+Ω+Ω+Ω

max 507 N m

τ

=⋅

7-17. Plot the following quantities for the motor in Problem 7-14 as slip varies from 0% to 10%: (a) ind

τ

(b)

conv

P (c) out

P (d) Efficiency

η

. At what slip does out

P equal the rated power of the machine?

S

OLUTION This problem is ideally suited to solution with a MATLAB program. An appropriate program is

shown below. It follows the calculations performed for Problem 7-14, but repeats them at many values of

slip, and then plots the results. Note that it plots all the specified values versus m

n, which varies from

2700 to 3000 r/min, corresponding to a range of 0 to 10% slip.

% M-file: prob7_17.m

% M-file create a plot of the induced torque, power

% converted, power out, and efficiency of the induction

% motor of Problem 7-14 as a function of slip.

% First, initialize the values needed in this program.

r1 = 0.075; % Stator resistance

x1 = 0.170; % Stator reactance

r2 = 0.065; % Rotor resistance

x2 = 0.170; % Rotor reactance

xm = 7.2; % Magnetization branch reactance

v_phase = 440 / sqrt(3); % Phase voltage

n_sync = 3000; % Synchronous speed (r/min)

w_sync = 314.2; % Synchronous speed (rad/s)

p_mech = 1000; % Mechanical losses (W)

p_core = 1100; % Core losses (W)

p_misc = 150; % Miscellaneous losses (W)

% Calculate the Thevenin voltage and impedance from Equations

% 7-41a and 7-43.

v_th = v_phase * ( xm / sqrt(r1^2 + (x1 + xm)^2) );

z_th = ((j*xm) * (r1 + j*x1)) / (r1 + j*(x1 + xm));

r_th = real(z_th);

x_th = imag(z_th);

189

% Now calculate the torque-speed characteristic for many

% slips between 0 and 0.1. Note that the first slip value

% is set to 0.001 instead of exactly 0 to avoid divide-

% by-zero problems.

s = (0:0.001:0.1); % Slip

s(1) = 0.001;

nm = (1 - s) * n_sync; % Mechanical speed

wm = nm * 2*pi/60; % Mechanical speed

% Calculate torque, P_conv, P_out, and efficiency

% versus speed

for ii = 1:length(s)

% Induced torque

t_ind(ii) = (3 * v_th^2 * r2 / s(ii)) / ...

(w_sync * ((r_th + r2/s(ii))^2 + (x_th + x2)^2) );

% Power converted

p_conv(ii) = t_ind(ii) * wm(ii);

% Power output

p_out(ii) = p_conv(ii) - p_mech - p_core - p_misc;

% Power input

zf = 1 / ( 1/(j*xm) + 1/(r2/s(ii)+j*x2) );

ia = v_phase / ( r1 + j*x1 + zf );

p_in(ii) = 3 * v_phase * abs(ia) * cos(atan(imag(ia)/real(ia)));

% Efficiency

eff(ii) = p_out(ii) / p_in(ii) * 100;

end

% Plot the torque-speed curve

figure(1);

plot(nm,t_ind,'b-','LineWidth',2.0);

xlabel('\bf\itn_{m} \rm\bf(r/min)');

ylabel('\bf\tau_{ind} \rm\bf(N-m)');

title ('\bfInduced Torque versus Speed');

grid on;

% Plot power converted versus speed

figure(2);

plot(nm,p_conv/1000,'b-','LineWidth',2.0);

xlabel('\bf\itn_{m} \rm\bf(r/min)');

ylabel('\bf\itP\rm\bf_{conv} (kW)');

title ('\bfPower Converted versus Speed');

grid on;

% Plot output power versus speed

figure(3);

plot(nm,p_out/1000,'b-','LineWidth',2.0);

xlabel('\bf\itn_{m} \rm\bf(r/min)');

ylabel('\bf\itP\rm\bf_{out} (kW)');

title ('\bfOutput Power versus Speed');

axis([2700 3000 0 180]);

190

grid on;

% Plot the efficiency

figure(4);

plot(nm,eff,'b-','LineWidth',2.0);

xlabel('\bf\itn_{m} \rm\bf(r/min)');

ylabel('\bf\eta (%)');

title ('\bfEfficiency versus Speed');

grid on;

The four plots are shown below:

2700 2750 2800 2850 2900 2950 3000

0

100

200

300

400

500

600

700

n

m

(r/min)

τ

ind

(N-m)

Induced Torque versus Speed

191

This machine is rated at 75 kW. It produces an output power of 75 kW at 3.1% slip, or a speed of 2907

r/min.

7-18. A 208-V, 60 Hz, six-pole Y-connected 25-hp design class B induction motor is tested in the laboratory,

with the following results:

No load: 208 V, 22.0 A, 1200 W, 60 Hz

Locked rotor: 24.6 V, 64.5 A, 2200 W, 15 Hz

DC test: 13.5 V, 64 A

Find the equivalent circuit of this motor, and plot its torque-speed characteristic curve.

192

S

OLUTION From the DC test,

1

13.5 V

264 A

R= ⇒ 10.105 R=Ω

R

1

R

1

R

1

V

DC

+

-

I

DC

In the no-load test, the line voltage is 208 V, so the phase voltage is 120 V. Therefore,

1

,nl

120 V 5.455 @ 60 Hz

22.0 A

M

A

V

XX I

φ

+= = = Ω

In the locked-rotor test, the line voltage is 24.6 V, so the phase voltage is 14.2 V. From the locked-rotor

test at 15 Hz,

LR LR LR

,LR

14.2 V 0.2202

64.5 A

A

V

ZRjXI

φ

=+ = = = Ω

′′

()()

11

LR

2200 W

cos cos 36.82

3 24.6 V 64.5 A

P

S

θ

−−

== =°

′

Therefore,

()()

LR LR LR

cos 0.2202 cos 36.82 0.176 RZ

θ

==Ω°=Ω′

⇒ 12

0.176 RR+= Ω

⇒ 20.071 R=Ω

()()

LR LR LR

in 0.2202 sin 36.82 0.132 XZs

θ

==Ω°=Ω′′

At a frequency of 60 Hz,

LR LR

60 Hz 0.528

15 Hz

XX

==Ω

′

For a Design Class B motor, the split is 10.211 X=Ω and 20.317 X=Ω. Therefore,

5.455 0.211 5.244

M

X=Ω−Ω= Ω

The resulting equivalent circuit is shown below:

193

0.105

Ω

j

0.211

Ω

+

-

V

φ

I

A

R

1

jX

1

R

2











−

s

R1

2

jX

2

jX

M

0.071

Ω

j

0.317

Ω

j

5.244

Ω

I

2

A MATLAB program to calculate the torque-speed characteristic of this motor is shown below:

% M-file: prob7_18.m

% M-file create a plot of the torque-speed curve of the

% induction motor of Problem 7-18.

% First, initialize the values needed in this program.

r1 = 0.105; % Stator resistance

x1 = 0.211; % Stator reactance

r2 = 0.071; % Rotor resistance

x2 = 0.317; % Rotor reactance

xm = 5.244; % Magnetization branch reactance

v_phase = 208 / sqrt(3); % Phase voltage

n_sync = 1200; % Synchronous speed (r/min)

w_sync = 125.7; % Synchronous speed (rad/s)

% Calculate the Thevenin voltage and impedance from Equations

% 7-41a and 7-43.

v_th = v_phase * ( xm / sqrt(r1^2 + (x1 + xm)^2) );

z_th = ((j*xm) * (r1 + j*x1)) / (r1 + j*(x1 + xm));

r_th = real(z_th);

x_th = imag(z_th);

% Now calculate the torque-speed characteristic for many

% slips between 0 and 1. Note that the first slip value

% is set to 0.001 instead of exactly 0 to avoid divide-

% by-zero problems.

s = (0:1:50) / 50; % Slip

s(1) = 0.001;

nm = (1 - s) * n_sync; % Mechanical speed

% Calculate torque versus speed

for ii = 1:51

t_ind(ii) = (3 * v_th^2 * r2 / s(ii)) / ...

(w_sync * ((r_th + r2/s(ii))^2 + (x_th + x2)^2) );

end

% Plot the torque-speed curve

figure(1);

plot(nm,t_ind,'b-','LineWidth',2.0);

xlabel('\bf\itn_{m}');

ylabel('\bf\tau_{ind}');

title ('\bfInduction Motor Torque-Speed Characteristic');

grid on;

194

The resulting plot is shown below:

7-19. A 460-V, four-pole, 50-hp, 60-Hz, Y-connected three-phase induction motor develops its full-load induced

torque at 3.8 percent slip when operating at 60 Hz and 460 V. The per-phase circuit model impedances of

the motor are

1

R = 0.33 Ω M

X = 30 Ω

1

X = 0.42 Ω 2

X = 0.42 Ω

Mechanical, core, and stray losses may be neglected in this problem.

(a) Find the value of the rotor resistance 2

R.

(b) Find max

τ

, max

s, and the rotor speed at maximum torque for this motor.

(c) Find the starting torque of this motor.

(d) What code letter factor should be assigned to this motor?

S

OLUTION The equivalent circuit for this motor is

0.33 Ωj0.42 Ω

+

-

V

φ

IAR1jX1R2











−

s

R1

2

jX2

jXM

??? Ωj0.42 Ω

j30 Ω

I2

The Thevenin equivalent of the input circuit is:

()

()( )

()

11

TH

11

30 0.33 0.42 0.321 0.418 0.527 52.5

0.33 0.42 30

M

jX R jX j j

Zj

RjXX j

+ΩΩ+Ω

== =+Ω=∠°Ω

++ Ω+ Ω+Ω

195

()

TH

11

30 265.6 0 V 262 0.6 V

0.33 0.42 30

M

j

jX

RjXX j

φ

Ω

== ∠°=∠°

++ Ω+ Ω+Ω

VV

(a) If losses are neglected, the induced torque in a motor is equal to its load torque. At full load, the

output power of this motor is 50 hp and its slip is 3.8%, so the induced torque is

()( )

1 0.038 1800 r/min 1732 r/min

m

n=− =

()( )

()

ind load

50 hp 746 W/hp 205.7 N m

2 rad 1min

1732 r/min 1 r 60 s

ττ π

== = ⋅

The induced torque is given by the equation

()()

2

TH 2

ind 22

sync TH 2 TH 2

3/

/

VRs

RRs X X

τω

=

+++

Substituting known values and solving for 2

/

Rs yields

()

2

22

2

3262 V /

205.7 N m

188.5 rad/s 0.321 / 0.418 0.42

Rs

⋅=

+++

()

2

205,932 /

38,774

0.321 / 0.702

Rs

=

++

()

2

22

0.321 / 0.702 5.311 /Rs Rs

++=

()

2

22 2

0.103 0.642 / / 0.702 5.311 /Rs Rs Rs

+++=

2

22

4.669 0.702 0

RR

ss

−+=

20.156, 4.513

R

s

=

20.0059 , 0.172 R=ΩΩ

These two solutions represent two situations in which the torque-speed curve would go through this specific

torque-speed point. The two curves are plotted below. As you can see, only the 0.172 Ω solution is

realistic, since the 0.0059 Ω solution passes through this torque-speed point at an unstable location on the

back side of the torque-speed curve.

196

1600 1620 1640 1660 1680 1700 1720 1740 1760 1780 1800

0

50

100

150

200

250

300

350

400

450

n

m

τ

ind

Induction Motor Torque-Speed Characteristic

R2 = 0.0059 ohms

R2 = 0.172 ohms

(b) The slip at pullout torque can be found by calculating the Thevenin equivalent of the input circuit

from the rotor back to the power supply, and then using that with the rotor circuit model. The Thevenin

equivalent of the input circuit was calculate in part (a). The slip at pullout torque is

()

2

max 2

2

TH TH 2

R

s

RXX

=++

()( )

max 22

0.172 0.192

0.321 0.418 0.420

sΩ

==

Ω+ Ω+ Ω

The rotor speed a maximum torque is

()( )

pullout sync

(1 ) 1 0.192 1800 r/min 1454 r/minnsn=− =− =

and the pullout torque of the motor is

()

2

TH

max 2

2

sync TH TH TH 2

3V

RRXX

τ

ω

=

2+++

()

() ()( )

2

max 22

3262 V

188.5 rad/s 0.321 0.321 0.418 0.420

τ

=

2Ω+Ω+Ω+Ω

max 448 N m

τ

=⋅

(c) The starting torque of this motor is the torque at slip s = 1. It is

()()

2

TH 2

ind 22

sync TH 2 TH 2

3/

/

VRs

RRs X X

τω

=

+++

()( )

()( )( )

2

ind 22

3 262 V 0.172 199 N m

188.5 rad/s 0.321 0.172 0.418 0.420

τ

Ω

==⋅

+Ω+ +

197

(d) To determine the starting code letter, we must find the locked-rotor kVA per horsepower, which is

equivalent to finding the starting kVA per horsepower. The easiest way to find the line current (or

armature current) at starting is to get the equivalent impedance F

Z

of the rotor circuit in parallel with

M

jX at starting conditions, and then calculate the starting current as the phase voltage divided by the sum

of the series impedances, as shown below.

0.33

Ω

j

0.42

Ω

+

-

V

φ

I

A,

start

R

1

jX

1

R

F

jX

F

The equivalent impedance of the rotor circuit in parallel with M

jX at starting conditions (s = 1.0) is:

,start

2

11

0.167 0.415 0.448 68.1

11 1 1

30 0.172 0.42

F

M

Zj

jX Z j j

== =+=∠°Ω

++

Ω+

The phase voltage is 460/ 3 = 266 V, so line current ,startL

I is

,start

11

266 0 V

0.33 0.42 0.167 0.415

LA

FF

RjXR jX j j

φ

∠°

== =

+++ Ω+ Ω+ Ω+ Ω

V

II

,start 274 59.2 A

LA

== ∠− °II

Therefore, the locked-rotor kVA of this motor is

()()

,rated

3 3 460 V 274 A 218 kVA

TL

SVI== =

and the kVA per horsepower is

218 kVA

kVA/hp 4.36 kVA/hp

50 hp

==

This motor would have starting code letter D, since letter D covers the range 4.00-4.50.

7-20. Answer the following questions about the motor in Problem 7-19.

(a) If this motor is started from a 460-V infinite bus, how much current will flow in the motor at starting?

(b) If transmission line with an impedance of 0.35 + j0.25 Ω per phase is used to connect the induction

motor to the infinite bus, what will the starting current of the motor be? What will the motor’s terminal

voltage be on starting?

(c) If an ideal 1.4:1 step-down autotransformer is connected between the transmission line and the motor,

what will the current be in the transmission line during starting? What will the voltage be at the motor

end of the transmission line during starting?

S

OLUTION

(a) The equivalent circuit of this induction motor is shown below:

198

0.33 Ωj0.42 Ω

+

-

V

φ

IAR1jX1R2











−

s

R1

2

jX2

jXM

0.172 Ωj0.42 Ω

j30 Ω

I2

The easiest way to find the line current (or armature current) at starting is to get the equivalent impedance

F

Z of the rotor circuit in parallel with M

jX at starting conditions, and then calculate the starting current

as the phase voltage divided by the sum of the series impedances, as shown below.

0.33

Ω

j

0.42

Ω

+

-

V

φ

I

A

R

1

jX

1

R

F

jX

F

The equivalent impedance of the rotor circuit in parallel with M

jX at starting conditions (s = 1.0) is:

2

11

0.167 0.415 0.448 68.0

11 1 1

30 0.172 0.42

F

M

Zj

jX Z j j

== =+=∠°Ω

++

Ω+

The phase voltage is 460/ 3 = 266 V, so line current L

I is

11

266 0 V

0.33 0.42 0.167 0.415

LA

FF

RjXR jX j j

φ

∠°

== =

+++ Ω+ Ω+ Ω+ Ω

V

II

273 59.2 A

LA

== ∠− °II

(b) If a transmission line with an impedance of 0.35 + j0.25 Ω per phase is used to connect the induction

motor to the infinite bus, its impedance will be in series with the motor’s impedances, and the starting

current will be

,bus

line line 1 1

LA

FF

RjXRjXRjX

φ

== +++++

V

II

266 0 V

0.35 0.25 0.33 0.42 0.167 0.415

LA jj j

∠°

== Ω+ Ω+ Ω+ Ω+ Ω+ Ω

II

193.2 52.0 A

LA

== ∠− °II

The voltage at the terminals of the motor will be

()

11AFF

RjXR jX

φ

=+++VI

()( )

194.1 52.3 A 0.33 0.42 0.167 0.415 jj

φ

= ∠− ° Ω+ Ω+ Ω+ ΩV

187.7 7.2 V

φ

=∠°V

Therefore, the terminal voltage will be

()

3 187.7 V 325 V=. Note that the terminal voltage sagged by

about 30% during motor starting, which would be unacceptable.

199

(c) If an ideal 1.4:1 step-down autotransformer is connected between the transmission line and the motor,

the motor’s impedances will be referred across the transformer by the square of the turns ratio a = 1.4. The

referred impedances are

()

2

11

1.96 0.33 0.647 RaR== Ω= Ω

′

()

2

11

1.96 0.42 0.823 XaX== Ω= Ω′

()

21.96 0.167 0.327

FF

RaR== Ω= Ω

′

()

21.96 0.415 0.813

FF

XaX== Ω=Ω′

Therefore, the starting current referred to the primary side of the transformer will be

,bus

line line 1 1

LA

FF

RjXRjXRjX

φ

==′′ +++++

′′′ ′

V

II

266 0 V

0.35 0.25 0.647 0.823 0.327 0.813

LA jj j

∠°

==

′′ Ω + Ω+ Ω+ Ω + Ω+ Ω

II

115.4 54.9 A

LA

== ∠− °

′′

II

The voltage at the motor end of the transmission line would be the same as the referred voltage at the

terminals of the motor

()

11AFF

RjXR jX

φ

=+++′′′ ′′ ′VI

()( )

115.4 54.9 A 0.647 0.823 0.327 0.813 jj

φ

= ∠− ° Ω+ Ω + Ω+ ΩV

219.7 4.3 V

φ

=∠°V

Therefore, the line voltage at the motor end of the transmission line will be

(

)

3 219.7 V 380.5 V=. Note

that this voltage sagged by 17.3% during motor starting, which is less than the 30% sag with case of

across-the-line starting.

7-21. In this chapter, we learned that a step-down autotransformer could be used to reduce the starting current

drawn by an induction motor. While this technique works, an autotransformer is relatively expensive. A

much less expensive way to reduce the starting current is to use a device called Y-∆ starter. If an induction

motor is normally ∆-connected, it is possible to reduce its phase voltage Vφ (and hence its starting current)

by simply re-connecting the stator windings in Y during starting, and then restoring the connections to ∆

when the motor comes up to speed. Answer the following questions about this type of starter.

(a) How would the phase voltage at starting compare with the phase voltage under normal running

conditions?

(b) How would the starting current of the Y-connected motor compare to the starting current if the motor

remained in a ∆-connection during starting?

S

OLUTION

(a) The phase voltage at starting would be 1 / 3 = 57.7% of the phase voltage under normal running

conditions.

(b) Since the phase voltage decreases to 1 / 3 = 57.7% of the normal voltage, the starting phase current

will also decrease to 57.7% of the normal starting current. However, since the line current for the original

delta connection was 3 times the phase current, while the line current for the Y starter connection is

equal to its phase current, the line current is reduced by a factor of 3 in a Y-∆ starter.

For the

∆-connection: ,,

3

L

II

φ

∆∆

=

200

For the Y-connection: ,Y ,YL

II

φ

=

But

,,Y

3 II

φφ

∆=, so ,,Y

3

LL

II

∆=

7-22. A 460-V 100-hp four-pole ∆-connected 60-Hz three-phase induction motor has a full-load slip of 5 percent,

an efficiency of 92 percent, and a power factor of 0.87 lagging. At start-up, the motor develops 1.9 times

the full-load torque but draws 7.5 times the rated current at the rated voltage. This motor is to be started

with an autotransformer reduced voltage starter.

(a) What should the output voltage of the starter circuit be to reduce the starting torque until it equals the

rated torque of the motor?

(b) What will the motor starting current and the current drawn from the supply be at this voltage?

S

OLUTION

(a) The starting torque of an induction motor is proportional to the square of TH

V,

22

start2 TH2 2

start1 TH1 1

T

VV

τ

==

If a torque of 1.9 rated

τ

is produced by a voltage of 460 V, then a torque of 1.00 rated

τ

would be produced

by a voltage of

2

rated 2

rated

1.00

1.90 460 V

T

V

τ

=

()

2

460 V 334 V

1.90

T

V==

(b) The motor starting current is directly proportional to the starting voltage, so

()()

()

2 1 1 rated rated

334 V 0.726 0.726 7.5 5.445

460 V

LLL

III II

=== =

The input power to this motor is

()( )

OUT

IN

100 hp 746 W/hp 81.1 kW

0.92

P

η

== =

The rated current is equal to

()

()()

IN

rated

81.1 kW 117 A

3 PF 3 460 V 0.87

T

P

IV

== =

Therefore, the motor starting current is

()( )

2 rated

5.445 5.445 117 A 637 A

L

II== =

The turns ratio of the autotransformer that produces this starting voltage is

460 V 1.377

334 V

SE C

C

NN

N

+==

so the current drawn from the supply will be

201

start

line

637 A 463 A

1.377 1.377

I

I== =

7-23. A wound-rotor induction motor is operating at rated voltage and frequency with its slip rings shorted and

with a load of about 25 percent of the rated value for the machine. If the rotor resistance of this machine is

doubled by inserting external resistors into the rotor circuit, explain what happens to the following:

(a) Slip s

(b) Motor speed nm

(c) The induced voltage in the rotor

(d) The rotor current

(e) ind

τ

(f) out

P

(g) RCL

P

(h) Overall efficiency

η

S

OLUTION

(a) The slip s will increase.

(b) The motor speed m

n will decrease.

(c) The induced voltage in the rotor will increase.

(d) The rotor current will increase.

(e) The induced torque will adjust to supply the load’s torque requirements at the new speed. This will

depend on the shape of the load’s torque-speed characteristic. For most loads, the induced torque will

decrease.

(f) The output power will generally decrease: OUT ind m

P

τω

=↓↓

(g) The rotor copper losses (including the external resistor) will increase.

202

(h) The overall efficiency

η

will decrease.

7-24. Answer the following questions about a 460-V ∆-connected two-pole 75-hp 60-Hz starting code letter E

induction motor:

(a) What is the maximum current starting current that this machine’s controller must be designed to

handle?

(b) If the controller is designed to switch the stator windings from a ∆ connection to a Y connection during

starting, what is the maximum starting current that the controller must be designed to handle?

(c) If a 1.25:1 step-down autotransformer starter is used during starting, what is the maximum starting

current that will be drawn from the line?

S

OLUTION

(a) Starting code letter E corresponds to a 4.50 – 5.00 kVA/hp, so the maximum starting kVA of this

motor is

()()

start 75 hp 5.00 375 kVAS==

Therefore,

()

start

375 kVA 471 A

3 3 460 V

T

S

IV

== =

(b) The line voltage will still be 460 V when the motor is switched to the Y-connection, but now the

phase voltage will be 460 / 3 = 266 V.

Before (in ∆):

()( )()( )

,

TH2TH2TH2TH2

460 V

V

IRRjX X RRjX X

φ

∆

∆==

++ + ++ +

But the line current in a ∆ connection is 3 times the phase current, so

()( )()( )

,

,,

TH 2 TH 2 TH 2 TH 2

3797 V

3

L

V

II

RRjX X RRjX X

φ

∆

∆∆

== =

++ + ++ +

After (in Y):

()( )()( )

,Y

,Y ,Y

TH 2 TH 2 TH 2 TH 2

265.6 V

L

V

II RRjX X RRjX X

φ

== =

++ + ++ +

Therefore the line current will decrease by a factor of 3 when using this starter. The starting current with a

∆-Y starter is

start

471 A 157 A

3

I==

(c) A 1.25:1 step-down autotransformer reduces the phase voltage on the motor by a factor 0.8. This

reduces the phase current and line current in the motor (and on the secondary side of the transformer) by a

factor of 0.8. However, the current on the primary of the autotransformer will be reduced by another factor

of 0.8, so the total starting current drawn from the line will be 64% of its original value. Therefore, the

maximum starting current drawn from the line will be

()( )

start 0.64 471 A 301 AI==

203

7-25. When it is necessary to stop an induction motor very rapidly, many induction motor controllers reverse the

direction of rotation of the magnetic fields by switching any two stator leads. When the direction of

rotation of the magnetic fields is reversed, the motor develops an induced torque opposite to the current

direction of rotation, so it quickly stops and tries to start turning in the opposite direction. If power is

removed from the stator circuit at the moment when the rotor speed goes through zero, then the motor has

been stopped very rapidly. This technique for rapidly stopping an induction motor is called plugging. The

motor of Problem 7-19 is running at rated conditions and is to be stopped by plugging.

(a) What is the slip s before plugging?

(b) What is the frequency of the rotor before plugging?

(c) What is the induced torque ind

τ

before plugging?

(d) What is the slip s immediately after switching the stator leads?

(e) What is the frequency of the rotor immediately after switching the stator leads?

(f) What is the induced torque ind

τ

immediately after switching the stator leads?

S

OLUTION

(a) The slip before plugging is 0.038 (see Problem 7-19).

(b) The frequency of the rotor before plugging is

()( )

0.038 60 Hz 2.28 Hz

re

fsf== =

(c) The induced torque before plugging is 205.7 N⋅m in the direction of motion (see Problem 7-19).

(d) After switching stator leads, the synchronous speed becomes –1800 r/min, while the mechanical speed

initially remains 1732 r/min. Therefore, the slip becomes

sync

1800 1732 1.962

1800

m

nn

sn

−−−

== =

−

(e) The frequency of the rotor after plugging is

()( )

1.962 60 Hz 117.72 Hz

re

fsf== =

(f) The induced torque immediately after switching the stator leads is

()()

2

TH 2

ind 22

sync TH 2 TH 2

3/

/

VRs

RRs X X

τω

=

+++

()( )

()( )( )

2

ind 22

3 262 V 0.172 /1.962

188.5 rad/s 0.321 0.172 /1.962 0.418 0.420

τ

Ω

=

+Ω + +

()( )

()( )( )

2

ind 22

3262 V 0.0877

188.5 rad/s 0.321 0.0877 0.418 0.420

τ

=

+++

ind 110 N m, opposite the direction of motion

τ

=⋅

204

Chapter 8: DC Machinery Fundamentals

8-1. The following information is given about the simple rotating loop shown in Figure 8-6:

0.8 T B= 24 V

B

V=

0.5 ml= 0.4 R=Ω

0.125 mr= 250 rad/s

ω

=

(a) Is this machine operating as a motor or a generator? Explain.

(b) What is the current i flowing into or out of the machine? What is the power flowing into or out of the

machine?

(c) If the speed of the rotor were changed to 275 rad/s, what would happen to the current flow into or out

of the machine?

(d) If the speed of the rotor were changed to 225 rad/s, what would happen to the current flow into or out

of the machine?

205

(a) If the speed of rotation

ω

of the shaft is 500 rad/s, then the voltage induced in the rotating loop will be

ind 2erlB

ω

=

(

)

(

)

(

)

(

)

ind 2 0.125 m 0.5 m 0.8 T 250 rad/s 25 Ve==

Since the external battery voltage is only 24 V, this machine is operating as a generator, charging the

battery.

(b) The current flowing out of the machine is approximately

ind 25 V 24 V 2.5 A

0.4

B

eV

iR

−−

== =

Ω

(Note that this value is the current flowing while the loop is under the pole faces. When the loop goes

beyond the pole faces, ind

e will momentarily fall to 0 V, and the current flow will momentarily reverse.

Therefore, the average current flow over a complete cycle will be somewhat less than 2.5 A.)

(c) If the speed of the rotor were increased to 275 rad/s, the induced voltage of the loop would increase to

ind 2erlB

ω

=

(

)

(

)

(

)

(

)

ind 2 0.125 m 0.5 m 0.8 T 275 rad/s 27.5 Ve==

and the current flow out of the machine will increase to

ind 27.5 V 24 V 8.75 A

0.4

B

eV

iR

−−

== =

Ω

(d) If the speed of the rotor were decreased to 450 rad/s, the induced voltage of the loop would fall to

ind 2erlB

ω

=

()()()( )

ind 2 0.125 m 0.5 m 0.8 T 225 rad/s 22.5 Ve==

Here,

ind

e is less than B

V, so current flows into the loop and the machine is acting as a motor. The current

flow into the machine would be

ind 24 V - 22.5 V 3.75 A

0.4

B

Ve

iR

−

== =

Ω

8-2. Refer to the simple two-pole eight-coil machine shown in Figure P8-1. The following information is given

about this machine:

B=10. T in air gap

l=03. m (length of coil sides)

coils) of (radiusm 08.0=r

CCWr/min 1700=n

The resistance of each rotor coil is 0.04 Ω.

(a) Is the armature winding shown a progressive or retrogressive winding?

(b) How many current paths are there through the armature of this machine?

(c) What are the magnitude and the polarity of the voltage at the brushes in this machine?

(d) What is the armature resistance RA of this machine?

206

(e) If a 10 Ω resistor is connected to the terminals of this machine, how much current flows in the

machine? Consider the internal resistance of the machine in determining the current flow.

(f) What are the magnitude and the direction of the resulting induced torque?

(g) Assuming that the speed of rotation and magnetic flux density are constant, plot the terminal voltage of

this machine as a function of the current drawn from it.

S

OLUTION

(a) This winding is progressive, since the ends of each coil are connected to the commutator segments

ahead of the segments that the beginnings of the coils are connected to.

(b) There are two current paths in parallel through the armature of this machine (this is a simplex lap

winding).

(c) The voltage is positive at brush x with respect to brush y, since the voltage in the conductors is

positive out of the page under the North pole face and positive into the page under the South pole face.

(d) There are 8 coils on this machine in two parallel paths, with each coil having a resistance of 0.04 Ω.

Therefore, the total resistance A

R is

()()

0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04

A

RΩ+ Ω+ Ω+ Ω Ω+ Ω+ Ω+ Ω

=Ω+ Ω+ Ω+ Ω+ Ω+ Ω+ Ω+ Ω

207

0.08

A

R=Ω

(e) The voltage produced by this machine can be found from Equations 8-32 and 8-33:

A

Z

vBl Zr Bl

Eaa

ω

==

where Z is the number of conductors under the pole faces, since the ones between the poles have no voltage

in them. There are 16 conductors in this machine, and about 12 of them are under the pole faces at any

given time.

()

2 rad 1 min

1700 r/min 178 rad/s

1 r 60 s

π

ω

==

()()( )()()

12 cond 0.08 m 178 rad/s 1.0 T 0.3 m 25.6 V

2 current paths

A

Zr Bl

Ea

ω

== =

Therefore, the current flowing in the machine will be

load

25.6 V 2.54 A

0.08 10

A

E

IRR

== =

+Ω+Ω

(f) The induced torque is given by Equation 8-46:

( )()()()()

ind

12 cond 0.08 m 0.3 m 1.0 T 2.54 A

2 current paths

A

ZrlBI

a

τ

==

CW m,N 366.0

ind ⋅=

τ

(opposite to the direction of rotation)

8-3. Prove that the equation for the induced voltage of a single simple rotating loop

ind

2 e

φ

ω

π

= (8-6)

is just a special case of the general equation for induced voltage in a dc machine

A

EK

φ

ω

= (8-38)

S

OLUTION From Equation 8-38,

A

EK

φ

ω

=

where

2

Z

P

Ka

π

=

For the simple rotation loop,

Z = 2 (There are 2 conductors)

P = 2 (There are 2 poles)

a = 1 (There is one current path through the machine)

Therefore,

()()

()

2 2 2

22 1

ZP

Ka

ππ π

==

and Equation 8-38 reduces to Equation 8-6.

8-4. A dc machine has 8 poles and a rated current of 100 A. How much current will flow in each path at rated

conditions if the armature is (a) simplex lap-wound, (b) duplex lap-wound, (c) simplex wave-wound?

S

OLUTION

208

(a) Simplex lap-wound:

()()

18 8 pathsamP== =

Therefore, the current per path is

100 A 12.5 A

8

A

I

Ia

== =

(b) Duplex lap-wound:

()()

2 8 16 pathsamP== =

Therefore, the current per path is

100 A 6.25 A

16

A

I

Ia

== =

(c) Simplex wave-wound:

()()

2212 pathsam== =

Therefore, the current per path is

100 A 50 A

2

A

I

Ia

== =

8-5. How many parallel current paths will there be in the armature of a 12-pole machine if the armature is (a)

simplex lap-wound, (b) duplex wave-wound, (c) triplex lap-wound, (d) quadruplex wave-wound?

S

OLUTION

(a) Simplex lap-wound:

(1)(12) 12 pathsamP

== =

(b) Duplex wave-wound:

2 (2)(2) 4 pathsam== =

(c) Triplex lap-wound:

(3)(12) 36 pathsamP== =

(d) Quadruplex wave-wound:

2 (2)(4) 8 pathsam== =

8-6. The power converted from one form to another within a dc motor was given by

conv indAA m

PEI

τω

==

Use the equations for A

E and ind

τ

[Equations (8-38) and (8-49)] to prove that AA

EI = m

ωτ

ind ; that is,

prove that the electric power disappearing at the point of power conversion is exactly equal to the

mechanical power appearing at that point.

S

OLUTION

conv AA

PEI=

Substituting Equation (8-38) for A

E

209

()

conv A

PKI

φω

=

()

conv

A

PKI

φ

ω

=

But from Equation (8-49), ind

A

KI

τφ

=, so

conv ind

P

τω

=

8-7. An eight-pole, 25-kW, 120-V DC generator has a duplex lap-wound armature, which has 64 coils with 16

turns per coil. Its rated speed is 2400 r/min.

(a) How much flux per pole is required to produce the rated voltage in this generator at no-load conditions?

(b) What is the current per path in the armature of this generator at the rated load?

(c) What is the induced torque in this machine at the rated load?

(d) How many brushes must this motor have? How wide must each one be?

(e) If the resistance of this winding is 0.011 Ω per turn, what is the armature resistance A

R of this

machine?

S

OLUTION

(a)

2

A

ZP

EK a

φ

ωφω

π

==

In this machine, the number of current paths is

()()

28 16amP== =

The number of conductor is

()( )( )

64 coils 16 turns/coil 2 conductors/turn 2048Z==

The equation for induced voltage is

2

A

ZP

Ea

φ

ω

π

=

so the required flux is

()()

()

2048 cond 8 poles 2 rad 1 min

120 V 2400 r/min

2 16 paths 1 r 60 s

π

φ

π

=

120 V 40,960

φ

=

0.00293 Wb

φ

=

(b) At rated load, the current flow in the generator would be

25 kW 208 A

120 V

A

I==

There are a = m P = (2)(8) = 16 parallel current paths through the machine, so the current per path is

208 A 13 A

16

A

I

Ia

== =

(c) The induced torque in this machine at rated load is

ind 2A

ZP I

a

τφ

π

=

210

()()

()

()()

ind

2048 cond 8 poles 0.00293 Wb 208 A

216 paths

τπ

=

ind 99.3 N m

τ

=⋅

(d) This motor must have 8 brushes, since it is lap-wound and has 8 poles. Since it is duplex-wound,

each brush must be wide enough to stretch across 2 complete commutator segments.

(e) There are a total of 1024 turns on the armature of this machine, so the number of turns per path is

1024 turns 64 turns/path

16 paths

P

N==

The total resistance per path is

()( )

64 0.011 0.704

P

R=Ω=Ω. Since there are 16 parallel paths through

the machine, the armature resistance of the generator is

0.704 0.044

16 paths

A

RΩ

==Ω

8-8. Figure P8-2 shows a small two-pole dc motor with eight rotor coils and four turns per coil. The flux per

pole in this machine is 0.0125 Wb.

(a) If this motor is connected to a 12-V dc car battery, what will the no-load speed of the motor be?

(b) If the positive terminal of the battery is connected to the rightmost brush on the motor, which way will

it rotate?

(c) If this motor is loaded down so that it consumes 50 W from the battery, what will the induced torque of

the motor be? (Ignore any internal resistance in the motor.)

S

OLUTION

(a) At no load, TA

VEK

φ

ω

== . If K is known, then the speed of the motor can be found. The constant

K is given by

2

Z

P

Ka

π

=

On the average, about 6 of the 8 coils are under the pole faces at any given time, so the average number of

active conductors is

Z = (6 coils)(4 turns/coil)(2 conductors/turn) = 48 conductors

211

There are two poles and two current paths, so

()()

()

48 cond 2 poles 7.64

2 2 2 paths

ZP

Ka

ππ

== =

The speed is given by

()( )

12 V 125.6 rad/s

7.64 0.0125 Wb

A

E

K

ωφ

== =

()

1 r 60 s

125.6 rad/s 1200 r/min

2 rad 1 min

m

n

π

==

(b) If the positive terminal of the battery is connected to the rightmost brush, current will flow into the

page under the South pole face, producing a CW torque ⇒ CW rotation.

(c) If the motor consumes 50 W from the battery, the current flow is

50 W 4.17 A

12 V

B

P

IV

== =

Therefore, the induced torque will be

()( )( )

ind 7.64 0.0125 Wb 4.17 A 0.40 N m, CW

A

KI

τφ

== = ⋅

8-9. Refer to the machine winding shown in Figure P8-3.

(a) How many parallel current paths are there through this armature winding?

(b) Where should the brushes be located on this machine for proper commutation? How wide should they

be?

(c) What is the plex of this machine?

(d) If the voltage on any single conductor under the pole faces in this machine is e, what is the voltage at

the terminals of this machine?

212

S

OLUTION

(a) This is a duplex, two-pole, lap winding, so there are 4 parallel current paths through the rotor.

(b) The brushes should be shorting out those windings lying between the two poles. At the time shown,

those windings are 1, 2, 9, and 10. Therefore, the brushes should be connected to short out commutator

segments b-c-d and j-k-l at the instant shown in the figure. Each brush should be two commutator

segments wide, since this is a duplex winding.

(c) Duplex (see above)

(d) There are 16 coils on the armature of this machine. Of that number, an average of 14 of them would

be under the pole faces at any one time. Therefore, there are 28 conductors divided among 4 parallel paths,

which produces 7 conductors per path. Therefore, TA VeE == 7 for no-load conditions.

213

8-10. Describe in detail the winding of the machine shown in Figure P8-4. If a positive voltage is applied to the

brush under the North pole face, which way will this motor rotate?

S

OLUTION This is a 2-pole, retrogressive, lap winding. If a positive voltage is applied to the brush under

the North pole face, the rotor will rotate in a counterclockwise direction.

214

Chapter 9: DC Motors and Generators

Problems 9-1 to 9-12 refer to the following dc motor:

P

rated = 15 hp IL,rated = 55 A

VT = 240 V NF = 2700 turns per pole

nrated = 1200 r/min NSE = 27 turns per pole

RA = 0.40 Ω RF = 100 Ω

RS = 0.04 Ω Radj = 100 to 400 Ω

Rotational losses = 1800 W at full load. Magnetization curve as shown in Figure P9-1.

Note: An electronic version of this magnetization curve can be found in file

p91_mag.dat, which can be used with MATLAB programs. Column 1

contains field current in amps, and column 2 contains the internal generated

voltage EA in volts.

In Problems 9-1 through 9-7, assume that the motor described above can be connected in shunt. The equivalent

circuit of the shunt motor is shown in Figure P9-2.

215

Note: Figure P9-2 shows incorrect values for RA and RF in the first printing of this

book. The correct values are given in the text, but shown incorrectly on the

figure. This will be corrected at the second printing.

9-1. If the resistor Radj is adjusted to 175 Ω what is the rotational speed of the motor at no-load conditions?

S

OLUTION At no-load conditions, 240 V

AT

EV== . The field current is given by

adj

240 V 240 V 0.873 A

175 100 250

T

F

V

IRR

== ==

+Ω+ΩΩ

From Figure P9-1, this field current would produce an internal generated voltage Ao

E of 271 V at a speed

o

n of 1200 r/min. Therefore, the speed n with a voltage A

E of 240 V would be

A

Ao o

En

=

()

240 V 1200 r/min 1063 r/min

271 V

A

o

Ao

E

nn

E

== =

9-2. Assuming no armature reaction, what is the speed of the motor at full load? What is the speed regulation of

the motor?

S

OLUTION At full load, the armature current is

adj

55 A 0.87 A 54.13 A

T

ALF L

F

V

IIIIRR

=−=− = − =

+

The internal generated voltage A

E is

()()

240 V 54.13 A 0.40 218.3 V

ATAA

EVIR=− = − Ω=

The field current is the same as before, and there is no armature reaction, so Ao

E is still 271 V at a speed

o

n of 1200 r/min. Therefore,

()

218.3 V 1200 r/min 967 r/min

271 V

A

o

Ao

E

nn

E

== =

The speed regulation is

nl fl

fl

1063 r/min 967 r/min

SR 100% 100% 9.9%

967 r/min

nn

n

−−

=×= ×=

216

9-3. If the motor is operating at full load and if its variable resistance adj

R is increased to 250 Ω, what is the

new speed of the motor? Compare the full-load speed of the motor with adj

R = 175 Ω to the full-load speed

with adj

R = 250 Ω. (Assume no armature reaction, as in the previous problem.)

S

OLUTION If adj

R is set to 250 Ω, the field current is now

adj

240 V 240 V 0.686 A

250 100 325

T

F

V

IRR

== ==

+Ω+ΩΩ

Since the motor is still at full load, A

E is still 218.3 V. From the magnetization curve (Figure P9-1), the

new field current F

I would produce a voltage Ao

E of 247 V at a speed o

n of 1200 r/min. Therefore,

()

218.3 V 1200 r/min 1061 r/min

247 V

A

o

Ao

E

nn

E

== =

Note that

adj

R has increased, and as a result the speed of the motor n increased.

9-4. Assume that the motor is operating at full load and that the variable resistor Radj is again 175 Ω. If the

armature reaction is 1200 A⋅turns at full load, what is the speed of the motor? How does it compare to the

result for Problem 9-2?

S

OLUTION The field current is again 0.87 A, and the motor is again at full load conditions. However, this

time there is an armature reaction of 1200 A⋅turns, and the effective field current is

*AR 1200 A turns

0.87 A 0.426 A

2700 turns

FF

F

II

N

⋅

=− = − =

From Figure P9-1, this field current would produce an internal generated voltage Ao

E of 181 V at a speed

o

n of 1200 r/min. The actual internal generated voltage A

E at these conditions is

()()

240 V 54.13 A 0.40 218.3 V

ATAA

EVIR=− = − Ω=

Therefore, the speed n with a voltage of 240 V would be

()

218.3 V 1200 r/min 1447 r/min

181 V

A

o

Ao

E

nn

E

== =

If all other conditions are the same, the motor with armature reaction runs at a higher speed than the motor

without armature reaction.

9-5. If Radj can be adjusted from 100 to 400 Ω, what are the maximum and minimum no-load speeds possible

with this motor?

S

OLUTION The minimum speed will occur when adj

R = 100 Ω, and the maximum speed will occur when

adj

R = 400 Ω. The field current when adj

R = 100 Ω is:

adj

240 V 240 V 1.20 A

100 100 200

T

F

V

IRR

== ==

+Ω+ΩΩ

From Figure P9-1, this field current would produce an internal generated voltage Ao

E of 287 V at a speed

o

n of 1200 r/min. Therefore, the speed n with a voltage of 240 V would be

217

A

Ao o

En

=

()

240 V 1200 r/min 1004 r/min

287 V

A

o

Ao

E

nn

E

== =

The field current when adj

R = 400 Ω is:

adj

240 V 240 V 0.480 A

400 100 500

T

F

V

IRR

== ==

+Ω+ΩΩ

From Figure P9-1, this field current would produce an internal generated voltage Ao

E of 199 V at a speed

o

n of 1200 r/min. Therefore, the speed n with a voltage of 240 V would be

A

Ao o

En

=

()

240 V 1200 r/min 1447 r/min

199 V

A

o

Ao

E

nn

E

== =

9-6. What is the starting current of this machine if it is started by connecting it directly to the power supply VT?

How does this starting current compare to the full-load current of the motor?

S

OLUTION The starting current of this machine (ignoring the small field current) is

,start

240 V 600 A

0.40

T

L

A

V

IR

== =

Ω

The rated current is 55 A, so the starting current is 10.9 times greater than the full-load current. This much

current is extremely likely to damage the motor.

9-7. Plot the torque-speed characteristic of this motor assuming no armature reaction, and again assuming a

full-load armature reaction of 1200 A⋅turns.

S

OLUTION This problem is best solved with MATLAB, since it involves calculating the torque-speed values

at many points. A MATLAB program to calculate and display both torque-speed characteristics is shown

below.

% M-file: prob9_7.m

% M-file to create a plot of the torque-speed curve of the

% the shunt dc motor with and without armature reaction.

% Get the magnetization curve. Note that this curve is

% defined for a speed of 1200 r/min.

load p91_mag.dat

if_values = p91_mag(:,1);

ea_values = p91_mag(:,2);

n_0 = 1200;

% First, initialize the values needed in this program.

v_t = 240; % Terminal voltage (V)

r_f = 100; % Field resistance (ohms)

r_adj = 175; % Adjustable resistance (ohms)

r_a = 0.40; % Armature resistance (ohms)

i_l = 0:1:55; % Line currents (A)

n_f = 2700; % Number of turns on field

218

f_ar0 = 1200; % Armature reaction @ 55 A (A-t/m)

% Calculate the armature current for each load.

i_a = i_l - v_t / (r_f + r_adj);

% Now calculate the internal generated voltage for

% each armature current.

e_a = v_t - i_a * r_a;

% Calculate the armature reaction MMF for each armature

% current.

f_ar = (i_a / 55) * f_ar0;

% Calculate the effective field current with and without

% armature reaction. Ther term i_f_ar is the field current

% with armature reaction, and the term i_f_noar is the

% field current without armature reaction.

i_f_ar = v_t / (r_f + r_adj) - f_ar / n_f;

i_f_noar = v_t / (r_f + r_adj);

% Calculate the resulting internal generated voltage at

% 1200 r/min by interpolating the motor's magnetization

% curve.

e_a0_ar = interp1(if_values,ea_values,i_f_ar);

e_a0_noar = interp1(if_values,ea_values,i_f_noar);

% Calculate the resulting speed from Equation (9-13).

n_ar = ( e_a ./ e_a0_ar ) * n_0;

n_noar = ( e_a ./ e_a0_noar ) * n_0;

% Calculate the induced torque corresponding to each

% speed from Equations (8-55) and (8-56).

t_ind_ar = e_a .* i_a ./ (n_ar * 2 * pi / 60);

t_ind_noar = e_a .* i_a ./ (n_noar * 2 * pi / 60);

% Plot the torque-speed curves

figure(1);

plot(t_ind_noar,n_noar,'b-','LineWidth',2.0);

hold on;

plot(t_ind_ar,n_ar,'k--','LineWidth',2.0);

xlabel('\bf\tau_{ind} (N-m)');

ylabel('\bf\itn_{m} \rm\bf(r/min)');

title ('\bfShunt DC Motor Torque-Speed Characteristic');

legend('No armature reaction','With armature reaction');

axis([ 0 125 800 1250]);

grid on;

hold off;

219

The resulting plot is shown below:

020 40 60 80 100 120

800

850

900

950

1000

1050

1100

1150

1200

1250

τ

ind

(N-m)

n

m

(r/min)

Shunt DC Motor Torque-Speed Characteristic

No armature reaction

With armature reaction

For Problems 9-8 and 9-9, the shunt dc motor is reconnected separately excited, as shown in Figure P9-3. It has a

fixed field voltage VF of 240 V and an armature voltage VA that can be varied from 120 to 240 V.

Note: Figure P9-3 shows incorrect values for RA and RF in the first printing of this

book. The correct values are given in the text, but shown incorrectly on the

figure. This will be corrected at the second printing.

9-8. What is the no-load speed of this separately excited motor when adj

R = 175 Ω and (a) A

V = 120 V, (b) A

V

= 180 V, (c) A

V = 240 V?

S

OLUTION At no-load conditions, AA

EV=. The field current is given by

adj

240 V 240 V 0.873 A

175 100 275

F

V

IRR

== ==

+Ω+ΩΩ

From Figure P9-1, this field current would produce an internal generated voltage Ao

E of 271 V at a speed

o

n of 1200 r/min. Therefore, the speed n with a voltage of 240 V would be

220

A

Ao o

En

=

A

o

Ao

E

nn

E

=

(a) If

A

V = 120 V, then A

E = 120 V, and

()

120 V 1200 r/min 531 r/min

271 V

n

==

(a) If

A

V = 180 V, then A

E = 180 V, and

()

180 V 1200 r/min 797 r/min

271 V

n

==

(a) If

A

V = 240 V, then A

E = 240 V, and

()

240 V 1200 r/min 1063 r/min

271 V

n

==

9-9. For the separately excited motor of Problem 9-8:

(a) What is the maximum no-load speed attainable by varying both A

V and adj

R?

(b) What is the minimum no-load speed attainable by varying both A

V and adj

R?

S

OLUTION

(a) The maximum speed will occur with the maximum A

V and the maximum adj

R. The field current

when adj

R = 400 Ω is:

adj

240 V 240 V 0.48 A

400 100 500

T

F

V

IRR

== ==

+Ω+ΩΩ

From Figure P9-1, this field current would produce an internal generated voltage Ao

E of 199 V at a speed

o

n of 1200 r/min. At no-load conditions, the maximum internal generated voltage AA

EV= = 240 V.

Therefore, the speed n with a voltage of 240 V would be

A

Ao o

En

=

()

240 V 1200 r/min 1447 r/min

199 V

A

o

Ao

E

nn

E

== =

(b) The minimum speed will occur with the minimum A

V and the minimum adj

R. The field current when

adj

R = 100 Ω is:

adj

240 V 240 V 1.2 A

100 100 200

T

F

V

IRR

== ==

+Ω+ΩΩ

From Figure P9-1, this field current would produce an internal generated voltage Ao

E of 287 V at a speed

o

n of 1200 r/min. At no-load conditions, the minimum internal generated voltage AA

EV= = 120 V.

Therefore, the speed n with a voltage of 120 V would be

221

A

Ao o

En

=

()

120 V 1200 r/min 502 r/min

287 V

A

o

Ao

E

nn

E

== =

9-10. If the motor is connected cumulatively compounded as shown in Figure P9-4 and if Radj = 175 Ω, what is

its no-load speed? What is its full-load speed? What is its speed regulation? Calculate and plot the torque-

speed characteristic for this motor. (Neglect armature effects in this problem.)

Note: Figure P9-4 shows incorrect values for RA + RS and RF in the first printing of

this book. The correct values are given in the text, but shown incorrectly on

the figure. This will be corrected at the second printing.

S

OLUTION At no-load conditions, 240 V

AT

EV== . The field current is given by

adj

240 V 240 V 0.873 A

175 100 275

F

V

IRR

== ==

+Ω+ΩΩ

From Figure P9-1, this field current would produce an internal generated voltage Ao

E of 271 V at a speed

o

n of 1200 r/min. Therefore, the speed n with a voltage of 240 V would be

A

Ao o

En

=

()

240 V 1200 r/min 1063 r/min

271 V

A

o

Ao

E

nn

E

== =

At full load conditions, the armature current is

adj

55 A 0.87 A 54.13 A

T

ALF L

F

V

IIIIRR

=−=− = − =

+

The internal generated voltage A

E is

()

()()

240 V 54.13 A 0.44 216.2 V

ATAA S

EVIRR=− + = − Ω=

The equivalent field current is

()

*SE 27 turns

0.873 A 54.13 A 1.41 A

2700 turns

FF A

F

N

II I

N

=+ = + =

222

From Figure P9-1, this field current would produce an internal generated voltage Ao

E of 290 V at a speed

o

n of 1200 r/min. Therefore,

()

216.2 V 1200 r/min 895 r/min

290 V

A

o

Ao

E

nn

E

== =

The speed regulation is

nl fl

fl

1063 r/min 895 r/min

SR 100% 100% 18.8%

895 r/min

nn

n

−−

=×= ×=

The torque-speed characteristic can best be plotted with a MATLAB program. An appropriate program is

shown below.

% M-file: prob9_10.m

% M-file to create a plot of the torque-speed curve of the

% a cumulatively compounded dc motor without

% armature reaction.

% Get the magnetization curve. Note that this curve is

% defined for a speed of 1200 r/min.

load p91_mag.dat

if_values = p91_mag(:,1);

ea_values = p91_mag(:,2);

n_0 = 1200;

% First, initialize the values needed in this program.

v_t = 240; % Terminal voltage (V)

r_f = 100; % Field resistance (ohms)

r_adj = 175; % Adjustable resistance (ohms)

r_a = 0.44; % Armature + series resistance (ohms)

i_l = 0:55; % Line currents (A)

n_f = 2700; % Number of turns on shunt field

n_se = 27; % Number of turns on series field

% Calculate the armature current for each load.

i_a = i_l - v_t / (r_f + r_adj);

% Now calculate the internal generated voltage for

% each armature current.

e_a = v_t - i_a * r_a;

% Calculate the effective field current for each armature

% current.

i_f = v_t / (r_f + r_adj) + (n_se / n_f) * i_a;

% Calculate the resulting internal generated voltage at

% 1200 r/min by interpolating the motor's magnetization

% curve.

e_a0 = interp1(if_values,ea_values,i_f);

% Calculate the resulting speed from Equation (9-13).

n = ( e_a ./ e_a0 ) * n_0;

% Calculate the induced torque corresponding to each

% speed from Equations (8-55) and (8-56).

223

t_ind = e_a .* i_a ./ (n * 2 * pi / 60);

% Plot the torque-speed curves

figure(1);

plot(t_ind,n,'b-','LineWidth',2.0);

xlabel('\bf\tau_{ind} (N-m)');

ylabel('\bf\itn_{m} \rm\bf(r/min)');

title ('\bfCumulatively-Compounded DC Motor Torque-Speed

Characteristic');

axis([0 125 800 1250]);

grid on;

The resulting plot is shown below:

Compare this torque-speed curve to that of the shunt motor in Problem 9-7. (Both curves are plotted on the

same scale to facilitate comparison.)

9-11. The motor is connected cumulatively compounded and is operating at full load. What will the new speed of

the motor be if adj

R is increased to 250 Ω? How does the new speed compared to the full-load speed

calculated in Problem 9-10?

S

OLUTION If adj

R is increased to 250 Ω, the field current is given by

adj

240 V 240 V 0.686 A

250 100 350

T

F

V

IRR

== ==

+Ω+ΩΩ

At full load conditions, the armature current is

55 A 0.686 A 54.3 A

ALF

III=−= − =

The internal generated voltage A

E is

()

(

)

(

)

240 V 54.3 A 0.44 216.1 V

ATAA S

EVIRR=− + = − Ω=

224

The equivalent field current is

()

*SE 27 turns

0.686 A 54.3 A 1.23 A

2700 turns

FF A

F

N

II I

N

=+ = + =

From Figure P9-1, this field current would produce an internal generated voltage Ao

E of 288 V at a speed

o

n of 1200 r/min. Therefore,

()

216.1 V 1200 r/min 900 r/min

288 V

A

o

Ao

E

nn

E

== =

The new full-load speed is higher than the full-load speed in Problem 9-10.

9-12. The motor is now connected differentially compounded.

(a) If Radj = 175 Ω, what is the no-load speed of the motor?

(b) What is the motor’s speed when the armature current reaches 20 A? 40 A? 60 A?

(c) Calculate and plot the torque-speed characteristic curve of this motor.

S

OLUTION

(a) At no-load conditions, 240 V

AT

EV== . The field current is given by

adj

240 V 240 V 0.873 A

175 100 275

F

V

IRR

== ==

+Ω+ΩΩ

From Figure P9-1, this field current would produce an internal generated voltage Ao

E of 271 V at a speed

o

n of 1200 r/min. Therefore, the speed n with a voltage of 240 V would be

A

Ao o

En

=

()

240 V 1200 r/min 1063 r/min

271 V

A

o

Ao

E

nn

E

== =

(b) At

A

I = 20A, the internal generated voltage A

E is

()

()( )

240 V 20 A 0.44 231.2 V

ATAA S

EVIRR=− + = − Ω=

The equivalent field current is

()

*SE 27 turns

0.873 A 20 A 0.673 A

2700 turns

FF A

F

N

II I

N

=− = − =

From Figure P9-1, this field current would produce an internal generated voltage Ao

E of 245 V at a speed

o

n of 1200 r/min. Therefore,

()

231.2 V 1200 r/min 1132 r/min

245 V

A

o

Ao

E

nn

E

== =

At

A

I = 40A, the internal generated voltage A

E is

()

()( )

240 V 40 A 0.44 222.4 V

ATAA S

EVIRR=− + = − Ω=

The equivalent field current is

225

()

*SE 27 turns

0.873 A 40 A 0.473 A

2700 turns

FF A

F

N

II I

N

=− = − =

From Figure P9-1, this field current would produce an internal generated voltage Ao

E of 197 V at a speed

o

n of 1200 r/min. Therefore,

()

227.4 V 1200 r/min 1385 r/min

197 V

A

o

Ao

E

nn

E

== =

At

A

I = 60A, the internal generated voltage A

E is

()

()( )

240 V 60 A 0.44 213.6 V

ATAA S

EVIRR=− + = − Ω=

The equivalent field current is

()

*SE 27 turns

0.873 A 60 A 0.273 A

2700 turns

FF A

F

N

II I

N

=− = − =

From Figure P9-1, this field current would produce an internal generated voltage Ao

E of 121 V at a speed

o

n of 1200 r/min. Therefore,

()

213.6 V 1200 r/min 2118 r/min

121 V

A

o

Ao

E

nn

E

== =

(c) The torque-speed characteristic can best be plotted with a MATLAB program. An appropriate

program is shown below.

% M-file: prob9_12.m

% M-file to create a plot of the torque-speed curve of the

% a differentially compounded dc motor withwithout

% armature reaction.

% Get the magnetization curve. Note that this curve is

% defined for a speed of 1200 r/min.

load p91_mag.dat

if_values = p91_mag(:,1);

ea_values = p91_mag(:,2);

n_0 = 1200;

% First, initialize the values needed in this program.

v_t = 240; % Terminal voltage (V)

r_f = 100; % Field resistance (ohms)

r_adj = 175; % Adjustable resistance (ohms)

r_a = 0.44; % Armature + series resistance (ohms)

i_l = 0:50; % Line currents (A)

n_f = 2700; % Number of turns on shunt field

n_se = 27; % Number of turns on series field

% Calculate the armature current for each load.

i_a = i_l - v_t / (r_f + r_adj);

% Now calculate the internal generated voltage for

% each armature current.

e_a = v_t - i_a * r_a;

% Calculate the effective field current for each armature

226

% current.

i_f = v_t / (r_f + r_adj) - (n_se / n_f) * i_a;

% Calculate the resulting internal generated voltage at

% 1200 r/min by interpolating the motor's magnetization

% curve.

e_a0 = interp1(if_values,ea_values,i_f);

% Calculate the resulting speed from Equation (9-13).

n = ( e_a ./ e_a0 ) * n_0;

% Calculate the induced torque corresponding to each

% speed from Equations (8-55) and (8-56).

t_ind = e_a .* i_a ./ (n * 2 * pi / 60);

% Plot the torque-speed curves

figure(1);

plot(t_ind,n,'b-','LineWidth',2.0);

xlabel('\bf\tau_{ind} (N-m)');

ylabel('\bf\itn_{m} \rm\bf(r/min)');

title ('\bfDifferentially-Compounded DC Motor Torque-Speed

Characteristic');

axis([0 100 800 1600]);

grid on;

The resulting plot is shown below:

Compare this torque-speed curve to that of the shunt motor in Problem 9-7 and the cumulatively-

compounded motor in Problem 9-10. (Note that this plot has a larger vertical scale to accommodate the

speed runaway of the differentially-compounded motor.)

9-13. A 7.5-hp 120-V series dc motor has an armature resistance of 0.2 Ω and a series field resistance of 0.16 Ω.

At full load, the current input is 58 A, and the rated speed is 1050 r/min. Its magnetization curve is shown

227

in Figure P9-5. The core losses are 200 W, and the mechanical losses are 240 W at full load. Assume that

the mechanical losses vary as the cube of the speed of the motor and that the core losses are constant.

Note: An electronic version of this magnetization curve can be found in file

p95_mag.dat, which can be used with MATLAB programs. Column 1

contains field current in amps, and column 2 contains the internal generated

voltage EA in volts.

(a) What is the efficiency of the motor at full load?

(b) What are the speed and efficiency of the motor if it is operating at an armature current of 35 A?

(c) Plot the torque-speed characteristic for this motor.

S

OLUTION

(a) The output power of this motor at full load is

()( )

OUT 7.5 hp 746 W/hp 5595 WP==

The input power is

228

()()

IN 120 V 58 A 6960 W

TL

PVI== =

Therefore the efficiency is

OUT

IN

5595 W

100% 100% 80.4%

6960 W

P

η

=× = × =

(b) If the armature current is 35 A, then the input power to the motor will be

()()

IN 120 V 35 A 4200 W

TL

PVI== =

The internal generated voltage at this condition is

()

()( )

2120 V 35 A 0.20 0.16 107.4 V

ATAAS

EVIRR=− + = − Ω+ Ω=

and the internal generated voltage at rated conditions is

()

()( )

1120 V 58 A 0.20 0.16 99.1 V

ATAAS

EVIRR=− + = − Ω+ Ω=

The final speed is given by the equation

,2 2

222

122,11

Ao

A

AAo

En

EK

EK En

φω

==

since the ratio ,2 ,1

/

Ao Ao

EE is the same as the ratio 21

/

φ

. Therefore, the final speed is

,1

2

21

1,2

Ao

A

AAo

E

nn

EE

=

From Figure P9-5, the internal generated voltage ,2Ao

E for a current of 35 A and a speed of o

n = 1200

r/min is ,2Ao

E = 115 V, and the internal generated voltage ,1Ao

E for a current of 58 A and a speed of o

n =

1200 r/min is ,1Ao

E = 134 V.

()

,1

2

21

1,2

107.4 V 134 V 1050 r/min 1326 r/min

99.1 V 115 V

Ao

A

AAo

E

nn

EE

== =

The power converted from electrical to mechanical form is

()()

conv 107.4 V 35 A 3759 W

AA

PEI== =

The core losses in the motor are 200 W, and the mechanical losses in the motor are 240 W at a speed of

1050 r/min. The mechanical losses in the motor scale proportionally to the cube of the rotational speedm

so the mechanical losses at 1326 r/min are

() ()

33

2

mech

1

1326 r/min

240 W 240 W 483 W

1050 r/min

n

Pn

== =

Therefore, the output power is

OUT conv mech core 3759 W 483 W 200 W 3076 WPPPP=−−= − − =

and the efficiency is

OUT

IN

3076 W

100% 100% 73.2%

4200 W

P

η

=× = × =

(c) A MATLAB program to plot the torque-speed characteristic of this motor is shown below:

229

% M-file: prob9_13.m

% M-file to create a plot of the torque-speed curve of the

% the series dc motor in Problem 9-13.

% Get the magnetization curve. Note that this curve is

% defined for a speed of 1200 r/min.

load p95_mag.dat

if_values = p95_mag(:,1);

ea_values = p95_mag(:,2);

n_0 = 1200;

% First, initialize the values needed in this program.

v_t = 120; % Terminal voltage (V)

r_a = 0.36; % Armature + field resistance (ohms)

i_a = 9:1:58; % Armature (line) currents (A)

% Calculate the internal generate voltage e_a.

e_a = v_t - i_a * r_a;

% Calculate the resulting internal generated voltage at

% 1200 r/min by interpolating the motor's magnetization

% curve. Note that the field current is the same as the

% armature current for this motor.

e_a0 = interp1(if_values,ea_values,i_a,'spline');

% Calculate the motor's speed, using the known fact that

% the motor runs at 1050 r/min at a current of 58 A. We

% know that

%

% Ea2 K' phi2 n2 Eao2 n2

% ----- = ------------ = ----------

% Ea1 K' phi1 n1 Eao1 n1

%

% Ea2 Eao1

% ==> n2 = ----- ------ n1

% Ea1 Eao2

%

% where Ea0 is the internal generated voltage at 1200 r/min

% for a given field current.

%

% Speed will be calculated by reference to full load speed

% and current.

n1 = 1050; % 1050 r/min at full load

Eao1 = interp1(if_values,ea_values,58,'spline');

Ea1 = v_t - 58 * r_a;

% Get speed

Eao2 = interp1(if_values,ea_values,i_a,'spline');

n = (e_a./Ea1) .* (Eao1 ./ Eao2) * n1;

% Calculate the induced torque corresponding to each

% speed from Equations (8-55) and (8-56).

t_ind = e_a .* i_a ./ (n * 2 * pi / 60);

% Plot the torque-speed curve

230

figure(1);

plot(t_ind,n,'b-','LineWidth',2.0);

hold on;

xlabel('\bf\tau_{ind} (N-m)');

ylabel('\bf\itn_{m} \rm\bf(r/min)');

title ('\bfSeries DC Motor Torque-Speed Characteristic');

grid on;

hold off;

The resulting torque-speed characteristic is shown below:

9-14. A 20-hp 240-V 76-A 900 r/min series motor has a field winding of 33 turns per pole. Its armature

resistance is 0.09 Ω, and its field resistance is 0.06 Ω. The magnetization curve expressed in terms of

magnetomotive force versus EA at 900 r/min is given by the following table:

EA, V 95 150 188 212 229 243

F, A turns⋅ 500 1000 1500 2000 2500 3000

Note: An electronic version of this magnetization curve can be found in file

prob9_14_mag.dat, which can be used with MATLAB programs. Column

1 contains magnetomotive force in ampere-turns, and column 2 contains the

internal generated voltage EA in volts.

Armature reaction is negligible in this machine.

(a) Compute the motor’s torque, speed, and output power at 33, 67, 100, and 133 percent of full-load

armature current. (Neglect rotational losses.)

(b) Plot the terminal characteristic of this machine.

S

OLUTION Note that this magnetization curve has been stored in a file called prob9_14_mag.dat. The

first column of the file is an array of mmf_values, and the second column is an array of ea_values.

These values are valid at a speed o

n = 900 r/min. Because the data in the file is relatively sparse, it is

231

important that interpolation be done using smooth curves, so be sure to specify the 'spline' option in

the MATLAB interp1 function:

load prob9_14_mag.dat;

mmf_values = prob9_14_mag(:,1);

ea_values = prob9_14_mag(:,2);

...

Eao = interp1(mmf_values,ea_values,mmf,'spline')

(a) Since full load corresponds to 76 A, this calculation must be performed for armature currents of 25.3

A, 50.7 A, 76 A, and 101.3 A.

If

A

I = 23.3 A, then

()

()( )

240 V 25.3 A 0.09 0.06 236.2 V

ATAA S

EVIRR=− + = − Ω+ Ω=

The magnetomotive force is

()()

33 turns 25.3 A 835 A turns

A

NI== = ⋅F, which produces a voltage Ao

E

of 134 V at o

n = 900 r/min. Therefore the speed of the motor at these conditions is

()

236.2 V 900 r/min 1586 r/min

134 V

A

o

Ao

E

nn

E

== =

The power converted from electrical to mechanical form is

()()

conv 236.2 V 25.3 A 5976 W

AA

PEI== =

Since the rotational losses are ignored, this is also the output power of the motor. The induced torque is

()

conv

ind

5976 W 36 N m

2 rad 1 min

1586 r/min 1 r 60 s

m

P

τπ

ω

== =⋅

If

A

I = 50.7 A, then

()

()( )

240 V 50.7 A 0.09 0.06 232.4 V

ATAA S

EVIRR=− + = − Ω+ Ω=

The magnetomotive force is

()()

33 turns 50.7 A 1672 A turns

A

NI== = ⋅F, which produces a voltage Ao

E

of 197 V at o

n = 900 r/min. Therefore the speed of the motor at these conditions is

()

232.4 V 900 r/min 1062 r/min

197 V

A

o

Ao

E

nn

E

== =

The power converted from electrical to mechanical form is

()()

conv 232.4 V 50.7 A 11,780 W

AA

PEI== =

Since the rotational losses are ignored, this is also the output power of the motor. The induced torque is

()

conv

ind

11,780 W 106 N m

2 rad 1 min

1062 r/min 1 r 60 s

m

P

τπ

ω

== = ⋅

If

A

I = 76 A, then

()

()( )

240 V 76 A 0.09 0.06 228.6 V

ATAA S

EVIRR=− + = − Ω+ Ω=

232

The magnetomotive force is

()()

33 turns 76 A 2508 A turns

A

NI== = ⋅F, which produces a voltage Ao

E

of 229 V at o

n = 900 r/min. Therefore the speed of the motor at these conditions is

()

228.6 V 900 r/min 899 r/min

229 V

A

o

Ao

E

nn

E

== =

The power converted from electrical to mechanical form is

()()

conv 228.6 V 76 A 17,370 W

AA

PEI== =

Since the rotational losses are ignored, this is also the output power of the motor. The induced torque is

()

conv

ind

17,370 W 185 N m

2 rad 1 min

899 r/min 1 r 60 s

m

P

τπ

ω

== = ⋅

If

A

I = 101.3 A, then

()

()( )

240 V 101.3 A 0.09 0.06 224.8 V

ATAA S

EVIRR=− + = − Ω+ Ω=

The magnetomotive force is

()()

33 turns 101.3 A 3343 A turns

A

NI== = ⋅F, which produces a voltage

Ao

E of 252 V at o

n = 900 r/min. Therefore the speed of the motor at these conditions is

()

224.8 V 900 r/min 803 r/min

252 V

A

o

Ao

E

nn

E

== =

The power converted from electrical to mechanical form is

()()

conv 224.8 V 101.3 A 22,770 W

AA

PEI== =

Since the rotational losses are ignored, this is also the output power of the motor. The induced torque is

()

conv

ind

22,770 W 271 N m

2 rad 1 min

803 r/min 1 r 60 s

m

P

τπ

ω

== = ⋅

(b) A MATLAB program to plot the torque-speed characteristic of this motor is shown below:

% M-file: series_ts_curve.m

% M-file to create a plot of the torque-speed curve of the

% the series dc motor in Problem 9-14.

% Get the magnetization curve. Note that this curve is

% defined for a speed of 900 r/min.

load prob9_14_mag.dat

mmf_values = prob9_14_mag(:,1);

ea_values = prob9_14_mag(:,2);

n_0 = 900;

% First, initialize the values needed in this program.

v_t = 240; % Terminal voltage (V)

r_a = 0.15; % Armature + field resistance (ohms)

i_a = 15:1:76; % Armature (line) currents (A)

n_s = 33; % Number of series turns on field

% Calculate the MMF for each load

233

f = n_s * i_a;

% Calculate the internal generate voltage e_a.

e_a = v_t - i_a * r_a;

% Calculate the resulting internal generated voltage at

% 900 r/min by interpolating the motor's magnetization

% curve. Specify cubic spline interpolation to provide

% good results with this sparse magnetization curve.

e_a0 = interp1(mmf_values,ea_values,f,'spline');

% Calculate the motor's speed from Equation (9-13).

n = (e_a ./ e_a0) * n_0;

% Calculate the induced torque corresponding to each

% speed from Equations (8-55) and (8-56).

t_ind = e_a .* i_a ./ (n * 2 * pi / 60);

% Plot the torque-speed curve

figure(1);

plot(t_ind,n,'b-','LineWidth',2.0);

hold on;

xlabel('\bf\tau_{ind} (N-m)');

ylabel('\bf\itn_{m} \rm\bf(r/min)');

title ('\bfSeries DC Motor Torque-Speed Characteristic');

%axis([ 0 700 0 5000]);

grid on;

hold off;

The resulting torque-speed characteristic is shown below:

9-15. A 300-hp 440-V 560-A, 863 r/min shunt dc motor has been tested, and the following data were taken:

Blocked-rotor test:

234

VA=16 3. V exclusive of brushes VF=440 V

IA=500 A IF=886. A

No-load operation:

VA=16 3. V including brushes IF=876. A

IA=231. A r/min 863=n

What is this motor’s efficiency at the rated conditions? [Note: Assume that (1) the brush voltage drop is 2

V; (2) the core loss is to be determined at an armature voltage equal to the armature voltage under full load;

and (3) stray load losses are 1 percent of full load.]

S

OLUTION The armature resistance of this motor is

,br

16.3 V 0.0326

500 A

A

V

RI

== = Ω

Under no-load conditions, the core and mechanical losses taken together (that is, the rotational losses) of

this motor are equal to the product of the internal generated voltage A

E and the armature current A

I, since

this is no output power from the motor at no-load conditions. Therefore, the rotational losses at rated speed

can be found as

()( )

brush 442 V 2 V 23.1 A 0.0326 439.2 V

AA AA

EVV IR=− − = − − Ω=

()()

rot conv 439.2 V 23.1 A 10.15 kW

AA

PP EI== = =

The input power to the motor at full load is

()()

IN 440 V 560 A 246.4 kW

TL

PVI== =

The output power from the motor at full load is

OUT IN CU rot brush stray

PPPPPP=−−− −

The copper losses are

()( )()( )

2

CU 560 A 0.0326 440 V 8.86 A 14.1 kW

AA FF

PIRVI=+= Ω+ =

The brush losses are

()( )

brush brush 2 V 560 A 1120 W

A

PVI== =

Therefore,

OUT IN CU rot brush stray

PPPPPP=−−− −

OUT 246.4 kW 14.1 kW 10.15 kW 1.12 kW 2.46 kW 218.6 kWP=−− −−=

The motor’s efficiency at full load is

OUT

IN

218.6 kW

100% 100% 88.7%

246.4 kW

P

η

=× = × =

Problems 9-16 to 9-19 refer to a 240-V 100-A dc motor which has both shunt and series windings. Its

characteristics are

RA = 0.14 Ω NF = 1500 turns

RS = 0.04 Ω NSE = 12 turns

235

RF = 200 Ω nm = 1200 r/min

Radj = 0 to 300 Ω, currently set to 120 Ω

This motor has compensating windings and interpoles. The magnetization curve for this motor at 1200 r/min is

shown in Figure P9-6.

Note: An electronic version of this magnetization curve can be found in file

p96_mag.dat, which can be used with MATLAB programs. Column 1

contains field current in amps, and column 2 contains the internal generated

voltage EA in volts.

9-16. The motor described above is connected in shunt.

(a) What is the no-load speed of this motor when Radj = 120 Ω?

(b) What is its full-load speed?

(c) Under no-load conditions, what range of possible speeds can be achieved by adjusting Radj ?

S

OLUTION Note that this magnetization curve has been stored in a file called p96_mag.dat. The first

column of the file is an array of ia_values, and the second column is an array of ea_values. These

values are valid at a speed o

n = 1200 r/min. These values can be used with the MATLAB interp1

function to look up an internal generated voltage as follows:

load p96_mag.dat;

236

if_values = p96_mag(:,1);

ea_values = p96_mag(:,2);

...

Ea = interp1(if_values,ea_values,if,'spline')

(a) If adj

R = 120 Ω, the total field resistance is 320 Ω, and the resulting field current is

adj

240 V 0.75 A

200 120

T

F

V

IRR

== =

+Ω+Ω

This field current would produce a voltage Ao

E of 256 V at a speed of o

n = 1200 r/min. The actual A

E is

240 V, so the actual speed will be

()

240 V 1200 r/min 1125 r/min

256 V

A

o

Ao

E

nn

E

== =

(b) At full load, 100 A 0.75 A 99.25 A

ALF

III=−= − = , and

()()

240 V 99.25 A 0.14 226.1 V

ATAA

EVIR=− = − Ω=

Therefore, the speed at full load will be

()

226.1 V 1200 r/min 1060 r/min

256 V

A

o

Ao

E

nn

E

== =

(c) If

adj

R is maximum at no-load conditions, the total resistance is 500 Ω, and

adj

240 V 0.48 A

200 300

T

F

V

IRR

== =

+Ω+Ω

This field current would produce a voltage Ao

E of 200 V at a speed of o

n = 1200 r/min. The actual A

E is

240 V, so the actual speed will be

()

240 V 1200 r/min 1440 r/min

200 V

A

o

Ao

E

nn

E

== =

If

Radj is minimum at no-load conditions, the total resistance is 200 Ω, and

adj

240 V 1.2 A

200 0

T

F

V

IRR

== =

+Ω+Ω

This field current would produce a voltage Ao

E of 287 V at a speed of o

n = 1200 r/min. The actual A

E is

240 V, so the actual speed will be

()

240 V 1200 r/min 1004 r/min

287 V

A

o

Ao

E

nn

E

== =

9-17. This machine is now connected as a cumulatively compounded dc motor with adj

R = 120 Ω.

(a) What is the full-load speed of this motor?

(b) Plot the torque-speed characteristic for this motor.

(c) What is its speed regulation?

S

OLUTION

237

(a) At full load, 100 A 0.75 A 99.25 A

ALF

III=−= − = , and

()

(

)

(

)

240 V 99.25 A 0.14 0.05 221.1 V

ATAA S

EVIRR=− + = − Ω+ Ω=

The actual field current will be

adj

240 V 0.75 A

200 120

T

F

V

IRR

== =

+Ω+Ω

and the effective field current will be

()

*SE 12 turns

0.75 A 99.25 A 1.54 A

1500 turns

FF A

F

N

II I

N

=+=+ =

This field current would produce a voltage Ao

E of 290 V at a speed of o

n = 1200 r/min. The actual A

E

is 240 V, so the actual speed at full load will be

()

221.1 V 1200 r/min 915 r/min

290 V

A

o

Ao

E

nn

E

== =

(b) A MATLAB program to calculate the torque-speed characteristic of this motor is shown below:

% M-file: prob9_17.m

% M-file to create a plot of the torque-speed curve of the

% a cumulatively compounded dc motor.

% Get the magnetization curve.

load p96_mag.dat;

if_values = p96_mag(:,1);

ea_values = p96_mag(:,2);

n_0 = 1200;

% First, initialize the values needed in this program.

v_t = 240; % Terminal voltage (V)

r_f = 200; % Field resistance (ohms)

r_adj = 120; % Adjustable resistance (ohms)

r_a = 0.19; % Armature + series resistance (ohms)

i_l = 0:2:100; % Line currents (A)

n_f = 1500; % Number of turns on shunt field

n_se = 12; % Number of turns on series field

% Calculate the armature current for each load.

i_a = i_l - v_t / (r_f + r_adj);

% Now calculate the internal generated voltage for

% each armature current.

e_a = v_t - i_a * r_a;

% Calculate the effective field current for each armature

% current.

i_f = v_t / (r_f + r_adj) + (n_se / n_f) * i_a;

% Calculate the resulting internal generated voltage at

% 1800 r/min by interpolating the motor's magnetization

% curve.

e_a0 = interp1(if_values,ea_values,i_f);

238

% Calculate the resulting speed from Equation (9-13).

n = ( e_a ./ e_a0 ) * n_0;

% Calculate the induced torque corresponding to each

% speed from Equations (8-55) and (8-56).

t_ind = e_a .* i_a ./ (n * 2 * pi / 60);

% Plot the torque-speed curves

figure(1);

plot(t_ind,n,'b-','LineWidth',2.0);

xlabel('\bf\tau_{ind} (N-m)');

ylabel('\bf\itn_{m} \rm\bf(r/min)');

title ('\bfCumulatively-Compounded DC Motor Torque-Speed

Characteristic');

axis([0 200 900 1600]);

grid on;

The resulting torque-speed characteristic is shown below:

(c) The no-load speed of this machine is the same as the no-load speed of the corresponding shunt dc

motor with adj

R = 120 Ω, which is 1125 r/min. The speed regulation of this motor is thus

nl fl

fl

1125 r/min - 915 r/min

SR 100% 100% 23.0%

915 r/min

nn

n

−

=×= ×=

9-18. The motor is reconnected differentially compounded with adj

R = 120 Ω. Derive the shape of its torque-

speed characteristic.

S

OLUTION A MATLAB program to calculate the torque-speed characteristic of this motor is shown below:

% M-file: prob9_18.m

% M-file to create a plot of the torque-speed curve of the

239

% a differentially compounded dc motor.

% Get the magnetization curve.

load p96_mag.dat;

if_values = p96_mag(:,1);

ea_values = p96_mag(:,2);

n_0 = 1200;

% First, initialize the values needed in this program.

v_t = 240; % Terminal voltage (V)

r_f = 200; % Field resistance (ohms)

r_adj = 120; % Adjustable resistance (ohms)

r_a = 0.19; % Armature + series resistance (ohms)

i_l = 0:2:40; % Line currents (A)

n_f = 1500; % Number of turns on shunt field

n_se = 12; % Number of turns on series field

% Calculate the armature current for each load.

i_a = i_l - v_t / (r_f + r_adj);

% Now calculate the internal generated voltage for

% each armature current.

e_a = v_t - i_a * r_a;

% Calculate the effective field current for each armature

% current.

i_f = v_t / (r_f + r_adj) - (n_se / n_f) * i_a;

% Calculate the resulting internal generated voltage at

% 1800 r/min by interpolating the motor's magnetization

% curve.

e_a0 = interp1(if_values,ea_values,i_f);

% Calculate the resulting speed from Equation (9-13).

n = ( e_a ./ e_a0 ) * n_0;

% Calculate the induced torque corresponding to each

% speed from Equations (8-55) and (8-56).

t_ind = e_a .* i_a ./ (n * 2 * pi / 60);

% Plot the torque-speed curves

figure(1);

plot(t_ind,n,'b-','LineWidth',2.0);

xlabel('\bf\tau_{ind} (N-m)');

ylabel('\bf\itn_{m} \rm\bf(r/min)');

title ('\bfDifferentially-Compounded DC Motor Torque-Speed

Characteristic');

axis([0 200 900 1600]);

grid on;

240

The resulting torque-speed characteristic is shown below:

This curve is plotted on the same scale as the torque-speed curve in Problem 6-17. Compare the two

curves.

9-19. A series motor is now constructed from this machine by leaving the shunt field out entirely. Derive the

torque-speed characteristic of the resulting motor.

S

OLUTION This motor will have extremely high speeds, since there are only a few series turns, and the flux

in the motor will be very small. A MATLAB program to calculate the torque-speed characteristic of this

motor is shown below:

% M-file: prob9_19.m

% M-file to create a plot of the torque-speed curve of the

% a series dc motor. This motor was formed by removing

% the shunt field from the cumulatively-compounded machine

% if Problem 9-17.

% Get the magnetization curve.

load p96_mag.dat;

if_values = p96_mag(:,1);

ea_values = p96_mag(:,2);

n_0 = 1200;

% First, initialize the values needed in this program.

v_t = 240; % Terminal voltage (V)

r_a = 0.19; % Armature + series resistance (ohms)

i_l = 20:1:45; % Line currents (A)

n_f = 1500; % Number of turns on shunt field

n_se = 12; % Number of turns on series field

% Calculate the armature current for each load.

i_a = i_l;

241

% Now calculate the internal generated voltage for

% each armature current.

e_a = v_t - i_a * r_a;

% Calculate the effective field current for each armature

% current. (Note that the magnetization curve is defined

% in terms of shunt field current, so we will have to

% translate the series field current into an equivalent

% shunt field current.

i_f = (n_se / n_f) * i_a;

% Calculate the resulting internal generated voltage at

% 1800 r/min by interpolating the motor's magnetization

% curve.

e_a0 = interp1(if_values,ea_values,i_f);

% Calculate the resulting speed from Equation (9-13).

n = ( e_a ./ e_a0 ) * n_0;

% Calculate the induced torque corresponding to each

% speed from Equations (8-55) and (8-56).

t_ind = e_a .* i_a ./ (n * 2 * pi / 60);

% Plot the torque-speed curves

figure(1);

plot(t_ind,n,'b-','LineWidth',2.0);

xlabel('\bf\tau_{ind} (N-m)');

ylabel('\bf\itn_{m} \rm\bf(r/min)');

title ('\bfSeries DC Motor Torque-Speed Characteristic');

grid on;

The resulting torque-speed characteristic is shown below:

242

The extreme speeds in this characteristic are due to the very light flux in the machine. To make a practical

series motor out of this machine, it would be necessary to include 20 to 30 series turns instead of 12.

9-20. An automatic starter circuit is to be designed for a shunt motor rated at 15 hp, 240 V, and 60 A. The

armature resistance of the motor is 0.15 Ω, and the shunt field resistance is 40 Ω. The motor is to start

with no more than 250 percent of its rated armature current, and as soon as the current falls to rated value,

a starting resistor stage is to be cut out. How many stages of starting resistance are needed, and how big

should each one be?

S

OLUTION The rated line current of this motor is 60 A, and the rated armature current is ALF

III=− = 60

A – 6 A = 54 A. The maximum desired starting current is (2.5)(54 A) = 135 A. Therefore, the total initial

starting resistance must be

start,1

240 V 1.778

135 A

A

RR+= = Ω

start,1 1.778 0.15 1.628 R=Ω−Ω=Ω

The current will fall to rated value when A

E rises to

()()

240 V 1.778 54 A 144 V

A

E=−Ω =

At that time, we want to cut out enough resistance to get the current back up to 135 A. Therefore,

start,2

240 V 144 V 0.711

135 A

A

RR −

+= =Ω

start,2 0.711 0.15 0.561 R=Ω−Ω=Ω

With this resistance in the circuit, the current will fall to rated value when A

E rises to

(

)

(

)

240 V 0.711 54 A 201.6 V

A

E=−Ω =

At that time, we want to cut out enough resistance to get the current back up to 185 A. Therefore,

start,3

240 V 201.6 V 0.284

135 A

A

RR −

+= = Ω

start,3 0.284 0.15 0.134 R=Ω−Ω=Ω

With this resistance in the circuit, the current will fall to rated value when A

E rises to

()()

240 V 0.284 54 A 224.7 V

A

E=−Ω =

If the resistance is cut out when A

E reaches 228,6 V, the resulting current is

240 V 224.7 V 102 A 135 A

0.15

A

I−

==<

Ω,

so there are only three stages of starting resistance. The three stages of starting resistance can be found

from the resistance in the circuit at each state during starting.

start,1 1 2 3 1.628 RRRR=++= Ω

start,2 2 3 0.561 RRR=+= Ω

start,3 3 0.134 RR== Ω

Therefore, the starting resistances are

11.067 R=Ω

20.427 R=Ω

30.134 R=Ω

243

9-21. A 15-hp 120-V 1800 r/min shunt dc motor has a full-load armature current of 60 A when operating at rated

conditions. The armature resistance of the motor is A

R = 0.15 Ω, and the field resistance F

R is 80 Ω.

The adjustable resistance in the field circuit adj

R may be varied over the range from 0 to 200 Ω and is

currently set to 90 Ω. Armature reaction may be ignored in this machine. The magnetization curve for this

motor, taken at a speed of 1800 r/min, is given in tabular form below:

EA, V 5 78 95 112 118 126

IF, A 0.00 0.80 1.00 1.28 1.44 2.88

Note: An electronic version of this magnetization curve can be found in file

prob9_21_mag.dat, which can be used with MATLAB programs. Column

1 contains field current in amps, and column 2 contains the internal generated

voltage EA in volts.

(a) What is the speed of this motor when it is running at the rated conditions specified above?

(b) The output power from the motor is 7.5 hp at rated conditions. What is the output torque of the motor?

(c) What are the copper losses and rotational losses in the motor at full load (ignore stray losses)?

(d) What is the efficiency of the motor at full load?

(e) If the motor is now unloaded with no changes in terminal voltage or Radj , what is the no-load speed of

the motor?

(f) Suppose that the motor is running at the no-load conditions described in part (e). What would happen

to the motor if its field circuit were to open? Ignoring armature reaction, what would the final steady-

state speed of the motor be under those conditions?

(g) What range of no-load speeds is possible in this motor, given the range of field resistance adjustments

available with adj

R?

S

OLUTION

(a) If adj

R = 90 Ω, the total field resistance is 170 Ω, and the resulting field current is

adj

230 V 1.35 A

90 80

T

F

V

IRR

== =

+Ω+Ω

This field current would produce a voltage Ao

E of 221 V at a speed of o

n = 1800 r/min. The actual A

E is

(

)

(

)

230 V 60 A 0.15 221 V

ATAA

EVIR=− = − Ω=

so the actual speed will be

()

221 V 1800 r/min 1800 r/min

221 V

A

o

Ao

E

nn

E

== =

(b) The output power is 7.5 hp and the output speed is 1800 r/min at rated conditions, therefore, the

torque is

()( )

()

out

15 hp 746 W/hp 59.4 N m

2 rad 1 min

1800 r/min 1 r 60 s

m

P

τπ

ω

== = ⋅

(c) The copper losses are

244

(

)

(

)

(

)

(

)

2

CU 60 A 0.15 230 V 1.35 A 851 W

AA FF

PIRVI=+= Ω+ =

The power converted from electrical to mechanical form is

()()

conv 221 V 60 A 13,260 W

AA

PEI== =

The output power is

()( )

OUT 15 hp 746 W/hp 11,190 WP==

Therefore, the rotational losses are

rot conv OUT 13,260 W 11,190 W 2070 WPP P=−= − =

(d) The input power to this motor is

()

()( )

IN 230 V 60 A 1.35 A 14,100 W

TA F

PVII=+= + =

Therefore, the efficiency is

OUT

IN

11,190 W

100% 100% 79.4%

14,100 W

P

η

=× = × =

(e) The no-load A

E will be 230 V, so the no-load speed will be

()

230 V 1800 r/min 1873 r/min

221 V

A

o

Ao

E

nn

E

== =

(f) If the field circuit opens, the field current would go to zero ⇒

φ

drops to res

φ

⇒ A

E↓ ⇒ A

I↑⇒

ind

τ

↑ ⇒ n↑ to a very high speed. If F

I = 0 A, Ao

E = 8.5 V at 1800 r/min, so

()

230 V 1800 r/min 48,700 r/min

8.5 V

A

o

Ao

E

nn

E

== =

(In reality, the motor speed would be limited by rotational losses, or else the motor will destroy itself first.)

(g) The maximum value of adj

R = 200 Ω, so

adj

230 V 0.821 A

200 80

T

F

V

IRR

== =

+Ω+Ω

This field current would produce a voltage Ao

E of 153 V at a speed of o

n = 1800 r/min. The actual A

E is

230 V, so the actual speed will be

()

230 V 1800 r/min 2706 r/min

153 V

A

o

Ao

E

nn

E

== =

The minimum value of adj

R = 0 Ω, so

adj

230 V 2.875 A

0 80

T

F

V

IRR

== =

+Ω+Ω

This field current would produce a voltage Ao

E of about 242 V at a speed of o

n = 1800 r/min. The actual

A

E is 230 V, so the actual speed will be

245

()

230 V 1800 r/min 1711 r/min

242 V

A

o

Ao

E

nn

E

== =

9-22. The magnetization curve for a separately excited dc generator is shown in Figure P9-7. The generator is

rated at 6 kW, 120 V, 50 A, and 1800 r/min and is shown in Figure P9-8. Its field circuit is rated at 5A.

The following data are known about the machine:

Note: An electronic version of this magnetization curve can be found in file

p97_mag.dat, which can be used with MATLAB programs. Column 1

contains field current in amps, and column 2 contains the internal generated

voltage EA in volts.

246

0.18

A

R=Ω 120 V

F

V=

adj 0 to 30 R=Ω 24

F

R=Ω

1000 turns per pole

F

N=

Answer the following questions about this generator, assuming no armature reaction.

(a) If this generator is operating at no load, what is the range of voltage adjustments that can be achieved

by changing Radj ?

(b) If the field rheostat is allowed to vary from 0 to 30 Ω and the generator’s speed is allowed to vary from

1500 to 2000 r/min, what are the maximum and minimum no-load voltages in the generator?

S

OLUTION

(a) If the generator is operating with no load at 1800 r/min, then the terminal voltage will equal the

internal generated voltage A

E. The maximum possible field current occurs when adj

R = 0 Ω. The current

is

,max

adj

120 V 5 A

24 0

F

V

IRR

== =

+Ω+Ω

From the magnetization curve, the voltage Ao

E at 1800 r/min is 129 V. Since the actual speed is 1800

r/min, the maximum no-load voltage is 129 V.

The minimum possible field current occurs when adj

R = 30 Ω. The current is

,max

adj

120 V 2.22 A

24 30

F

V

IRR

== =

+Ω+Ω

From the magnetization curve, the voltage Ao

E at 1800 r/min is 87.4 V. Since the actual speed is 1800

r/min, the minimum no-load voltage is 87 V.

(b) The maximum voltage will occur at the highest current and speed, and the minimum voltage will

occur at the lowest current and speed. The maximum possible field current occurs when adj

R = 0 Ω. The

current is

,max

adj

120 V 5 A

24 0

F

V

IRR

== =

+Ω+Ω

From the magnetization curve, the voltage Ao

E at 1800 r/min is 129 V. Since the actual speed is 2000

r/min, the maximum no-load voltage is

247

A

Ao o

En

=

()

2000 r/min 129 V 143 V

1800 r/min

AAo

o

n

EE

n

== =

The minimum possible field current occurs when adj

R = 30 Ω. The current is

,max

adj

120 V 2.22 A

24 30

F

V

IRR

== =

+Ω+Ω

From the magnetization curve, the voltage Ao

E at 1800 r/min is 87.4 V. Since the actual speed is 1500

r/min, the maximum no-load voltage is

A

Ao o

En

=

()

1500 r/min 87.4 V 72.8 V

1800 r/min

AAo

o

n

EE

n

== =

9-23. If the armature current of the generator in Problem 9-22 is 50 A, the speed of the generator is 1700 r/min,

and the terminal voltage is 106 V, how much field current must be flowing in the generator?

S

OLUTION The internal generated voltage of this generator is

(

)

(

)

106 V 50 A 0.18 115 V

ATAA

EVIR=+ = + Ω=

at a speed of 1700 r/min. This corresponds to an Ao

E at 1800 r/min of

A

Ao o

En

=

()

1800 r/min 115 V 121.8 V

1700 r/min

o

Ao A

n

EE

n

== =

From the magnetization curve, this value of Ao

E requires a field current of 4.2 A.

9-24. Assuming that the generator in Problem 9-22 has an armature reaction at full load equivalent to 400

A⋅turns of magnetomotive force, what will the terminal voltage of the generator be when F

I = 5 A, m

n =

1700 r/min, and A

I = 50 A?

S

OLUTION When F

I is 5 A and the armature current is 50 A, the magnetomotive force in the generator is

(

)

(

)

net AR 1000 turns 5 A 400 A turns 4600 A turns

F

NI=−= − ⋅ = ⋅FF

or

*

net

/

4600 A turns / 1000 turns 4.6 A

FF

IN==⋅ =F

The equivalent internal generated voltage Ao

E of the generator at 1800 r/min would be 126 V. The actual

voltage at 1700 r/min would be

()

1700 r/min 126 V 119 V

1800 r/min

AAo

o

n

EE

n

== =

Therefore, the terminal voltage would be

(

)

(

)

119 V 50 A 0.18 110 V

TAAA

VEIR=− = − Ω=

248

9-25. The machine in Problem 9-22 is reconnected as a shunt generator and is shown in Figure P9-9. The shunt

field resistor Radj is adjusted to 10 Ω, and the generator’s speed is 1800 r/min.

(a) What is the no-load terminal voltage of the generator?

(b) Assuming no armature reaction, what is the terminal voltage of the generator with an armature current

of 20 A? 40 A?

(c) Assuming an armature reaction equal to 200 A⋅turns at full load, what is the terminal voltage of the

generator with an armature current of 20 A? 40 A?

(d) Calculate and plot the terminal characteristics of this generator with and without armature reaction.

S

OLUTION

(a) The total field resistance of this generator is 34 Ω, and the no-load terminal voltage can be found

from the intersection of the resistance line with the magnetization curve for this generator. The

magnetization curve and the field resistance line are plotted below. As you can see, they intersect at a

terminal voltage of 112 V.

249

(b) At an armature current of 20 A, the internal voltage drop in the armature resistance is

()( )

V 6.3 0.18 A 20 =Ω . As shown in the figure below, there is a difference of 3.6 V between A

E and

T

V at a terminal voltage of about 106 V.

A MATLAB program to locate the position where the triangle exactly fits between the A

E and T

V lines is

shown below. This program created the plot shown above. Note that there are actually two places where

the difference between the A

E and T

V lines is 3.6 volts, but the low-voltage one of them is unstable. The

code shown in bold face below prevents the program from reporting that first (unstable) point.

% M-file: prob9_25b.m

% M-file to create a plot of the magnetization curve and the

% field current curve of a shunt dc generator, determining

% the point where the difference between them is 3.6 V.

% Get the magnetization curve. This file contains the

% three variables if_values, ea_values, and n_0.

clear all

load p97_mag.dat;

if_values = p97_mag(:,1);

ea_values = p97_mag(:,2);

n_0 = 1800;

% First, initialize the values needed in this program.

r_f = 24; % Field resistance (ohms)

r_adj = 10; % Adjustable resistance (ohms)

r_a = 0.19; % Armature + series resistance (ohms)

i_f = 0:0.02:6; % Field current (A)

n = 1800; % Generator speed (r/min)

% Calculate Ea versus If

Ea = interp1(if_values,ea_values,i_f);

250

% Calculate Vt versus If

Vt = (r_f + r_adj) * i_f;

% Find the point where the difference between the two

% lines is 3.6 V. This will be the point where the line

% line "Ea - Vt - 3.6" goes negative. That will be a

% close enough estimate of Vt.

diff = Ea - Vt - 3.6;

% This code prevents us from reporting the first (unstable)

% location satisfying the criterion.

was_pos = 0;

for ii = 1:length(i_f);

if diff(ii) > 0

was_pos = 1;

end

if ( diff(ii) < 0 & was_pos == 1 )

break;

end;

% We have the intersection. Tell user.

disp (['Ea = ' num2str(Ea(ii)) ' V']);

disp (['Vt = ' num2str(Vt(ii)) ' V']);

disp (['If = ' num2str(i_f(ii)) ' A']);

% Plot the curves

figure(1);

plot(i_f,Ea,'b-','LineWidth',2.0);

hold on;

plot(i_f,Vt,'k--','LineWidth',2.0);

% Plot intersections

plot([i_f(ii) i_f(ii)], [0 Ea(ii)], 'k-');

plot([0 i_f(ii)], [Vt(ii) Vt(ii)],'k-');

plot([0 i_f(ii)], [Ea(ii) Ea(ii)],'k-');

xlabel('\bf\itI_{F} \rm\bf(A)');

ylabel('\bf\itE_{A} \rm\bf or \itV_{T}');

title ('\bfPlot of \itE_{A} \rm\bf and \itV_{T} \rm\bf vs field

current');

axis ([0 5 0 150]);

set(gca,'YTick',[0 10 20 30 40 50 60 70 80 90 100 110 120 130 140

150]')

set(gca,'XTick',[0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0]')

legend ('Ea line','Vt line',4);

hold off;

grid on;

At an armature current of 40 A, the internal voltage drop in the armature resistance is

()( )

V 2.7 0.18 A 40 =Ω . As shown in the figure below, there is a difference of 7.2 V between A

E and

T

V at a terminal voltage of about 98 V.

251

(c) The rated current of this generated is 50 A, so 20 A is 40% of full load. If the full load armature

reaction is 200 A⋅turns, and if the armature reaction is assumed to change linearly with armature current,

then the armature reaction will be 80 A⋅turns. The figure below shows that a triangle consisting of 3.6 V

and (80 A⋅turns)/(1000 turns) = 0.08 A fits exactly between the A

E and T

V lines at a terminal voltage of

103 V.

252

The rated current of this generated is 50 A, so 40 A is 80% of full load. If the full load armature reaction

is 200 A⋅turns, and if the armature reaction is assumed to change linearly with armature current, then the

armature reaction will be 160 A⋅turns. There is no point where a triangle consisting of 3.6 V and (80

A⋅turns)/(1000 turns) = 0.16 A fits exactly between the A

E and T

V lines, so this is not a stable operating

condition.

(c) A MATLAB program to calculate the terminal characteristic of this generator without armature

reaction is shown below:

% M-file: prob9_25d.m

% M-file to calculate the terminal characteristic of a shunt

% dc generator without armature reaction.

% Get the magnetization curve. This file contains the

% three variables if_values, ea_values, and n_0.

load p97_mag.dat;

if_values = p97_mag(:,1);

ea_values = p97_mag(:,2);

n_0 = 1800;

% First, initialize the values needed in this program.

r_f = 24; % Field resistance (ohms)

r_adj = 10; % Adjustable resistance (ohms)

r_a = 0.18; % Armature + series resistance (ohms)

i_f = 0:0.005:6; % Field current (A)

n = 1800; % Generator speed (r/min)

253

% Calculate Ea versus If

Ea = interp1(if_values,ea_values,i_f);

% Calculate Vt versus If

Vt = (r_f + r_adj) * i_f;

% Find the point where the difference between the two

% lines is exactly equal to i_a*r_a. This will be the

% point where the line line "Ea - Vt - i_a*r_a" goes

% negative.

i_a = 0:1:50;

for jj = 1:length(i_a)

% Get the voltage difference

diff = Ea - Vt - i_a(jj)*r_a;

% This code prevents us from reporting the first (unstable)

% location satisfying the criterion.

was_pos = 0;

for ii = 1:length(i_f);

if diff(ii) > 0

was_pos = 1;

end

if ( diff(ii) < 0 & was_pos == 1 )

break;

end;

% Save terminal voltage at this point

v_t(jj) = Vt(ii);

i_l(jj) = i_a(jj) - v_t(jj) / ( r_f + r_adj);

end;

% Plot the terminal characteristic

figure(1);

plot(i_l,v_t,'b-','LineWidth',2.0);

xlabel('\bf\itI_{L} \rm\bf(A)');

ylabel('\bf\itV_{T} \rm\bf(V)');

title ('\bfTerminal Characteristic of a Shunt DC Generator');

hold off;

axis( [ 0 50 0 120]);

grid on;

254

The resulting terminal characteristic is shown below:

A MATLAB program to calculate the terminal characteristic of this generator with armature reaction is

shown below:

% M-file: prob9_25d2.m

% M-file to calculate the terminal characteristic of a shunt

% dc generator with armature reaction.

% Get the magnetization curve. This file contains the

% three variables if_values, ea_values, and n_0.

clear all

load p97_mag.dat;

if_values = p97_mag(:,1);

ea_values = p97_mag(:,2);

n_0 = 1800;

% First, initialize the values needed in this program.

r_f = 24; % Field resistance (ohms)

r_adj = 10; % Adjustable resistance (ohms)

r_a = 0.18; % Armature + series resistance (ohms)

i_f = 0:0.005:6; % Field current (A)

n = 1800; % Generator speed (r/min)

n_f = 1000; % Number of field turns

% Calculate Ea versus If

Ea = interp1(if_values,ea_values,i_f);

% Calculate Vt versus If

Vt = (r_f + r_adj) * i_f;

% Find the point where the difference between the Ea

% armature reaction line and the Vt line is exactly

% equal to i_a*r_a. This will be the point where

255

% the line "Ea_ar - Vt - i_a*r_a" goes negative.

i_a = 0:1:37;

for jj = 1:length(i_a)

% Calculate the equivalent field current due to armature

% reaction.

i_ar = (i_a(jj) / 50) * 200 / n_f;

% Calculate the Ea values modified by armature reaction

Ea_ar = interp1(if_values,ea_values,i_f - i_ar);

% Get the voltage difference

diff = Ea_ar - Vt - i_a(jj)*r_a;

% This code prevents us from reporting the first (unstable)

% location satisfying the criterion.

was_pos = 0;

for ii = 1:length(i_f);

if diff(ii) > 0

was_pos = 1;

end

if ( diff(ii) < 0 & was_pos == 1 )

break;

end;

% Save terminal voltage at this point

v_t(jj) = Vt(ii);

i_l(jj) = i_a(jj) - v_t(jj) / ( r_f + r_adj);

end;

% Plot the terminal characteristic

figure(1);

plot(i_l,v_t,'b-','LineWidth',2.0);

xlabel('\bf\itI_{L} \rm\bf(A)');

ylabel('\bf\itV_{T} \rm\bf(V)');

title ('\bfTerminal Characteristic of a Shunt DC Generator w/AR');

hold off;

axis([ 0 50 0 120]);

grid on;

256

The resulting terminal characteristic is shown below:

9-26. If the machine in Problem 9-25 is running at 1800 r/min with a field resistance Radj = 10 Ω and an

armature current of 25 A, what will the resulting terminal voltage be? If the field resistor decreases to 5 Ω

while the armature current remains 25 A, what will the new terminal voltage be? (Assume no armature

reaction.)

S

OLUTION If A

I = 25 A, then

(

)

(

)

25 A 0.18

AA

IR =Ω

= 4.5 V. The point where the distance between the

A

E and T

V curves is exactly 4.5 V corresponds to a terminal voltage of 104 V, as shown below.

257

If

Radj decreases to 5 Ω, the total field resistance becomes 29 Ω, and the terminal voltage line gets

shallower. The new point where the distance between the A

E and T

V curves is exactly 4.5 V corresponds

to a terminal voltage of 115 V, as shown below.

Note that decreasing the field resistance of the shunt generator increases the terminal voltage.

9-27. A 120-V 50-A cumulatively compounded dc generator has the following characteristics:

0.21

AS

RR+= Ω

1000 turns

F

N=

20

F

R=Ω

SE 20 turnsN=

adj 0 to 30 , set to 10 R=Ω Ω

1800 r/min

m

n=

The machine has the magnetization curve shown in Figure P9-7. Its equivalent circuit is shown in Figure

P9-10. Answer the following questions about this machine, assuming no armature reaction.

(a) If the generator is operating at no load, what is its terminal voltage?

(b) If the generator has an armature current of 20 A, what is its terminal voltage?

258

(c) If the generator has an armature current of 40 A, what is its terminal voltage'?

(d) Calculate and plot the terminal characteristic of this machine.

S

OLUTION

(a) The total field resistance of this generator is 30 Ω, and the no-load terminal voltage can be found

from the intersection of the resistance line with the magnetization curve for this generator. The

magnetization curve and the field resistance line are plotted below. As you can see, they intersect at a

terminal voltage of 121 V.

(b) If the armature current is 20 A, then the effective field current contribution from the armature current

()

SE 20 20 A 0.4 A

1000

A

F

NI

N==

and the

()

AA S

IR R+ voltage drop is

()

(

)

(

)

20 A 0.21 4.2 V

AA S

IR R+= Ω= . The location where the

triangle formed by SE

A

F

NI

N and AA

IR exactly fits between the A

E and T

V lines corresponds to a terminal

voltage of 120 V, as shown below.

259

(c) If the armature current is 40 A, then the effective field current contribution from the armature current

()

A 6.0A 40

1000

15

SE ==

A

F

I

N

and the

()

SAA RRI + voltage drop is

()()()

V 8 20.0 A 80 =Ω=+ SAA RRI . The location where the

triangle formed by A

F

I

N

NSE and AA RI exactly fits between the A

E and T

V lines corresponds to a terminal

voltage of 116 V, as shown below.

260

A MATLAB program to locate the position where the triangle exactly fits between the A

E and T

V lines is

shown below. This program created the plot shown above.

% M-file: prob9_27b.m

% M-file to create a plot of the magnetization curve and the

% field current curve of a cumulatively-compounded dc generator

% when the armature current is 20 A.

% Get the magnetization curve. This file contains the

% three variables if_values, ea_values, and n_0.

clear all

load p97_mag.dat;

if_values = p97_mag(:,1);

ea_values = p97_mag(:,2);

n_0 = 1800;

% First, initialize the values needed in this program.

r_f = 20; % Field resistance (ohms)

r_adj = 10; % Adjustable resistance (ohms)

r_a = 0.21; % Armature + series resistance (ohms)

i_f = 0:0.02:6; % Field current (A)

n = 1800; % Generator speed (r/min)

n_f = 1000; % Shunt field turns

n_se = 20; % Series field turns

% Calculate Ea versus If

Ea = interp1(if_values,ea_values,i_f);

% Calculate Vt versus If

Vt = (r_f + r_adj) * i_f;

% Calculate the Ea values modified by mmf due to the

% armature current

261

i_a = 20;

Ea_a = interp1(if_values,ea_values,i_f + i_a * n_se/n_f);

% Find the point where the difference between the

% enhanced Ea line and the Vt line is 4 V. This will

% be the point where the line "Ea_a - Vt - 4" goes

% negative.

diff = Ea_a - Vt - 4;

% This code prevents us from reporting the first (unstable)

% location satisfying the criterion.

was_pos = 0;

for ii = 1:length(i_f);

if diff(ii) > 0

was_pos = 1;

end

if ( diff(ii) < 0 & was_pos == 1 )

break;

end;

% We have the intersection. Tell user.

disp (['Ea_a = ' num2str(Ea_a(ii)) ' V']);

disp (['Ea = ' num2str(Ea(ii)) ' V']);

disp (['Vt = ' num2str(Vt(ii)) ' V']);

disp (['If = ' num2str(i_f(ii)) ' A']);

disp (['If_a = ' num2str(i_f(ii)+ i_a * n_se/n_f) ' A']);

% Plot the curves

figure(1);

plot(i_f,Ea,'b-','LineWidth',2.0);

hold on;

plot(i_f,Vt,'k--','LineWidth',2.0);

% Plot intersections

plot([i_f(ii) i_f(ii)], [0 Vt(ii)], 'k-');

plot([0 i_f(ii)], [Vt(ii) Vt(ii)],'k-');

plot([0 i_f(ii)+i_a*n_se/n_f], [Ea_a(ii) Ea_a(ii)],'k-');

% Plot compounding triangle

plot([i_f(ii) i_f(ii)+i_a*n_se/n_f],[Vt(ii) Vt(ii)],'b-');

plot([i_f(ii) i_f(ii)+i_a*n_se/n_f],[Vt(ii) Ea_a(ii)],'b-');

plot([i_f(ii)+i_a*n_se/n_f i_f(ii)+i_a*n_se/n_f],[Vt(ii)

Ea_a(ii)],'b-');

xlabel('\bf\itI_{F} \rm\bf(A)');

ylabel('\bf\itE_{A} \rm\bf or \itE_{A} \rm\bf(V)');

title ('\bfPlot of \itE_{A} \rm\bf and \itV_{T} \rm\bf vs field

current');

axis ([0 5 0 150]);

set(gca,'YTick',[0 10 20 30 40 50 60 70 80 90 100 110 120 130 140

150]')

set(gca,'XTick',[0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0]')

legend ('Ea line','Vt line',4);

hold off;

grid on;

262

(d) A MATLAB program to calculate and plot the terminal characteristic of this generator is shown

below.

% M-file: prob9_27d.m

% M-file to calculate the terminal characteristic of a

% cumulatively compounded dc generator without armature

% reaction.

% Get the magnetization curve. This file contains the

% three variables if_values, ea_values, and n_0.

clear all

load p97_mag.dat;

if_values = p97_mag(:,1);

ea_values = p97_mag(:,2);

n_0 = 1800;

% First, initialize the values needed in this program.

r_f = 20; % Field resistance (ohms)

r_adj = 10; % Adjustable resistance (ohms)

r_a = 0.21; % Armature + series resistance (ohms)

i_f = 0:0.02:6; % Field current (A)

n = 1800; % Generator speed (r/min)

n_f = 1000; % Shunt field turns

n_se = 20; % Series field turns

% Calculate Ea versus If

Ea = interp1(if_values,ea_values,i_f);

% Calculate Vt versus If

Vt = (r_f + r_adj) * i_f;

% Find the point where the difference between the two

% lines is exactly equal to i_a*r_a. This will be the

% point where the line line "Ea - Vt - i_a*r_a" goes

% negative.

i_a = 0:1:50;

for jj = 1:length(i_a)

% Calculate the Ea values modified by mmf due to the

% armature current

Ea_a = interp1(if_values,ea_values,i_f + i_a(jj)*n_se/n_f);

% Get the voltage difference

diff = Ea_a - Vt - i_a(jj)*r_a;

% This code prevents us from reporting the first (unstable)

% location satisfying the criterion.

was_pos = 0;

for ii = 1:length(i_f);

if diff(ii) > 0

was_pos = 1;

end

if ( diff(ii) < 0 & was_pos == 1 )

break;

end;

263

% Save terminal voltage at this point

v_t(jj) = Vt(ii);

i_l(jj) = i_a(jj) - v_t(jj) / ( r_f + r_adj);

end;

% Plot the terminal characteristic

figure(1);

plot(i_l,v_t,'b-','LineWidth',2.0);

xlabel('\bf\itI_{L} \rm\bf(A)');

ylabel('\bf\itV_{T} \rm\bf(V)');

string = ['\bfTerminal Characteristic of a Cumulatively ' ...

'Compounded DC Generator'];

title (string);

hold off;

axis([ 0 50 0 130]);

grid on;

The resulting terminal characteristic is shown below. Compare it to the terminal characteristics of the

shunt dc generators in Problem 9-25 (d).

9-28. If the machine described in Problem 9-27 is reconnected as a differentially compounded dc generator, what

will its terminal characteristic look like? Derive it in the same fashion as in Problem 9-27.

S

OLUTION A MATLAB program to calculate and plot the terminal characteristic of this generator is shown

below.

% M-file: prob9_28.m

% M-file to calculate the terminal characteristic of a

% differentially compounded dc generator without armature

% reaction.

% Get the magnetization curve. This file contains the

264

% three variables if_values, ea_values, and n_0.

clear all

load p97_mag.dat;

if_values = p97_mag(:,1);

ea_values = p97_mag(:,2);

n_0 = 1800;

% First, initialize the values needed in this program.

r_f = 20; % Field resistance (ohms)

r_adj = 10; % Adjustable resistance (ohms)

r_a = 0.21; % Armature + series resistance (ohms)

i_f = 0:0.02:6; % Field current (A)

n = 1800; % Generator speed (r/min)

n_f = 1000; % Shunt field turns

n_se = 20; % Series field turns

% Calculate Ea versus If

Ea = interp1(if_values,ea_values,i_f);

% Calculate Vt versus If

Vt = (r_f + r_adj) * i_f;

% Find the point where the difference between the two

% lines is exactly equal to i_a*r_a. This will be the

% point where the line line "Ea - Vt - i_a*r_a" goes

% negative.

i_a = 0:1:26;

for jj = 1:length(i_a)

% Calculate the Ea values modified by mmf due to the

% armature current

Ea_a = interp1(if_values,ea_values,i_f - i_a(jj)*n_se/n_f);

% Get the voltage difference

diff = Ea_a - Vt - i_a(jj)*r_a;

% This code prevents us from reporting the first (unstable)

% location satisfying the criterion.

was_pos = 0;

for ii = 1:length(i_f);

if diff(ii) > 0

was_pos = 1;

end

if ( diff(ii) < 0 & was_pos == 1 )

break;

end;

% Save terminal voltage at this point

v_t(jj) = Vt(ii);

i_l(jj) = i_a(jj) - v_t(jj) / ( r_f + r_adj);

end;

% Plot the terminal characteristic

figure(1);

265

plot(i_l,v_t,'b-','LineWidth',2.0);

xlabel('\bf\itI_{L} \rm\bf(A)');

ylabel('\bf\itV_{T} \rm\bf(V)');

string = ['\bfTerminal Characteristic of a Cumulatively ' ...

'Compounded DC Generator'];

title (string);

hold off;

axis([ 0 50 0 120]);

grid on;

The resulting terminal characteristic is shown below. Compare it to the terminal characteristics of the

cumulatively compounded dc generator in Problem 9-28 and the shunt dc generators in Problem 9-25 (d).

9-29. A cumulatively compounded dc generator is operating properly as a flat-compounded dc generator. The

machine is then shut down, and its shunt field connections are reversed.

(a) If this generator is turned in the same direction as before, will an output voltage be built up at its

terminals? Why or why not?

(b) Will the voltage build up for rotation in the opposite direction? Why or why not?

(c) For the direction of rotation in which a voltage builds up, will the generator be cumulatively or

differentially compounded?

S

OLUTION

(a) The output voltage will not build up, because the residual flux now induces a voltage in the opposite

direction, which causes a field current to flow that tends to further reduce the residual flux.

(b) If the motor rotates in the opposite direction, the voltage will build up, because the reversal in voltage

due to the change in direction of rotation causes the voltage to produce a field current that increases the

residual flux, starting a positive feedback chain.

(c) The generator will now be differentially compounded.

266

9-30. A three-phase synchronous machine is mechanically connected to a shunt dc machine, forming a motor-

generator set, as shown in Figure P9-11. The dc machine is connected to a dc power system supplying a

constant 240 V, and the ac machine is connected to a 480-V 60-Hz infinite bus.

The dc machine has four poles and is rated at 50 kW and 240 V. It has a per-unit armature resistance of

0.04. The ac machine has four poles and is Y-connected. It is rated at 50 kVA, 480 V, and 0.8 PF, and its

saturated synchronous reactance is 2.0 Ω per phase.

All losses except the dc machine’s armature resistance may be neglected in this problem. Assume that the

magnetization curves of both machines are linear.

(a) Initially, the ac machine is supplying 50 kVA at 0.8 PF lagging to the ac power system.

1. How much power is being supplied to the dc motor from the dc power system?

2. How large is the internal generated voltage A

E of the dc machine?

3. How large is the internal generated voltage A

E of the ac machine?

(b) The field current in the ac machine is now increased by 5 percent. What effect does this change have

on the real power supplied by the motor-generator set? On the reactive power supplied by the motor-

generator set? Calculate the real and reactive power supplied or consumed by the ac machine under

these conditions. Sketch the ac machine’s phasor diagram before and after the change in field current.

(c) Starting from the conditions in part (b), the field current in the dc machine is now decreased by 1

percent. What effect does this change have on the real power supplied by the motor-generator set? On

the reactive power supplied by the motor-generator set? Calculate the real and reactive power supplied

or consumed by the ac machine under these conditions. Sketch the ac machine’s phasor diagram before

and after the change in the dc machine’s field current.

(d) From the above results, answer the following questions:

1. How can the real power flow through an ac-dc motor-generator set be controlled?

2. How can the reactive power supplied or consumed by the ac machine be controlled without

affecting the real power flow?

S

OLUTION

(a) The power supplied by the ac machine to the ac power system is

()()

AC cos 50 kVA 0.8 40 kWPS

θ

== =

267

and the reactive power supplied by the ac machine to the ac power system is

() ()

1

AC sin 50 kVA sin cos 0.8 30 kvarQS

θ

−

== =

The power out of the dc motor is thus 40 kW. This is also the power converted from electrical to

mechanical form in the dc machine, since all other losses are neglected. Therefore,

()

conv 40 kW

AA T A A A

PEIVIRI==− =

240 kW 0

TA A A

VI I R−− =

The base resistance of the dc machine is

()

2

,base

base,dc

base

230 V 1.058

50 kW

T

V

RP

== =Ω

Therefore, the actual armature resistance is

()( )

0.04 1.058 0.0423

A

R=Ω=Ω

Continuing to solve the equation for conv

P, we get

2

0.0423 230 40,000 0

AA

II−+ =

2

5434.8 945180 0

AA

II−+=

179.9 A

A

I=

and

A

E = 222.4 V.

Therefore, the power into the dc machine is 41.38 kW

TA

VI =, while the power converted from electrical

to mechanical form (which is equal to the output power) is

()()

222.4 V 179.9 A 40 kW

AA

EI ==. The

internal generated voltage A

E of the dc machine is 222.4 V.

The armature current in the ac machine is

()

50 kVA 60.1 A

3 3 480 V

A

S

IV

φ

== =

60.1 36.87 A

A=∠− °I

Therefore, the internal generated voltage A

E of the ac machine is

ASA

jX

φ

=+EV I

()( )

277 0 V 2.0 60.1 36.87 A 362 15.4 V

Aj=∠°+ Ω ∠− °=∠°E

(b) When the field current of the ac machine is increased by 5%, it has no effect on the real power

supplied by the motor-generator set. This fact is true because P

τω

=, and the speed is constant (since the

MG set is tied to an infinite bus). With the speed unchanged, the dc machine’s torque is unchanged, so the

total power supplied to the ac machine’s shaft is unchanged.

If the field current is increased by 5% and the OCC of the ac machine is linear, A

E increases to

(

)

(

)

1.05 262 V 380 V

A

E==′

The new torque angle

δ

can be found from the fact that since the terminal voltage and power of the ac

machine are constant, the quantity sin

A

E

δ

must be constant.

268

sin sin

AA

EE

δδ

=′′

11

362 V

sin sin sin sin15.4 14.7

380 V

A

E

δδ

−−

== °=°

′

Therefore, the armature current will be

380 14.7 V 277 0 V 66.1 43.2 A

2.0

A

S

jX j

φ

−∠°−∠°

== =∠−°

Ω

EV

I

The resulting reactive power is

(

)

(

)

3 sin 3 480 V 66.1 A sin 43.2 37.6 kvar

TL

QVI

θ

== °=

The reactive power supplied to the ac power system will be 37.6 kvar, compared to 30 kvar before the ac

machine field current was increased. The phasor diagram illustrating this change is shown below.

V

φ

E

A

1

jX

S

I

A

E

A

2

I

A

2

I

A

1

(c) If the dc field current is decreased by 1%, the dc machine’s flux will decrease by 1%. The internal

generated voltage in the dc machine is given by the equation

A

EK

φ

ω

=, and

ω

is held constant by the

infinite bus attached to the ac machine. Therefore, A

E on the dc machine will decrease to (0.99)(222.4 V)

= 220.2 V. The resulting armature current is

,dc

230 V 220.2 V 231.7 A

0.0423

TA

A

VE

IR

−−

== =

Ω

The power into the dc motor is now (230 V)(231.7 A) = 53.3 kW, and the power converted from electrical

to mechanical form in the dc machine is (220.2 V)(231.7 A) = 51 kW. This is also the output power of the

dc machine, the input power of the ac machine, and the output power of the ac machine, since losses are

being neglected.

The torque angle of the ac machine now can be found from the equation

ac

3sin

A

S

VE

PX

φ

δ

=

()()

11

ac 51 kW 2.0

sin sin 18.9

3 3 277 V 380 V

S

A

PX

VE

φ

δ

−− Ω

== =°

The new

A

E of this machine is thus 380 18.9 V∠°, and the resulting armature current is

380 18.9 V 277 0 V 74.0 33.8 A

2.0

A

S

jX j

φ

−∠°−∠°

== =∠−°

Ω

EV

I

The real and reactive powers are now

(

)

(

)

3 cos 3 480 V 74.0 A cos 33.8 51 kW

TL

PVI

θ

== °=

(

)

(

)

3 sin 3 480 V 74.0 A sin 33.8 34.2 kvar

TL

QVI

θ

== °=

269

The phasor diagram of the ac machine before and after the change in dc machine field current is shown

below.

V

φ

E

A

1

jX

S

I

A

E

A

2

I

A

2

I

A

1

(d) The real power flow through an ac-dc motor-generator set can be controlled by adjusting the field

current of the dc machine. (Note that changes in power flow also have some effect on the reactive power of

the ac machine: in this problem, Q dropped from 35 kvar to 30 kvar when the real power flow was

adjusted.)

The reactive power flow in the ac machine of the MG set can be adjusted by adjusting the ac machine’s

field current. This adjustment has basically no effect on the real power flow through the MG set.

270

Chapter 10: Single-Phase and Special-Purpose Motors

10-1. A 120-V, 1/3-hp 60-Hz, four-pole, split-phase induction motor has the following impedances:

1

R = 1.80 Ω 1

X = 2.40 Ω M

X = 60 Ω

2

R = 2.50 Ω 2

X = 2.40 Ω

At a slip of 0.05, the motor’s rotational losses are 51 W. The rotational losses may be assumed constant

over the normal operating range of the motor. If the slip is 0.05, find the following quantities for this

motor:

(a) Input power

(b) Air-gap power

(c) P

conv

(d) P

out

(e) ind

τ

(f) load

τ

(g) Overall motor efficiency

(h) Stator power factor

S

OLUTION The equivalent circuit of the motor is shown below

1.8 Ωj2.4 Ω

+

-

V

I1R1jX1

s

R2

5.0

j0.5X2

j0.5XM

j1.20 Ω

j30 Ω

s

R

−2

5.0 2

jX2

j1.20 Ω

j0.5XM

j30 Ω

{

Forward

Reverse

0.5ZB

0.5ZF

()()

22

/

M

F

M

Rs jX jX

ZR s jX jX

+

=++

(

)

(

)

50 2.40 60 28.15 24.87

50 2.40 60

F

jj

Zj

jj

+

==+Ω

++

()

22

/2

M

B

M

R s jX jX

ZR s jX jX

=−+ +

271

()()

1.282 2.40 60 1.185 2.332

1.282 2.40 60

B

jj

Zj

jj

+

==+Ω

++

(a) The input current is

1

11

I0.5 0.5

FB

RjX Z Z

=++ +

V

()( )( )

1

120 0 V

I5.2344.2 A

1.80 2.40 0.5 28.15 24.87 0.5 1.185 2.332jj j

∠°

==∠−°

++ + + +

()( )

IN cos 120 V 5.23 A cos 44.2 450 WPVI

θ

== °=

(b) The air-gap power is

()

()()

2

AG, 1 0.5 5.23 A 14.1 386 W

FF

PIR== Ω=

()

()( )

2

AG, 1 0.5 5.23 A 0.592 16.2 W

BB

PIR== Ω=

AG AG, AG, 386 W 14.8 W 371 W

FB

PP P=−= − =

(c) The power converted from electrical to mechanical form is

() ( )( )

conv, AG,

1 1 0.05 386 W 367 W

FF

PsP=− =− =

() ( )( )

conv, AG,

1 1 0.05 16.2 W 15.4 W

BB

PsP=− =− =

conv conv, conv, 367 W 15.4 W 352 W

FB

PP P=−= − =

(d) The output power is

OUT conv rot 352 W 51 W 301 WPPP=−= − =

(e) The induced torque is

()

AG

ind

sync

371 W 1.97 N m

2 rad 1 min

1800 r/min 1 r 60 s

P

τπ

ω

== = ⋅

(f) The load torque is

()( )

OUT

load

301 W 1.68 N m

2 rad 1 min

0.95 1800 r/min 1 r 60 s

m

P

τπ

ω

== = ⋅

(g) The overall efficiency is

OUT

IN

301 W

100% 100% 66.9%

450 W

P

η

=× = × =

(h) The stator power factor is

PF cos 44.2 0.713 lagging

=°=

10-2. Repeat Problem 10-1 for a rotor slip of 0.025.

()()

22

/

M

F

M

Rs jX jX

ZR s jX jX

+

=++

()()

100 2.40 60 28.91 43.83

1002.4060

F

jj

Zj

jj

+

==+Ω

++

272

()

22

/2

M

B

M

R s jX jX

ZR s jX jX

=−+ +

()()

1.282 2.40 60 1.170 2.331

1.282 2.40 60

B

jj

Zj

jj

+

==+Ω

++

(a) The input current is

1

11

I0.5 0.5

FB

RjX Z Z

=++ +

V

()( )( )

1

120 0 V

I4.0359.0 A

1.80 2.40 0.5 25.91 43.83 0.5 1.170 2.331jj j

∠°

==∠−°

++ + + +

()( )

IN cos 120 V 4.03 A cos 59.0 249 WPVI

θ

== °=

(b) The air-gap power is

()

(

)

(

)

2

AG, 1 0.5 4.03 A 12.96 210.5 W

FF

PIR== Ω=

()

(

)

(

)

2

AG, 1 0.5 4.03 A 0.585 9.5 W

BB

PIR== Ω=

AG AG, AG, 210.5 W 9.5 W 201 W

FB

PP P=−= − =

(c) The power converted from electrical to mechanical form is

(

)

(

)

(

)

conv, AG,

1 1 0.025 210.5 W 205 W

FF

PsP=− =− =

() ( )( )

conv, AG,

1 1 0.025 9.5 W 9.3 W

BB

PsP=− =− =

conv conv, conv, 205 W 9.3 W 196 W

FB

PP P=−= − =

(d) The output power is

OUT conv rot 205 W 51 W 154 WPPP=−= − =

(e) The induced torque is

()

AG

ind

sync

210.5 W 1.12 N m

2 rad 1 min

1800 r/min 1 r 60 s

P

τπ

ω

== = ⋅

(f) The load torque is

()( )

OUT

load

154 W 0.84 N m

2 rad 1 min

0.975 1800 r/min 1 r 60 s

m

P

τπ

ω

== = ⋅

(g) The overall efficiency is

OUT

IN

154 W

100% 100% 61.8%

249 W

P

η

=× = × =

(h) The stator power factor is

PF cos 59.0 0.515 lagging

=°=

10-3. Suppose that the motor in Problem 10-1 is started and the auxiliary winding fails open while the rotor is

accelerating through 400 r/min. How much induced torque will the motor be able to produce on its main

273

winding alone? Assuming that the rotational losses are still 51 W, will this motor continue accelerating or

will it slow down again? Prove your answer.

S

OLUTION At a speed of 400 r/min, the slip is

1800 r/min 400 r/min 0.778

1800 r/min

s−

==

()()

22

/

M

F

M

Rs jX jX

ZR s jX jX

+

=++

()()

100 2.40 60 2.96 2.46

1002.4060

F

jj

Zj

jj

+

==+Ω

++

()

22

/2

M

B

M

R s jX jX

ZR s jX jX

=−+ +

()()

1.282 2.40 60 1.90 2.37

1.282 2.40 60

B

jj

Zj

jj

+

==+Ω

++

The input current is

1

11

I0.5 0.5

FB

RjX Z Z

=++ +

V

()()()

1

120 0 V

I 18.73 48.7 A

1.80 2.40 0.5 2.96 2.46 0.5 1.90 2.37jjj

∠°

==∠−°

++ ++ +

The air-gap power is

()

()()

2

AG, 1 0.5 18.73 A 1.48 519.2 W

FF

PIR== Ω=

()

()()

2

AG, 1 0.5 18.73 A 0.945 331.5 W

BB

PIR== Ω=

AG AG, AG, 519.2 W 331.5 W 188 W

FB

PP P=−= − =

The power converted from electrical to mechanical form is

() ( )( )

conv, AG,

1 1 0.778 519.2 W 115.2 W

FF

PsP=− =− =

()()()

conv, AG,

1 1 0.778 331.5 W 73.6 W

BB

PsP=− =− =

conv conv, conv, 115.2 W 73.6 W 41.6 W

FB

PP P=−= − =

The induced torque is

()

AG

ind

sync

188 W 1.00 N m

2 rad 1 min

1800 r/min 1 r 60 s

P

τπ

ω

== = ⋅

Assuming that the rotational losses are still 51 W, this motor is not producing enough torque to keep

accelerating. conv

P is 41.6 W, while the rotational losses are 51 W, so there is not enough power to make

up the rotational losses. The motor will slow down5.

10-4. Use MATLAB to calculate and plot the torque-speed characteristic of the motor in Problem 10-1, ignoring

the starting winding.

5 Note that in the real world, rotational losses decrease with decreased shaft speed. Therefore, the losses will really

be less than 51 W, and this motor might just be able to keep on accelerating slowly—it is a close thing either way.

274

S

OLUTION This problem is best solved with MATLAB, since it involves calculating the torque-speed values

at many points. A MATLAB program to calculate and display both torque-speed characteristics is shown

below. Note that this program shows the torque-speed curve for both positive and negative directions of

rotation. Also, note that we had to avoid calculating the slip at exactly 0 or 2, since those numbers would

produce divide-by-zero errors in F

Z

and B

Z

respectively.

% M-file: torque_speed_curve3.m

% M-file create a plot of the torque-speed curve of the

% single-phase induction motor of Problem 10-4.

% First, initialize the values needed in this program.

r1 = 1.80; % Stator resistance

x1 = 2.40; % Stator reactance

r2 = 2.50; % Rotor resistance

x2 = 2.40; % Rotor reactance

xm = 60; % Magnetization branch reactance

v = 120; % Single-Phase voltage

n_sync = 1800; % Synchronous speed (r/min)

w_sync = 188.5; % Synchronous speed (rad/s)

% Specify slip ranges to plot

s = 0:0.01:2.0;

% Offset slips at 0 and 2 slightly to avoid divide by zero errors

s(1) = 0.0001;

s(201) = 1.9999;

% Get the corresponding speeds in rpm

nm = ( 1 - s) * n_sync;

% Caclulate Zf and Zb as a function of slip

zf = (r2 ./ s + j*x2) * (j*xm) ./ (r2 ./ s + j*x2 + j*xm);

zb = (r2 ./(2-s) + j*x2) * (j*xm) ./ (r2 ./(2-s) + j*x2 + j*xm);

% Calculate the current flowing at each slip

i1 = v ./ ( r1 + j*x1 + zf + zb);

% Calculate the air-gap power

p_ag_f = abs(i1).^2 .* 0.5 .* real(zf);

p_ag_b = abs(i1).^2 .* 0.5 .* real(zb);

p_ag = p_ag_f - p_ag_b;

% Calculate torque in N-m.

t_ind = p_ag ./ w_sync;

% Plot the torque-speed curve

figure(1)

plot(nm,t_ind,'Color','b','LineWidth',2.0);

xlabel('\itn_{m} \rm(r/min)');

ylabel('\tau_{ind} \rm(N-m)');

title ('Single Phase Induction motor torque-speed

characteristic','FontSize',12);

grid on;

hold off;

275

The resulting torque-speed characteristic is shown below:

10-5. A 220-V, 1.5-hp 50-Hz, two-pole, capacitor-start induction motor has the following main-winding

impedances:

1

R = 1.40 Ω 1

X = 2.01 Ω M

X = 105 Ω

2

R = 1.50 Ω 2

X = 2.01 Ω

At a slip of 0.05, the motor’s rotational losses are 291 W. The rotational losses may be assumed constant

over the normal operating range of the motor. Find the following quantities for this motor at 5 percent slip:

(a) Stator current

(b) Stator power factor

(c) Input power

(d) AG

P

(e) P

conv

(f) out

P

(g) ind

τ

(h) load

τ

(i) Efficiency

S

OLUTION The equivalent circuit of the motor is shown below

276

1.4 Ωj1.9 Ω

+

-

V = 220∠0° V

I1R1jX1

s

R2

5.0

j0.5X2

j0.5XM

j1.90 Ω

j30 Ω

s

R

−2

5.0 2

jX2

j1.90 Ω

j0.5XM

j100 Ω

{

Forward

Reverse

0.5ZB

0.5ZF

()()

22

/

M

F

M

Rs jX jX

ZR s jX jX

+

=++

()()

30 1.90 100 26.59 9.69

30 1.90 100

F

jj

Zj

jj

+

==+Ω

++

()

22

/2

M

B

M

R s jX jX

ZR s jX jX

=−+ +

()()

0.769 1.90 100 0.741 1.870

0.769 1.90 100

B

jj

Zj

jj

+

==+Ω

++

(a) The input stator current is

1

11

I0.5 0.5

FB

RjX Z Z

=++ +

V

()( )( )

1

220 0 V

I 13.0 27.0 A

1.40 1.90 0.5 26.59 9.69 0.5 0.741 1.870jjj

∠°

==∠−°

++ ++ +

(b) The stator power factor is

PF cos 27 0.891 lagging

=°=

(c) The input power is

()()

IN cos 220 V 13.0 A cos 27 2548 WPVI

θ

== °=

(d) The air-gap power is

()

()( )

2

AG, 1 0.5 13.0 A 13.29 2246 W

FF

PIR== Ω=

()

()( )

2

AG, 1 0.5 13.0 A 0.370 62.5 W

BB

PIR== Ω=

AG AG, AG, 2246 W 62.5 W 2184 W

FB

PP P=−= − =

277

(e) The power converted from electrical to mechanical form is

() ( )( )

conv, AG,

1 1 0.05 2246 W 2134 W

FF

PsP=− =− =

() ( )( )

conv, AG,

1 1 0.05 62.5 W 59 W

BB

PsP=− =− =

conv conv, conv, 2134 W 59 W 2075 W

FB

PP P=−= −=

(f) The output power is

OUT conv rot 2134 W 291 W 1843 WPPP=−= − =

(g) The induced torque is

()

AG

ind

sync

2184 W 6.95 N m

2 rad 1 min

3000 r/min 1 r 60 s

P

τπ

ω

== = ⋅

(h) The load torque is

()( )

OUT

load

1843 W 6.18 N m

2 rad 1 min

0.95 3000 r/min 1 r 60 s

m

P

τπ

ω

== = ⋅

(i) The overall efficiency is

OUT

IN

1843 W

100% 100% 72.3%

2548 W

P

η

=× = × =

10-6. Find the induced torque in the motor in Problem 10-5 if it is operating at 5 percent slip and its terminal

voltage is (a) 190 V, (b) 208 V, (c) 230 V.

()()

22

/

M

F

M

Rs jX jX

ZR s jX jX

+

=++

()()

30 1.90 100 26.59 9.69

30 1.90 100

F

jj

Zj

jj

+

==+Ω

++

()

22

/2

M

B

M

R s jX jX

ZR s jX jX

=−+ +

()()

0.769 1.90 100 0.741 1.870

0.769 1.90 100

B

jj

Zj

jj

+

==+Ω

++

(a) If T

V = 190∠0° V,

1

11

I0.5 0.5

FB

RjX Z Z

=++ +

V

()( )( )

1

190 0 V

I 11.2 27.0 A

1.40 1.90 0.5 26.59 9.69 0.5 0.741 1.870jjj

∠°

==∠−°

++ ++ +

()

()( )

2

AG, 1 0.5 11.2 A 13.29 1667 W

FF

PIR== Ω=

()

()( )

2

AG, 1 0.5 11.2 A 0.370 46.4 W

BB

PIR== Ω=

AG AG, AG, 1667 W 46.4 W 1621 W

FB

PP P=−= − =

278

()

AG

ind

sync

1621 W 5.16 N m

2 rad 1 min

3000 r/min 1 r 60 s

P

τπ

ω

== = ⋅

(b) If T

V = 208∠0° V,

1

11

I0.5 0.5

FB

RjX Z Z

=++ +

V

()( )( )

1

208 0 V

I12.327.0 A

1.40 1.90 0.5 26.59 9.69 0.5 0.741 1.870jjj

∠°

==∠−°

++ ++ +

()

()( )

2

AG, 1 0.5 12.3 A 13.29 2010 W

FF

PIR== Ω=

()

()( )

2

AG, 1 0.5 12.3 A 0.370 56 W

BB

PIR== Ω=

AG AG, AG, 2010 W 56 W 1954 W

FB

PP P=−= − =

()

AG

ind

sync

1954 W 6.22 N m

2 rad 1 min

3000 r/min 1 r 60 s

P

τπ

ω

== = ⋅

(c) If T

V = 230∠0° V,

1

11

I0.5 0.5

FB

RjX Z Z

=++ +

V

()( )( )

1

230 0 V

I 13.6 27.0 A

1.40 1.90 0.5 26.59 9.69 0.5 0.741 1.870jjj

∠°

==∠−°

++ ++ +

()

()( )

2

AG, 1 0.5 13.6 A 13.29 2458 W

FF

PIR== Ω=

()

()( )

2

AG, 1 0.5 13.6 A 0.370 68 W

BB

PIR== Ω=

AG AG, AG, 2458 W 68 W 2390 W

FB

PP P=−= − =

()

AG

ind

sync

2390 W 7.61 N m

2 rad 1 min

3000 r/min 1 r 60 s

P

τπ

ω

== = ⋅

Note that the induced torque is proportional to the square of the terminal voltage.

10-7. What type of motor would you select to perform each of the following jobs? Why?

(a) Vacuum cleaner (b) Refrigerator

(c) Air conditioner compressor (d) Air conditioner fan

(e) Variable-speed sewing machine (f) Clock

(g) Electric drill

S

OLUTION

(a) Universal motor—for its high torque

(b) Capacitor start or Capacitor start and run—For its high starting torque and relatively constant

speed at a wide variety of loads

(c) Same as (b) above

279

(d) Split-phase—Fans are low-starting-torque applications, and a split-phase motor is appropriate

(e) Universal Motor—Direction and speed are easy to control with solid-state drives

(f) Hysteresis motor—for its easy starting and operation at sync

n. A reluctance motor would also do

nicely.

(g) Universal Motor—for easy speed control with solid-state drives, plus high torque under loaded

conditions.

10-8. For a particular application, a three-phase stepper motor must be capable of stepping in 10° increments.

How many poles must it have?

S

OLUTION From Equation (10-18), the relationship between mechanical angle and electrical angle in a

three-phase stepper motor is

2

me

P

θθ

=

so 60

2 2 12 poles

10

e

m

P

θ

°

== =

°

10-9. How many pulses per second must be supplied to the control unit of the motor in Problem 10-7 to achieve a

rotational speed of 600 r/min?

S

OLUTION From Equation (10-20),

pulses

1

3

m

nn

P

=

so

()( )

pulses 3 3 12 poles 600 r/min 21,600 pulses/min 360 pulses/s

m

nPn== = =

10-10. Construct a table showing step size versus number of poles for three-phase and four-phase stepper motors.

S

OLUTION For 3-phase stepper motors, °= 60

e

θ

, and for 4-phase stepper motors, °= 45

e

θ

. Therefore,

Number of poles Mechanical Step Size

3-phase ( 60

e

θ

=°) 4-phase ( 45

e

θ

=°)

2 60° 45°

4 30° 22.5°

6 20° 15°

8 15° 11.25°

10 12° 9

°

12 10° 7.5°

280

Appendix A: Review of Three-Phase Circuits

A-1. Three impedances of 4 + j3 Ω are ∆-connected and tied to a three-phase 208-V power line. Find Iφ, IL,

P, Q, S, and the power factor of this load.

S

OLUTION

Z

φ

Z

φ

Z

φ

+

-

240 V

I

L

I

φ

Ω+= 43 jZ

φ

Here, 208 V

L

VV

φ

== , and 4 3 5 36.87 Zj

φ

=+ Ω=∠ °Ω, so

208 V 41.6 A

5

V

IZ

φ

== =

Ω

()

3 3 41.6 A 72.05 A

L

II

φ

== =

()

2

2208 V

3 cos 3 cos 36.87 20.77 kW

5

V

PZ

φ

θ

== °=

Ω

()

2

2208 V

3 sin 3 sin 36.87 15.58 kvar

5

V

QZ

φ

θ

== °=

Ω

22

25.96 kVASPQ=+=

PF cos 0.8 lagging

θ

==

A-2. Figure PA-1 shows a three-phase power system with two loads. The ∆-connected generator is producing a

line voltage of 480 V, and the line impedance is 0.09 + j0.16 Ω. Load 1 is Y-connected, with a phase

impedance of 2.5∠36.87° Ω and load 2 is ∆-connected, with a phase impedance of 5∠-20° Ω.

281

(a) What is the line voltage of the two loads?

(b) What is the voltage drop on the transmission lines?

(c) Find the real and reactive powers supplied to each load.

(d) Find the real and reactive power losses in the transmission line.

(e) Find the real power, reactive power, and power factor supplied by the generator.

S

OLUTION To solve this problem, first convert the two deltas to equivalent wyes, and get the per-phase

equivalent circuit.

+

-

277

∠

0° V

Line

0.090

Ω

j

0.16

Ω

1

φ

Z2

φ

Z

Ω°∠= 87.365.2

1

φ

Z

Ω°−∠= 2067.1

2

φ

Z

load,

φ

V

+

-

(a) The phase voltage of the equivalent Y-loads can be found by nodal analysis.

,load ,load ,load

277 0 V 0

0.09 0.16 2.5 36.87 1.67 20 j

φφφ

−∠°

++=

+Ω ∠°Ω∠−°Ω

VVV

()

(

)

()()

,load ,load ,load

5.443 60.6 277 0 V 0.4 36.87 0.6 20 0

φφφ

∠− ° −∠°+∠− ° +∠° =VVV

()

,load

5.955 53.34 1508 60.6

φ

∠− ° = ∠− °V

,load 253.2 7.3 V

φ

=∠−°V

282

Therefore, the line voltage at the loads is 3 439 V

L

VV

φ

=.

(b) The voltage drop in the transmission lines is

line ,gen ,load 277 0 V 253.2 -7.3 41.3 52 V

φφ

∆=−=∠°−∠°=∠°VV V

(c) The real and reactive power of each load is

()

2

1

253.2 V

3 cos 3 cos 36.87 61.6 kW

2.5

V

PZ

φ

θ

== °=

Ω

()

2

1

253.2 V

3 sin 3 sin 36.87 46.2 kvar

2.5

V

QZ

φ

θ

== °=

Ω

()

2

253.2 V

3 cos 3 cos -20 108.4 kW

1.67

V

PZ

φ

θ

== °=

Ω

()

2

253.2 V

3 sin 3 sin -20 39.5 kvar

1.67

V

QZ

φ

θ

== °=−

Ω

(d) The line current is

line

41.3 52 V 225 8.6 A

0.09 0.16 Zj

∆∠°

== =∠−°

+Ω

V

I

Therefore, the loses in the transmission line are

()( )

2

line line line

3 3 225 A 0.09 13.7 kWPIR== Ω=

()( )

2

line line line

3 3 225 A 0.16 24.3 kvarQIX== Ω=

(e) The real and reactive power supplied by the generator is

gen line 1 2 13.7 kW 61.6 kW 108.4 kW 183.7 kWPPPP=++= + + =

gen line 1 2 24.3 kvar 46.2 kvar 39.5 kvar 31 kvarQQQQ=++= + − =

The power factor of the generator is

gen

-1 1

gen

31 kvar

PF cos tan cos tan 0.986 lagging

183.7 kW

Q

P

−

== =

A-3. Figure PA-2 shows a one-line diagram of a simple power system containing a single 480 V generator and

three loads. Assume that the transmission lines in this power system are lossless, and answer the following

questions.

(a) Assume that Load 1 is Y-connected. What are the phase voltage and currents in that load?

(b) Assume that Load 2 is ∆-connected. What are the phase voltage and currents in that load?

(c) What real, reactive, and apparent power does the generator supply when the switch is open?

(d) What is the total line current L

I when the switch is open?

(e) What real, reactive, and apparent power does the generator supply when the switch is closed?

(f) What is the total line current L

I when the switch is closed?

(g) How does the total line current L

I compare to the sum of the three individual currents 123

III++? If

they are not equal, why not?

283

S

OLUTION Since the transmission lines are lossless in this power system, the full voltage generated by 1

G

will be present at each of the loads.

(a) Since this load is Y-connected, the phase voltage is

1

480 V 277 V

3

V

φ

==

The phase current can be derived from the equation 3cosPVI

φφ

θ

= as follows:

()()

1

100 kW 133.7 A

3 cos 3 277 V 0.9

P

IV

φ

θ

== =

(b) Since this load is ∆-connected, the phase voltage is

2480 VV

φ

=

The phase current can be derived from the equation 3SVI

φ

= as follows:

()

2

80 kVA 55.56 A

3 3 480 V

S

IV

φ

== =

(c) The real and reactive power supplied by the generator when the switch is open is just the sum of the

real and reactive powers of Loads 1 and 2.

1100 kWP=

(

)

(

)

(

)

1

1tan tan cos PF 100 kW tan 25.84 48.4 kvarQP P

θ

−

== = °=

(

)

(

)

2cos 80 kVA 0.8 64 kWPS

θ

== =

(

)

(

)

2sin 80 kVA 0.6 48 kvarQS

θ

== =

12

100 kW 64 kW 164 kW

G

PPP=+= + =

12

48.4 kvar 48 kvar 96.4 kvar

G

QQQ=+= + =

(d) The line current when the switch is open is given by 3 cos

L

P

IV

θ

=, where 1

tan G

G

Q

P

θ

−

=.

11

96.4 kvar

tan tan 30.45

164 kW

G

Q

P

θ

−−

== =°

()()

164 kW 228.8 A

3 cos 3 480 V cos 30.45

L

P

IV

θ

== =

°

284

(e) The real and reactive power supplied by the generator when the switch is closed is just the sum of the

real and reactive powers of Loads 1, 2, and 3. The powers of Loads 1 and 2 have already been calculated.

The real and reactive power of Load 3 are:

380 kWP=

(

)

(

)

(

)

1

3tan tan cos PF 80 kW tan 31.79 49.6 kvarQP P

θ

−

== =

12 3

100 kW 64 kW 80 kW 244 kW

G

PPPP=++= + + =

123

48.4 kvar 48 kvar 49.6 kvar 46.8 kvar

G

QQQQ=++= + − =

(f) The line current when the switch is closed is given by 3 cos

L

P

IV

θ

=, where 1

tan G

G

Q

P

θ

−

=.

11

46.8 kvar

tan tan 10.86

244 kW

G

Q

P

θ

−−

== =°

()()

244 kW 298.8 A

3 cos 3 480 V cos 10.86

L

P

IV

θ

== =

°

(g) The total line current from the generator is 298.8 A. The line currents to each individual load are:

()()

1

100 kW 133.6 A

3 cos 3 480 V 0.9

L

P

IV

θ

== =

()

2

80 kVA 96.2 A

3 3 480 V

L

S

IV

== =

()()

3

80 kW 113.2 A

3 cos 3 480 V 0.85

L

P

IV

θ

== =

The sum of the three individual line currents is 343 A, while the current supplied by the generator is 298.8

A. These values are not the same, because the three loads have different impedance angles. Essentially,

Load 3 is supplying some of the reactive power being consumed by Loads 1 and 2, so that it does not have

to come from the generator.

A-4. Prove that the line voltage of a Y-connected generator with an acb phase sequence lags the corresponding

phase voltage by 30°. Draw a phasor diagram showing the phase and line voltages for this generator.

S

OLUTION If the generator has an acb phase sequence, then the three phase voltages will be

0

an V

φ

=∠°V

240

bn V

φ

=∠− °V

120

cn V

φ

=∠− °V

The relationship between line voltage and phase voltage is derived below. By Kirchhoff’s voltage law, the

line-to-line voltage ab

V is given by

ab a b

=−VVV

0 240

ab VV

φφ

=∠°− ∠− °V

1333

2222

ab VVjVVjV

φ

φφφφ

=−− + = −

V

31

322

ab Vj

φ

=−

V

330

ab V

φ

=∠−°V

285

Thus the line voltage lags the corresponding phase voltage by 30°. The phasor diagram for this connection

is shown below.

Van

Vbn

Vcn

Vab

Vbc

A-5. Find the magnitudes and angles of each line and phase voltage and current on the load shown in Figure P2-

3.

S

OLUTION Note that because this load is ∆-connected, the line and phase voltages are identical.

120 0 V 120 120 V 208 30 V

ab an bn --=−=∠° ∠ °=∠°VVV

120 120 V 120 240 V 208 90 V

bc bn cn -- -=−=∠−° ∠ °=∠°VVV

120 240 V 120 0 V 208 150 V

ca cn an -=−=∠−° ∠°=∠°VVV

286

208 30 V 20.8 10 A

10 20

ab

ab Z

φ

∠°

== =∠°

∠°Ω

V

I

208 90 V 20.8 110 A

10 20

bc

bc Z

φ

∠− °

== =∠−°

∠°Ω

V

I

208 150 V 20.8 130 A

10 20

ca

ca Z

φ

∠°

== =∠°

∠°Ω

V

I

20.8 10 A 20.8 130 A 36 20 A

aabca --=−= ∠° ∠°=∠°II I

20.8 110 A 20.8 10 A 36 140 A

bbcab --=−= ∠−° ∠°=∠ °II I

20.8 130 A 20.8 -110 A 36 100 A

ccabc -=−= ∠° ∠ °=∠°II I

A-6. Figure PA-4 shows a small 480-V distribution system. Assume that the lines in the system have zero

impedance.

(a) If the switch shown is open, find the real, reactive, and apparent powers in the system. Find the total

current supplied to the distribution system by the utility.

(b) Repeat part (a) with the switch closed. What happened to the total current supplied? Why?

S

OLUTION

(a) With the switch open, the power supplied to each load is

()

kW 86.9530 cos

10

V 804

3cos3

2

1=°

Ω

==

θ

φ

Z

V

P

()

2

1

480 V

3 sin 3 sin 30 34.56 kvar

10

V

QZ

φ

θ

== °=

Ω

()

kW 04.4636.87 cos

4

V 277

3cos3

2

2=°

Ω

==

θ

φ

Z

V

P

()

2

277 V

3 sin 3 sin 36.87 34.53 kvar

4

V

QZ

φ

θ

== °=

Ω

kW 105.9 kW 46.04 kW 86.59

21TOT =+=+= PPP

TOT 1 2 34.56 kvar 34.53 kvar 69.09 kvarQQQ=+= + =

The apparent power supplied by the utility is

22

TOT TOT TOT 126.4 kVASPQ=+=

The power factor supplied by the utility is

287

-1 1

TOT

69.09 kvar

PF cos tan cos tan 0.838 lagging

105.9 kW

Q

P

−

== =

The current supplied by the utility is

()()

TOT 105.9 kW 152 A

3 PF 3 480 V 0.838

L

T

P

IV

== =

(b) With the switch closed, 3

P is added to the circuit. The real and reactive power of 3

P is

()

kW 090 cos

5

V 277

3cos3

2

3=°

Ω

== -

Z

V

P

θ

φ

()()

2

3

277 V

3 sin 3 sin 90 46.06 kvar

5

V

P-

Z

φ

θ

== °=−

Ω

TOT 1 2 3 59.86 kW 46.04 kW 0 kW 105.9 kWPPPP=++= + + =

TOT 1 2 3 34.56 kvar 34.53 kvar 46.06 kvar 23.03 kvarQQQQ=++= + − =

The apparent power supplied by the utility is

22

TOT TOT TOT 108.4 kVASPQ=+=

The power factor supplied by the utility is

-1 1

TOT

23.03 kVAR

PF cos tan cos tan 0.977 lagging

105.9 kW

Q

P

−

== =

The current supplied by the utility is

()()

TOT 105.9 kW 130.4 A

3 PF 3 480 V 0.977

L

T

P

IV

== =

(c) The total current supplied by the power system drops when the switch is closed because the capacitor

bank is supplying some of the reactive power being consumed by loads 1 and 2.

288

Appendix B: Coil Pitch and Distributed Windings

B-1. A 2-slot three-phase stator armature is wound for two-pole operation. If fractional-pitch windings are to be

used, what is the best possible choice for winding pitch if it is desired to eliminate the fifth-harmonic

component of voltage?

S

OLUTION The pitch factor of a winding is given by Equation (B-19):

2

sin

υ

ρ

=

p

k

To eliminate the fifth harmonic, we want to select

ρ

so that 0

2

5

sin =

ρ

. This implies that

()

n 180

2

5°=

ρ

, where n = 0, 1, 2, 3, …

or

()

... ,144 ,72

5

1802°°=

°

=n

ρ

These are acceptable pitches to eliminate the fifth harmonic. Expressed as fractions of full pitch, these

pitches are 2/5, 4/5, 6/5, etc. Since the desire is to have the maximum possible fundamental voltage, the

best choice for coil pitch would be 4/5 or 6/5. The closest that we can approach to a 4/5 pitch in a 24-slot

winding is 10/12 pitch, so that is the pitch that we would use.

At 10/12 pitch,

966.0

2

150

sin =

°

=

p

k for the fundamental frequency

()( )

259.0

2

150 5

sin =

°

=

p

k for the fifth harmonic

289

B-2. Derive the relationship for the winding distribution factor kd in Equation B-22.

S

OLUTION The above illustration shows the case of 5 slots per phase, but the results are general. If there

are 5 slots per phase, each with voltage Ai

E, where the phase angle of each voltage increases by γ° from

slot to slot, then the total voltage in the phase will be

AnAAAAAA EEEEEEE ++++++= ...

54321

The resulting voltage A

E can be found from geometrical considerations. These “n” phases, when

drawn end-to-end, form equally-spaced chords on a circle of radius R. If a line is drawn from the center of

a chord to the origin of the circle, it forma a right triangle with the radius at the end of the chord (see

voltage 5A

E above). The hypotenuse of this right triangle is R, its opposite side is 2/E, and its smaller

angle is 2/

γ

. Therefore,

R

E2/

2

sin =

γ

⇒

2

sin

2

1

γ

E

R= (1)

The total voltage A

E also forms a chord on the circle, and dropping a line from the center of that chord to

the origin forms a right triangle. For this triangle, the hypotenuse is R, the opposite side is 2/

A

E, and the

angle is 2/

γ

n. Therefore,

R

En A2/

2

sin =

γ

⇒

2

sin

2

1

γ

n

E

R

A

= (2)

Combining (1) and (2) yields

290

2

sin

2

1

2

sin

2

1

γγ

n

EE A

=

2

sin

2

sin

γ

n

E

EA=

Finally,

2

sin

2

sin

γ

n

nE

E

kA

d==

since

d

k is defined as the ratio of the total voltage produced to the sum of the magnitudes of each

component voltage.

B-3. A three-phase four-pole synchronous machine has 96 stator slots. The slots contain a double-layer winding

(two coils per slot) with four turns per coil. The coil pitch is 19/24.

(a) Find the slot and coil pitch in electrical degrees.

(b) Find the pitch, distribution, and winding factors for this machine.

(c) How well will this winding suppress third, fifth, seventh, ninth, and eleventh harmonics? Be sure to

consider the effects of both coil pitch and winding distribution in your answer.

S

OLUTION

(a) The coil pitch is 19/24 or 142.5°. Note that these are electrical degrees. Since this is a 4-pole

machine, the coil pitch would be 71.25 mechanical degrees.

There are 96 slots on this stator, so the slot pitch is 360°/96 = 3.75 mechanical degrees or 7.5 electrical

degrees.

(b) The pitch factor of this winding is

947.0

2

5.142

sin

2

sin =

°

==

ρ

p

k

The distribution factor is

2

sin

2

sin

γ

n

kd=

The electrical angle γ between slots is 7.5°, and each phase group occupies 8 adjacent slots. Therefore, the

distribution factor is

291

()( )

956.0

2

15

sin 8

2

158

sin

2

sin

2

sin

=

°

==

γ

n

kd

The winding factor is

()()

905.00.956 947.0 === dpw kkk

B-4. A three-phase four-pole winding of the double-layer type is to be installed on a 48-slot stator. The pitch of

the stator windings is 5/6, and there are 10 turns per coil in the windings. All coils in each phase are

connected in series, and the three phases are connected in ∆. The flux per pole in the machine is 0.054 Wb,

and the speed of rotation of the magnetic field is 1800 r/min.

(a) What is the pitch factor of this winding?

(b) What is the distribution factor of this winding?

(c) What is the frequency of the voltage produced in this winding?

(d) What are the resulting phase and terminal voltages of this stator?

S

OLUTION

(a) The pitch factor of this winding is

966.0

2

150

sin

2

sin =

°

==

ρ

p

k

(b) The coils in each phase group of this machine cover 4 slots, and the slot pitch is 360/48 = 7.5

mechanical degrees or 15 electrical degrees. Therefore, the distribution factor is

()( )

958.0

2

15

sin 4

2

15 4

sin

2

sin

2

sin

=

°

==

γ

n

kd

(c) The frequency of the voltage produces by this winding is

()()

Hz60

120

poles 4r/min 1800

120 === Pn

fm

e

(d) There are 48 slots on this stator, with two coils sides in each slot. Therefore, there are 48 coils on the

machine. They are divided into 12 phase groups, so there are 4 coils per phase. There are 10 turns per

coil, so there are 40 turns per phase group. The voltage in one phase group is

( )()()( )( )

V 533 Hz60 Wb 0.054 0.958 0.966 turns4022 ===

πφπ

edpPG fkkNE

There are two phase groups per phase, connected in series (this is a 4-pole machine), so the total phase

voltage is V 10662== G

EV

φ

. Since the machine is ∆-connected,

V 1066==

φ

VVT

B-5. A three-phase Y-connected six-pole synchronous generator has six slots per pole on its stator winding. The

winding itself is a chorded (fractional-pitch) double-layer winding with eight turns per coil. The

distribution factor kd = 0.956, and the pitch factor kp = 0.981. The flux in the generator is 0.02 Wb per

292

pole, and the speed of rotation is 1200 r/min. What is the line voltage produced by this generator at these

conditions?

S

OLUTION There are 6 slots per pole × 6 poles = 36 slots on the stator of this machine. Therefore, there are

36 coils on the machine, or 12 coils per phase. The electrical frequency produced by this winding is

()()

Hz60

120

poles 6r/min 1200

120 === Pn

fm

e

The phase voltage is

()()()( )()

V 480 Hz60 Wb 0.02 0.956 0.981 turns9622 ===

πφπ

φ

edpP fkkNV

Therefore, the line voltage is

V 8313==

φ

VVL

B-6. A three-phase Y-connected 50-Hz two-pole synchronous machine has a stator with 18 slots. Its coils form

a double-layer chorded winding (two coils per slot), and each coil has 60 turns. The pitch of the stator coils

is 8/9.

(a) What rotor flux would be required to produce a terminal (line-to-line) voltage of 6 kV?

(b) How effective are coils of this pitch at reducing the fifth-harmonic component of voltage? The seventh-

harmonic component of voltage?

S

OLUTION

(a) The pitch of this winding is 8/9 = 160°, so the pitch factor is

985.0

2

160

sin =

°

=

p

k

The phase groups in this machine cover three slots each, and the slot pitch is 20 mechanical or 20 electrical

degrees. Thus the distribution factor is

()( )

960.0

2

20

sin 3

2

20 3

sin

2

sin

2

sin

=

°

==

γ

n

kd

The phase voltage of this machine will be

()( )()()()

Hz50 0.960 0.985 l turns/coi60 coils 622

φπφπ

φ

== edpP fkkNV

φ

75621=V

The desired phase voltage is 6 kV / 3= 3464 V, so

Wb 046.0

75621

V 3464 ==

φ

(b) The

fifth harmonic:

()( )

643.0

2

160 5

sin =

°

=

p

k

The

seventh harmonic:

()( )

342.0

2

160 7

sin −=

°

=

p

k

293

Since the fundamental voltage is reduced by 0.985, the fifth and seventh harmonics are suppressed relative

to the fundamental by the fractions:

5

th: 653.0

985.0

643.0 =

7

th: 347.0

985.0

342.0 =

In other words, the 5th harmonic is suppressed by 34.7% relative to the fundamental, and the 7th harmonic is

suppressed by 65.3% relative to the fundamental frequency.

B-7. What coil pitch could be used to completely eliminate the seventh-harmonic component of voltage in ac

machine armature (stator)? What is the minimum number of slots needed on an eight-pole winding to

exactly achieve this pitch? What would this pitch do to the fifth-harmonic component of voltage?

S

OLUTION To totally eliminate the seventh harmonic of voltage in an ac machine armature, the pitch factor

for that harmonic must be zero.

2

7

sin0

ρ

==

p

k

⇒

()

n 180

2

7°=

ρ

, n = 0, 1, 2, …

()

7

1802n°

=

ρ

In order to maximize the fundamental voltage while canceling out the seventh harmonic, we pick the value

of n that makes

ρ

as nearly 180° as possible. If n = 3, then

ρ

= 154.3°, and the pitch factor for the

fundamental frequency would be

975.0

2

3.154

sin =

°

=

p

k

This pitch corresponds to a ratio of 6/7. For a two-pole machine, a ratio of 6/7 could be implemented with

a total of 14 slots. If that ratio is desired in an 8-pole machine, then 56 slots would be needed.

The fifth harmonic would be suppressed by this winding as follows:

()( )

434.0

2

3.154 5

sin =

°

=

p

k

B-8. A 13.8-kV Y-connected 60-Hz 12-pole three-phase synchronous generator has 180 stator slots with a

double-layer winding and eight turns per coil. The coil pitch on the stator is 12 slots. The conductors from

all phase belts (or groups) in a given phase are connected in series.

(a) What flux per pole would be required to give a no-load terminal (line) voltage of 13.8 kV?

(b) What is this machine’s winding factor kw?

S

OLUTION

(a) The stator pitch is 12/15 = 4/5, so °= 144

ρ

, and

951.0

2

144

sin =

°

=

p

k

294

Each phase belt consists of (180 slots)/(12 poles)(6) = 2.5 slots per phase group. The slot pitch is 2

mechanical degrees or 24 electrical degrees. The corresponding distribution factor is

()( )

962.0

2

24

sin .52

2

24 5.2

sin

2

sin

2

sin

=

°

==

γ

n

kd

Since there are 60 coils in each phase and 8 turns per coil, all connected in series, there are 480 turns per

phase. The resulting voltage is

()()()()

Hz60 0.962 0.951 turns48022

φπφπ

φ

== edpP fkkNV

061,117

φ

=V

The phase voltage of this generator must be V 79673 / kV 8.13 =, so the flux must be

Wb 068.0

117,061

V 9677==

φ

(b) The machine’s winding factor is

()()

915.00.962 0.951 === dpw kkk

295

Appendix C: Salient Pole Theory of Synchronous Machines

C-1. A 480-V 200-kVA 0.8-PF-lagging 60-Hz four-pole Y-connected synchronous generator has a direct-axis

reactance of 0.25 Ω, a quadrature-axis reactance of 0.18 Ω, and an armature resistance of 0.03 Ω.

Friction, windage, and stray losses may be assumed negligible. The generator’s open-circuit characteristic

is given by Figure P5-1.

(a) How much field current is required to make VT equal to 480 V when the generator is running at no

load?

(b) What is the internal generated voltage of this machine when it is operating at rated conditions? How

does this value of EA compare to that of Problem 5-2b?

(c) What fraction of this generator’s full-load power is due to the reluctance torque of the rotor?

S

OLUTION

(a) If the no-load terminal voltage is 480 V, the required field current can be read directly from the open-

circuit characteristic. It is 4.55 A.

(b) At rated conditions, the line and phase current in this generator is

()

A 6.240

V 4803

kVA 200

3 ====

L

LA V

P

II at an angle of –36.87°

296

′′ = + +EV I I

AAAqA

RjX

φ

()()()()

A 87.366.240 18.0A 87.366.240 03.00277 °−∠Ω+°−∠Ω+°∠=

′′ j

A

E

V 61.5310 °∠=

′′

A

E

Therefore, the torque angle

δ

is 5.61°. The direct-axis current is

()

°−∠+= 90 sin

δδθ

Ad II

()()

°−∠°= 4.84 48.42sinA 6.240

d

I

A 4.84 5.162 °−∠=

d

I

The quadrature-axis current is

()

δδθ

∠+= cos

Aq II

()()

°∠°= 61.5 48.42cosA 6.240

q

I

A 61.5 4.177 °∠=

q

I

Therefore, the internal generated voltage of the machine is

qqddAAA jXjXRIIIVE +++=

φ

()( )()( )()( )

°∠+°−∠+°−∠+°∠= 61.54.17718.04.845.16225.087.366.24003.00277 jj

A

E

V 61.5322 °∠=

A

E

A

E is approximately the same magnitude here as in Problem 5-2b, but the angle is about 2.2° different.

(c) The power supplied by this machine is given by the equation

PVE

X

VX X

XX

A

d

dq

=+

−













33

2

φφ

δδ sin sin 2

()() ()

()()

°











−

+°5= 1.221 sin

18.025.0

2

2773

.61 sin

25.0

32227732

P

kW 139.4kW 8.34kW 6.104

=

+

=

P

The cylindrical rotor term is 104.6 kW, and the reluctance term is 34.8 kW, so the reluctance torque

accounts for about 25% of the power in this generator.

297

C-2. A 14-pole Y-connected three-phase water-turbine-driven generator is rated at 120 MVA, 13.2 kV, 0.8 PF

lagging, and 60 Hz. Its direct-axis reactance is 0.62 Ω and its quadrature- axis reactance is 0.40 Ω. All

rotational losses may be neglected.

(a) What internal generated voltage would be required for this generator to operate at the rated conditions?

(b) What is the voltage regulation of this generator at the rated conditions?

(c) Sketch the power-versus-torque-angle curve for this generator. At what angle δ is the power of the

generator maximum?

(d) How does the maximum power out of this generator compare to the maximum power available if it

were of cylindrical rotor construction?

S

OLUTION

(a) At rated conditions, the line and phase current in this generator is

()

A 5249

kV 2.133

MVA120

3 ====

L

LA V

P

II at an angle of –36.87°

′′ =+ +

EV I I

AAAqA

RjX

φ

()( )

A 87.365249 40.0007621 °−∠Ω++°∠=

′′ j

A

E

V 7.109038 °∠=

′′

A

E

Therefore, the torque angle

δ

is 10.7°. The direct-axis current is

()

°−∠+= 90 sin

δδθ

Ad II

()()

°−∠°= 3.79 57.47sinA 2495

d

I

A 3.79 3874 °−∠=

d

I

The quadrature-axis current is

()

δδθ

∠+= cos

Aq II

()()

°∠°= 7.10 57.47cosA 2495

q

I

A 7.10 3541 °∠=

q

I

Therefore, the internal generated voltage of the machine is

qqddAAA jXjXRIIIVE +++=

φ

()( )()( )

°∠+°−∠++°∠= 7.103541 40.03.793874 62.0007621 jj

A

E

V 7.109890 °∠=

A

E

(b) The voltage regulation of this generator is

%8.29%100

7621

76219890

%100

fl

flnl =×

−

=×

−

V

VV

(c) The power supplied by this machine is given by the equation

PVE

X

VX X

XX

A

d

dq

=+

−













33

2

φφ

δδ sin sin 2

298

()() ()

()()

δδ

2 sin

40.0 62.0

40.062.0

2

76213

sin

62.0

9890762132











−

+=P

MW2 sin 3.77 sin 7.364

δ

+=P

A plot of power supplied as a function of torque angle is shown below:

The peak power occurs at an angle of 70.6°, and the maximum power that the generator can supply is

392.4 MW.

(d) If this generator were non-salient, MAX

P would occur when

δ

= 90°, and MAX

P would be 364.7 MW.

Therefore, the salient-pole generator has a higher maximum power than an equivalent non-salint pole

generator.

C-3. Suppose that a salient-pole machine is to be used as a motor.

(a) Sketch the phasor diagram of a salient-pole synchronous machine used as a motor.

(b) Write the equations describing the voltages and currents in this motor.

(c) Prove that the torque angle δ between EA and Vφ on this motor is given by

δθθ

θθ

φ

=+

tan cos - sin

sin + cos

-1 IX IR

VIX IR

Aq AA

S

OLUTION

299

300

C-4. If the machine in Problem C-1 is running as a motor at the rated conditions, what is the maximum torque

that can be drawn from its shaft without it slipping poles when the field current is zero?

S

OLUTION When the field current is zero, A

E = 0, so











−

=

δ

φ

2 sin

2

32

qd

XX

XXV

P

()

()()

kW 2 sin2 sin

18.025.0

2

27732

δδ

179=











−

=P

At °= 45

δ

, 179 kW can be drawn from the motor.

301

Appendix D: Errata for Electric Machinery Fundamentals 4/e

(Current at 10 January 2004)

Please note that some or all of the following errata may be corrected in future reprints of the

book, so they may not appear in your copy of the text. PDF pages with these corrections are attached to

this appendix; please provide them to your students.

1. Page 56, Problem 1-6, there are 400 turns of wire on the coil, as shown on Figure P1-3. The body of

the problem incorrectly states that there are 300 turns.

2. Page 56, Problem 1-7, there are 400 turns of wire on the left-hand coil, and 300 turns on the right-

hand coil, as shown on Figure P1-4. The body of the problem is incorrect.

3. Page 62, Problem 1-19, should state: “Figure P1-14 shows a simple single-phase ac power system

with three loads. The voltage source is 120 0 V=∠°V, and the three loads are …”

4. Page 64, Problem 1-22, should state: “If the bar runs off into a region where the flux density falls to

0.30 T… ”. Also, the load should be 10 N, not 20.

5. Page 147, Problem 2-10, should state that the transformer bank is Y-∆, not ∆-Y.

6. Page 226, Problem 3-10, the holding current H

I should be 8 mA.

7. Page 342, Figure p5-2, the generator for Problems 5-11 through 5-21, the OCC and SCC curves are

in error. The correct curves are given below. Note that the voltage scale and current scales were

both off by a factor of 2.

302

8. Page 344, Problem 5-28, the voltage of the infinite bus is 12.2 kV.

9. Page 377, Problem 6-11, the armature resistance is 0.08 Ω, and the synchronous reactance is 1.0 Ω.

10. Page 470, Problem 7-20 (a), the holding the infinite bus is 460-V.

303

11. Page 623, Figure P9-2 and Figure P9-3, 0.40

A

R=Ω and 100

F

R=Ω. Values are stated correctly

in the text but shown incorrectly on the figure.

12. Page 624, Figure P9-4, 0.44

AS

RR+= Ω and 100

F

R=Ω. Values are stated correctly in the text

but shown incorrectly on the figure.

13. Page 627, Problem 9-21, adj

R is currently set to 90 Ω. Also, the magnetization curve is taken at 1800

r/min.

14. Page 627, Problem 9-22, A

R is 0.18 Ω.

15. Page 630, Figure P9-10, 0.21

AS

RR+= Ω

SE

N is 20 turns. Values are stated correctly in the text

but shown incorrectly on the figure.

16. Page 680, Problem 10-6, refers to Problem 10-5 instead of Problem 10-4.

56 ELECTRIC MACHINERY FUNDAMENTALS

1–4. A motor is supplying 60 N • m of torque to its load. If the motor’s shaft is turning at

1800 r/min, what is the mechanical power supplied to the load in watts? In horse-

power?

1–5. A ferromagnetic core is shown in Figure P1–2. The depth of the core is 5 cm. The

other dimensions of the core are as shown in the figure. Find the value of the current

that will produce a flux of 0.005 Wb. With this current, what is the flux density at

the top of the core? What is the flux density at the right side of the core? Assume

that the relative permeability of the core is 1000.

1–6. A ferromagnetic core with a relative permeability of 1500 is shown in Figure P1–3.

The dimensions are as shown in the diagram, and the depth of the core is 7 cm. The

air gaps on the left and right sides of the core are 0.070 and 0.050 cm, respectively.

Because of fringing effects, the effective area of the air gaps is 5 percent larger than

their physical size. If there are 400 turns in the coil wrapped around the center leg

of the core and if the current in the coil is 1.0 A, what is the flux in each of the left,

center, and right legs of the core? What is the flux density in each air gap?

1–7. A two-legged core is shown in Figure P1–4. The winding on the left leg of the core

(N1) has 400 turns, and the winding on the right (N2) has 300 turns. The coils are

wound in the directions shown in the figure. If the dimensions are as shown, then

what flux would be produced by currents i10.5 A and i20.75 A? Assume



r

1000 and constant.

1–8. A core with three legs is shown in Figure P1–5. Its depth is 5 cm, and there are 200

turns on the leftmost leg. The relative permeability of the core can be assumed to be

1500 and constant. What flux exists in each of the three legs of the core? What is the

flux density in each of the legs? Assume a 4 percent increase in the effective area of

the air gap due to fringing effects.

15 cm

5 cm

20 cm10 cm

i

400 turns

Core depth  5 cm

φ

15 cm

+

–

FIGURE P1–2

The core of Problems 1–5 and 1–16.

cha65239_ch01.qxd 10/16/2003 9:54 AM Page 56

62 ELECTRIC MACHINERY FUNDAMENTALS

(d) Calculate the reactive power consumed or supplied by this load. Does the load

consume reactive power from the source or supply it to the source?

1–19. Figure P1–14 shows a simple single-phase ac power system with three loads. The

voltage source is V = 120∠0° V, and the impedances of the three loads are

Z1530° Z2545° Z3590° 

Answer the following questions about this power system.

(a) Assume that the switch shown in the figure is open, and calculate the current I,

the power factor, and the real, reactive, and apparent power being supplied by

the load.

N turns

l

c

= 48 cm

l

r

= 4 cm

4 cm

Depth = 4 cm

4 cm

l

g

= 0.05 cm

i

N = ?

FIGURE P1–13

The core of Problem 1–17.

t (ms)

12345678

0

φ

(Wb)

0.010

0.005

–0.005

–0.010

FIGURE P1–12

Plot of flux



as a function of time for Problem 1–16.

cha65239_ch01.qxd 10/23/2003 9:22 AM Page 62

64 ELECTRIC MACHINERY FUNDAMENTALS

(a) If this bar has a load of 10 N attached to it opposite to the direction of motion,

what is the steady-state speed of the bar?

(b) If the bar runs off into a region where the flux density falls to 0.30 T, what hap-

pens to the bar? What is its final steady-state speed?

(c) Suppose VBis now decreased to 80 V with everything else remaining as in

part b. What is the new steady-state speed of the bar?

(d) From the results for parts band c, what are two methods of controlling the

speed of a linear machine (or a real dc motor)?

REFERENCES

1. Alexander, Charles K., and Matthew N. O. Sadiku: Fundamentals of Electric Circuits, McGraw-

Hill, 2000.

2. Beer, F., and E. Johnston, Jr.: Vector Mechanics for Engineers: Dynamics, 6th ed., McGraw-Hill,

New York, 1997.

3. Hayt, William H.: Engineering Electromagnetics, 5th ed., McGraw-Hill, New York, 1989.

4. Mulligan, J. F.: Introductory College Physics, 2nd ed., McGraw-Hill, New York, 1991.

5. Sears, Francis W., Mark W. Zemansky, and Hugh D. Young: University Physics, Addison-Wesley,

Reading, Mass., 1982.

cha65239_ch01.qxd 10/16/2003 9:54 AM Page 64

TRANSFORMERS 147

2–8. A 200-MVA, 15/200-kV single-phase power transformer has a per-unit resistance of

1.2 percent and a per-unit reactance of 5 percent (data taken from the transformer’s

nameplate). The magnetizing impedance is j80 per unit.

(a) Find the equivalent circuit referred to the low-voltage side of this transformer.

(b) Calculate the voltage regulation of this transformer for a full-load current at

power factor of 0.8 lagging.

(c) Assume that the primary voltage of this transformer is a constant 15 kV, and

plot the secondary voltage as a function of load current for currents from no

load to full load. Repeat this process for power factors of 0.8 lagging, 1.0, and

0.8 leading.

2–9. A three-phase transformer bank is to handle 600 kVA and have a 34.5/13.8-kV volt-

age ratio. Find the rating of each individual transformer in the bank (high voltage,

low voltage, turns ratio, and apparent power) if the transformer bank is connected to

(a) Y–Y, ( b) Y–, (c) –Y, ( d) –, (e) open , (f) open Y–open .

2–10. A 13,800/480-V three-phase Y--connected transformer bank consists of three

identical 100-kVA 7967/480-V transformers. It is supplied with power directly from

a large constant-voltage bus. In the short-circuit test, the recorded values on the

high-voltage side for one of these transformers are

VSC 560 V ISC 12.6 A PSC 3300 W

(a) If this bank delivers a rated load at 0.85 PF lagging and rated voltage, what is

the line-to-line voltage on the high-voltage side of the transformer bank?

(b) What is the voltage regulation under these conditions?

(c) Assume that the primary voltage of this transformer is a constant 13.8 kV, and

plot the secondary voltage as a function of load current for currents from no-

load to full-load. Repeat this process for power factors of 0.85 lagging, 1.0, and

0.85 leading.

(d) Plot the voltage regulation of this transformer as a function of load current for

currents from no-load to full-load. Repeat this process for power factors of 0.85

lagging, 1.0, and 0.85 leading.

2–11. A 100,000-kVA, 230/115-kV –three-phase power transformer has a resistance of

0.02 pu and a reactance of 0.055 pu. The excitation branch elements are RC110 pu

and XM20 pu.

(a) If this transformer supplies a load of 80 MVA at 0.85 PF lagging, draw the pha-

sor diagram of one phase of the transformer.

(b) What is the voltage regulation of the transformer bank under these conditions?

(c) Sketch the equivalent circuit referred to the low-voltage side of one phase of this

transformer. Calculate all the transformer impedances referred to the low-voltage

side.

2–12. An autotransformer is used to connect a 13.2-kV distribution line to a 13.8-kV dis-

tribution line. It must be capable of handling 2000 kVA. There are three phases, con-

nected Y–Y with their neutrals solidly grounded.

(a) What must the NC/NSE turns ratio be to accomplish this connection?

(b) How much apparent power must the windings of each autotransformer handle?

(c) If one of the autotransformers were reconnected as an ordinary transformer,

what would its ratings be?

2–13. Two phases of a 13.8-kV three-phase distribution line serve a remote rural road (the

neutral is also available). A farmer along the road has a 480-V feeder supplying

cha65239_ch02.qxd 10/16/2003 12:20 PM Page 147

226 ELECTRIC MACHINERY FUNDAMENTALS

3–10. A series-capacitor forced commutation chopper circuit supplying a purely resistive

load is shown in Figure P3–5.

(a) When SCR1 is turned on, how long will it remain on? What causes it to turn off?

(b) When SCR1 turns off, how long will it be until the SCR can be turned on again?

(Assume that 3 time constants must pass before the capacitor is discharged.)

(c) What problem or problems do these calculations reveal about this simple series-

capacitor forced-commutation chopper circuit?

(d) How can the problem(s) described in part cbe eliminated?

3–11. A parallel-capacitor forced-commutation chopper circuit supplying a purely resis-

tive load is shown in Figure P3–6.

(a) When SCR1is turned on, how long will it remain on? What causes it to turn off?

(b) What is the earliest time that SCR1can be turned off after it is turned on?

(Assume that 3 time constants must pass before the capacitor is charged.)

(c) When SCR1turns off, how long will it be until the SCR can be turned on again?

(d) What problem or problems do these calculations reveal about this simple parallel-

capacitor forced-commutation chopper circuit?

(e) How can the problem(s) described in part dbe eliminated?

3–12. Figure P3–7 shows a single-phase rectifier-inverter circuit. Explain how this circuit

functions. What are the purposes of C1and C2? What controls the output frequency

of the inverter?

VDC 120 V

IH5 mA

VBO 250 V

R120 k

Rload 250 

C15



F

VDC 120 V

IH

 8 mA

VBO 200 V

R120 k

Rload 250 

C 150



F

+

–

+

–

+

–

V

DC

R1

R

LOAD

C

D

SCR

Load

v

load

v

c











FIGURE P3–5

The simple series-capacitor forced-commutation circuit of Problem 3–10.

cha65239_ch03.qxd 10/30/2003 1:15 PM Page 226

342

0 0.1

Open-circuit voltage, V

Field current, A

1200

1100

1000

900

800

700

600

500

400

300

200

100

00.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

(a)

0

Armature current, A

Field current, A

1600

1400

1200

1000

800

600

400

200

00.2 0.4 0.6 0.8 1 1.2 1.4

(b)

Open Circuit Characteristic

Short Circuit Characteristic

FIGURE P5–2

(a) Open-circuit characteristic curve for the generator in Problems 5–11 to 5–21. (b) Short-circuit

characteristic curve for the generator in Problems 5–11 to 5–21.

cha65239_ch05.qxd 11/5/2003 2:14 PM Page 342

344 ELECTRIC MACHINERY FUNDAMENTALS

5–27. A 25-MVA, three-phase, 13.8-kV, two-pole, 60-Hz Y-connected synchronous gen-

erator was tested by the open-circuit test, and its air-gap voltage was extrapolated

with the following results:

Open-circuit test

Field current, A 320 365 380 475 570

Line voltage, kV 13.0 13.8 14.1 15.2 16.0

Extrapolated air-gap voltage, kV 15.4 17.5 18.3 22.8 27.4

The short-circuit test was then performed with the following results:

Short-circuit test

Field current, A 320 365 380 475 570

Armature current, A 1040 1190 1240 1550 1885

The armature resistance is 0.24 per phase.

(a) Find the unsaturated synchronous reactance of this generator in ohms per phase

and per unit.

(b) Find the approximate saturated synchronous reactance XSat a field current of

380 A. Express the answer both in ohms per phase and per unit.

(c) Find the approximate saturated synchronous reactance at a field current of

475 A. Express the answer both in ohms per phase and in per-unit.

(d) Find the short-circuit ratio for this generator.

5–28. A 20-MVA, 12.2-kV, 0.8-PF-lagging, Y-connected synchronous generator has a neg-

ligible armature resistance and a synchronous reactance of 1.1 per unit. The gener-

ator is connected in parallel with a 60-Hz, 12.2-kV infinite bus that is capable of sup-

plying or consuming any amount of real or reactive power with no change in

frequency or terminal voltage.

(a) What is the synchronous reactance of the generator in ohms?

(b) What is the internal generated voltage EAof this generator under rated conditions?

(c) What is the armature current IAin this machine at rated conditions?

(d) Suppose that the generator is initially operating at rated conditions. If the inter-

nal generated voltage EAis decreased by 5 percent, what will the new armature

current IAbe?

(e) Repeat part dfor 10, 15, 20, and 25 percent reductions in EA.

(f) Plot the magnitude of the armature current IAas a function of EA. (You may wish

to use MATLAB to create this plot.)

cha65239_ch05.qxd 11/5/2003 2:14 PM Page 344

SYNCHRONOUS MOTORS 377

6–9. Figure P6–2 shows a synchronous motor phasor diagram for a motor operating at a

leading power factor with no RA. For this motor, the torque angle is given by

Derive an equation for the torque angle of the synchronous motor if the armature re-

sistance is included.

6–10. A 480-V, 375-kVA, 0.8-PF-lagging, Y-connected synchronous generator has a syn-

chronous reactance of 0.4  and a negligible armature resistance. This generator is

supplying power to a 480-V, 80-kW, 0.8-PF-leading, Y-connected synchronous mo-

tor with a synchronous reactance of 1.1  and a negligible armature resistance. The

synchronous generator is adjusted to have a terminal voltage of 480 V when the mo-

tor is drawing the rated power at unity power factor.

(a) Calculate the magnitudes and angles of EAfor both machines.

(b) If the flux of the motor is increased by 10 percent, what happens to the termi-

nal voltage of the power system? What is its new value?

(c) What is the power factor of the motor after the increase in motor flux?

6–11. A 480-V, 100-kW, 50-Hz, four-pole, Y-connected synchronous motor has a rated

power factor of 0.85 leading. At full load, the efficiency is 91 percent. The armature

resistance is 0.08 , and the synchronous reactance is 1.0 . Find the following

quantities for this machine when it is operating at full load:

(a) Output torque

(b) Input power

(c)nm

(d)EA

(e)|IA|

(f)P

conv

(g)P

mech P

core P

stray



tan1

(

XSIA cos



V



XSIA sin



)

tan



XSIA cos



V



XSIA sin



= tan

–1

(

V

E

A

I

A

j

X

S

I

A

X

S

I

A

cos

X

S

I

A

sin

X

S

I

A

cos

––—————–

V + X

S

I

A

sin

FIGURE P6–2

Phasor diagram of a motor at a leading power factor.

cha65239_ch06.qxd 11/5/2003 2:35 PM Page 377

470 ELECTRIC MACHINERY FUNDAMENTALS

(a) The line current IL

(b) The stator power factor

(c) The rotor power factor

(d) The stator copper losses PSCL

(e) The air-gap power PAG

(f) The power converted from electrical to mechanical form Pconv

(g) The induced torque



ind

(h) The load torque



load

(i) The overall machine efficiency



(j) The motor speed in revolutions per minute and radians per second

7–15. For the motor in Problem 7–14, what is the pullout torque? What is the slip at the

pullout torque? What is the rotor speed at the pullout torque?

7–16. If the motor in Problem 7–14 is to be driven from a 440-V, 60-Hz power supply,

what will the pullout torque be? What will the slip be at pullout?

7–17. Plot the following quantities for the motor in Problem 7–14 as slip varies from 0 to

10 percent: (a)



ind; (b) Pconv; (c) Pout; (d) efficiency



. At what slip does Pout equal

the rated power of the machine?

7–18. A 208-V, 60 Hz six-pole, Y-connected, 25-hp design class B induction motor is

tested in the laboratory, with the following results:

No load: 208 V, 22.0 A, 1200 W, 60 Hz

Locked rotor: 24.6 V, 64.5 A, 2200 W, 15 Hz

DC test: 13.5 V, 64 A

Find the equivalent circuit of this motor, and plot its torque–speed characteristic

curve.

7–19. A 460-V, four-pole, 50-hp, 60-Hz, Y-connected, three-phase induction motor devel-

ops its full-load induced torque at 3.8 percent slip when operating at 60 Hz and 460

V. The per-phase circuit model impedances of the motor are

R10.33 XM30 

X10.42 X20.42 

Mechanical, core, and stray losses may be neglected in this problem.

(a) Find the value of the rotor resistance R2.

(b) Find



max, smax, and the rotor speed at maximum torque for this motor.

(c) Find the starting torque of this motor.

(d) What code letter factor should be assigned to this motor?

7–20. Answer the following questions about the motor in Problem 7–19.

(a) If this motor is started from a 460-V infinite bus, how much current will flow in

the motor at starting?

(b) If transmission line with an impedance of 0.35 j0.25 per phase is used to

connect the induction motor to the infinite bus, what will the starting current of

the motor be? What will the motor’s terminal voltage be on starting?

(c) If an ideal 1.4:1 step-down autotransformer is connected between the transmis-

sion line and the motor, what will the current be in the transmission line during

starting? What will the voltage be at the motor end of the transmission line dur-

ing starting?

cha65239_ch07.qxd 11/5/2003 3:02 PM Page 470

DC MOTORS AND GENERATORS 623

9–10. If the motor is connected cumulatively compounded as shown in Figure P9–4 and if

Radj 175 , what is its no-load speed? What is its full-load speed? What is its

speed regulation? Calculate and plot the torque–speed characteristic for this motor.

(Neglect armature effects in this problem.)

9–11. The motor is connected cumulatively compounded and is operating at full load.

What will the new speed of the motor be if Radj is increased to 250 ? How does the

new speed compare to the full-load speed calculated in Problem 9–10?

9–12. The motor is now connected differentially compounded.

(a) If Radj 175 , what is the no-load speed of the motor?

(b) What is the motor’s speed when the armature current reaches 20A? 40 A? 60 A?

(c) Calculate and plot the torque–speed characteristic curve of this motor.

9–13. A 7.5-hp, 120-V series dc motor has an armature resistance of 0.2  and a series field

resistance of 0.16 . At full load, the current input is 58 A, and the rated speed is

RA

EA

VT = 240 V

100

+

–

IF

IL

IA

LF

RF

Radj

0.40

FIGURE P9–2

The equivalent circuit of the shunt motor in Problems 9–1 to 9–7.

R

A

E

A

V

F

= 240 V V

A

= 120 to 240 V

+

–

I

L

I

F

I

A

+

–

+

–

R

F

= 100

L

F

R

adj

0.40

FIGURE P9–3

The equivalent circuit of the separately excited motor in Problems 9–8 and 9–9.

cha65239_ch09.qxd 11/14/03 10:10 AM Page 623

624 ELECTRIC MACHINERY FUNDAMENTALS

1050 r/min. Its magnetization curve is shown in Figure P9–5. The core losses are 200

W, and the mechanical losses are 240 W at full load. Assume that the mechanical

losses vary as the cube of the speed of the motor and that the core losses are constant.

(a) What is the efficiency of the motor at full load?

(b) What are the speed and efficiency of the motor if it is operating at an armature

current of 35 A?

(c) Plot the torque–speed characteristic for this motor.

9–14. A 20-hp, 240-V, 76-A, 900 r/min series motor has a field winding of 33 turns per

pole. Its armature resistance is 0.09 , and its field resistance is 0.06 . The mag-

netization curve expressed in terms of magnetomotive force versus EAat 900 r/min

is given by the following table:

EA, V 95 150 188 212 229 243

, A • turns 500 1000 1500 2000 2500 3000

Armature reaction is negligible in this machine.

(a) Compute the motor’s torque, speed, and output power at 33, 67, 100, and 133

percent of full-load armature current. (Neglect rotational losses.)

(b) Plot the torque–speed characteristic of this machine.

9–15. A 300-hp, 440-V, 560-A, 863 r/min shunt dc motor has been tested, and the follow-

ing data were taken:

Blocked-rotor test:

VA16.3 V exclusive of brushes VF440 V

IA500 A IF8.86 A

No-load operation:

VA16.3 V including brushes IF8.76 A

IA23.1 A n863 r/min

E

A

V

T

= 240 V

= Cumulatively compounded

= Differentially compounded

= R

A

+ R

S

100

+

–

I

F

I

L

I

A

L

F

R

F

L

S

R

adj

+

–

0.44

FIGURE P9–4

The equivalent circuit of the compounded motor in Problems 9–10 to 9–12.

cha65239_ch09.qxd 11/14/03 10:10 AM Page 624

DC MOTORS AND GENERATORS 627

9–19. A series motor is now constructed from this machine by leaving the shunt field out

entirely. Derive the torque–speed characteristic of the resulting motor.

9–20. An automatic starter circuit is to be designed for a shunt motor rated at 15 hp, 240

V, and 60 A. The armature resistance of the motor is 0.15 , and the shunt field re-

sistance is 40 . The motor is to start with no more than 250 percent of its rated ar-

mature current, and as soon as the current falls to rated value, a starting resistor

stage is to be cut out. How many stages of starting resistance are needed, and how

big should each one be?

9–21. A 15-hp, 230-V, 1800 r/min shunt dc motor has a full-load armature current of 60 A

when operating at rated conditions. The armature resistance of the motor is RA

0.15 , and the field resistance RFis 80 .The adjustable resistance in the field cir-

cuit Radj

may be varied over the range from 0 to 200  and is currently set to 90 .

Armature reaction may be ignored in this machine. The magnetization curve for this

motor, taken at a speed of 1800 r/min, is given in tabular form below:

EA, V 8.5 150 180 215 226 242

IF, A 0.00 0.80 1.00 1.28 1.44 2.88

(a) What is the speed of this motor when it is running at the rated conditions spec-

ified above?

(b) The output power from the motor is 7.5 hp at rated conditions. What is the out-

put torque of the motor?

(c) What are the copper losses and rotational losses in the motor at full load (ignore

stray losses)?

(d) What is the efficiency of the motor at full load?

(e) If the motor is now unloaded with no changes in terminal voltage or Radj, what

is the no-load speed of the motor?

(f) Suppose that the motor is running at the no-load conditions described in part e.

What would happen to the motor if its field circuit were to open? Ignoring ar-

mature reaction, what would the final steady-state speed of the motor be under

those conditions?

(g) What range of no-load speeds is possible in this motor, given the range of field

resistance adjustments available with Radj?

9–22. The magnetization curve for a separately excited dc generator is shown in Figure

P9–7. The generator is rated at 6 kW, 120 V, 50 A, and 1800 r/min and is shown in

Figure P9–8. Its field circuit is rated at 5A. The following data are known about the

machine:

RA

 0.18  VF

 120 V

Radj 0 to 30 RF24 

NF1000 turns per pole

Answer the following questions about this generator, assuming no armature reaction.

(a) If this generator is operating at no load, what is the range of voltage adjustments

that can be achieved by changing Radj?

(b) If the field rheostat is allowed to vary from 0 to 30  and the generator’s speed

is allowed to vary from 1500 to 2000 r/min, what are the maximum and mini-

mum no-load voltages in the generator?

cha65239_ch09.qxd 11/14/03 10:10 AM Page 627

630 ELECTRIC MACHINERY FUNDAMENTALS

The machine has the magnetization curve shown in Figure P9–7. Its equivalent cir-

cuit is shown in Figure P9–10. Answer the following questions about this machine,

assuming no armature reaction.

(a) If the generator is operating at no load, what is its terminal voltage?

(b) If the generator has an armature current of 20 A, what is its terminal voltage?

(c) If the generator has an armature current of 40 A, what is its terminal voltage?

(d) Calculate and plot the terminal characteristic of this machine.

9–28. If the machine described in Problem 9–27 is reconnected as a differentially com-

pounded dc generator, what will its terminal characteristic look like? Derive it in the

same fashion as in Problem 9–27.

9–29. A cumulatively compounded dc generator is operating properly as a flat-

compounded dc generator. The machine is then shut down, and its shunt field con-

nections are reversed.

(a) If this generator is turned in the same direction as before, will an output voltage

be built up at its terminals? Why or why not?

(b) Will the voltage build up for rotation in the opposite direction? Why or why

not?

(c) For the direction of rotation in which a voltage builds up, will the generator be

cumulatively or differentially compounded?

9–30. A three-phase synchronous machine is mechanically connected to a shunt dc ma-

chine, forming a motor–generator set, as shown in Figure P9–11. The dc machine is

connected to a dc power system supplying a constant 240 V, and the ac machine is

connected to a 480-V, 60-Hz infinite bus.

The dc machine has four poles and is rated at 50 kW and 240 V. It has a per-unit

armature resistance of 0.04. The ac machine has four poles and is Y-connected. It is

rated at 50 kVA, 480 V, and 0.8 PF, and its saturated synchronous reactance is 2.0 

per phase.

All losses except the dc machine’s armature resistance may be neglected in this

problem. Assume that the magnetization curves of both machines are linear.

(a) Initially, the ac machine is supplying 50 kVA at 0.8 PF lagging to the ac power

system.

RA + RS

Nse = 20 turns

EAVT

20

+

–

IF

IL

IA

LFNF = 1000

turns

RF

LS

Radj

+

–

0.21

FIGURE P9–10

The compounded dc generator in Problems 9–27 and 9–28.

cha65239_ch09.qxd 11/14/03 10:10 AM Page 630

680 ELECTRIC MACHINERY FUNDAMENTALS

(f) Pout

(g)



ind

(h)



load

(i) Efficiency

10–6. Find the induced torque in the motor in Problem 10–5 if it is operating at 5 percent

slip and its terminal voltage is (a) 190 V, (b) 208 V, (c) 230 V.

10–7. What type of motor would you select to perform each of the following jobs? Why?

(a) Vacuum cleaner

(b) Refrigerator

(c) Air conditioner compressor

(d) Air conditioner fan

(e) Variable-speed sewing machine

(f) Clock

(g) Electric drill

10–8. For a particular application, a three-phase stepper motor must be capable of step-

ping in 10°increments. How many poles must it have?

10–9. How many pulses per second must be supplied to the control unit of the motor in

Problem 10–8 to achieve a rotational speed of 600 r/min?

10–10. Construct a table showing step size versus number of poles for three-phase and

four-phase stepper motors.

REFERENCES

1. Fitzgerald, A. E., and C. Kingsley, Jr. Electric Machinery. New York: McGraw-Hill, 1952.

2. National Electrical Manufacturers Association. Motors and Generators, Publication No. MG1-

1993. Washington, D.C.: NEMA, 1993.

3. Veinott, G. C. Fractional and Subfractional Horsepower Electric Motors. New York: McGraw-

Hill, 1970.

4. Werninck, E. H. (ed.). Electric Motor Handbook. London: McGraw-Hill, 1978.

cha65239_ch10.qxd 11/14/03 12:41 PM Page 680

ISM For Electrical Machinery Fundamentals 4e (Instructor's Manual)

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