Electric Motors and Drives
Electric Motors and Drives
Fundamentals, Types and Applications
Third edition
Austin Hughes
Senior Fellow, School of Electronic and Electrical Engineering,
University of Leeds
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First edition 1990
Second edition 1993
Third edition 2006
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CONTENTS
Preface
xvi
1 ELECTRIC MOTORS
1
Introduction
1
Producing Rotation
2
Magnetic field and magnetic flux
3
Magnetic flux density
4
Force on a conductor
6
Magnetic Circuits
7
Magnetomotive force (MMF)
9
Electric circuit analogy
10
The air-gap
11
Reluctance and air-gap flux densities
12
Saturation
14
Magnetic circuits in motors
15
Torque Production
16
Magnitude of torque
18
The beauty of slotting
19
Specific Loadings and Specific Output
21
Specific loadings
21
Torque and motor volume
23
Specific output power – importance of speed
23
Energy Conversion – Motional EMF
25
Elementary motor – stationary conditions
26
Power relationships – conductor moving at
constant speed
28
Equivalent Circuit
30
Motoring condition
32
Behaviour with no mechanical load
32
Behaviour with a mechanical load
35
Relative magnitudes of V and E, and efficiency
37
Analysis of primitive motor – conclusions
38
General Properties of Electric Motors
39
Operating temperature and cooling
39
Torque per unit volume
40
Power per unit volume – importance of speed
41
Size effects – specific torque and efficiency
41
Efficiency and speed
41
Rated voltage
41
Short-term overload
42
Review Questions
42
2 POWER ELECTRONIC CONVERTERS FOR
MOTOR DRIVES
45
Introduction
45
General arrangement of drives
45
Voltage Control – D.C. Output from D.C. Supply
47
Switching control
48
Transistor chopper
49
Chopper with inductive load – overvoltage
protection
52
Features of power electronic converters
54
D.C. from A.C. – Controlled Rectification
55
The thyristor
55
Single-pulse rectifier
56
Single-phase fully controlled converter – output
voltage and control
57
3-phase fully controlled converter
62
Output voltage range
64
Firing circuits
64
A.C. from D.C. SP – SP Inversion
65
Single-phase inverter
65
Output voltage control
67
Sinusoidal PWM
68
3-phase inverter
69
vi
Contents
Forced and natural commutation – historical
perspective
69
Matrix converters
70
Inverter Switching Devices
72
Bipolar junction transistor (BJT)
72
Metal oxide semiconductor field effect
transistor (MOSFET)
73
Insulated gate bipolar transistor (IGBT)
74
Gate turn-off thyristor (GTO)
74
Converter Waveforms and Acoustic Noise
75
Cooling of Power Switching Devices
75
Thermal resistance
75
Arrangement of heatsinks and forced air cooling
77
Cooling fans
78
Review Questions
79
3 CONVENTIONAL D.C. MOTORS
82
Introduction
82
Torque Production
84
Function of the commutator
86
Operation of the commutator – interpoles
88
Motional E.M.F.
90
Equivalent circuit
94
D.C. motor – Steady-State Characteristics
95
No-load speed
95
Performance calculation – example
96
Behaviour when loaded
98
Base speed and field weakening
103
Armature reaction
105
Maximum output power
106
Transient Behaviour – Current Surges
107
Dynamic behaviour and time-constants
108
Shunt, Series and Compound Motors
111
Shunt motor – steady-state operating
characteristics
113
Series motor – steady-state operating
characteristics
115
Contents
vii
Universal motors
118
Compound motors
119
Four-Quadrant Operation and Regenerative Braking
119
Full speed regenerative reversal
122
Dynamic braking
124
Toy Motors
124
Review Questions
126
4 D.C. MOTOR DRIVES
133
Introduction
133
Thyristor D.C. Drives – General
134
Motor operation with converter supply
136
Motor current waveforms
136
Discontinuous current
139
Converter output impedance: overlap
141
Four-quadrant operation and inversion
143
Single-converter reversing drives
144
Double SP-converter reversing drives
146
Power factor and supply effects
146
Control Arrangements for D.C. Drives
148
Current control
150
Torque control
152
Speed control
152
Overall operating region
154
Armature voltage feedback and IR
compensation
155
Drives without current control
155
Chopper-Fed D.C. Motor Drives
155
Performance of chopper-fed d.c. motor drives
156
Torque–speed characteristics and
control arrangements
159
D.C. Servo Drives
159
Servo motors
160
Position control
162
Digitally Controlled Drives
163
Review Questions
164
viii
Contents
5 INDUCTION MOTORS – ROTATING FIELD,
SLIP AND TORQUE
167
Introduction
167
Outline of approach
168
The Rotating Magnetic Field
170
Production of rotating magnetic field
172
Field produced by each phase winding
172
Resultant field
176
Direction of rotation
177
Main (air-gap) flux and leakage flux
177
Magnitude of rotating flux wave
179
Excitation power and VA
182
Summary
183
Torque Production
183
Rotor construction
183
Slip
185
Rotor induced e.m.f., current and torque
185
Rotor currents and torque – small slip
187
Rotor currents and torque – large slip
189
Influence of Rotor Current on Flux
191
Reduction of flux by rotor current
192
Stator Current-Speed Characteristics
193
Review Questions
196
6 OPERATING CHARACTERISTICS OF
INDUCTION MOTORS
198
Methods of Starting Cage Motors
198
Direct Starting – Problems
198
Star/delta (wye/mesh) starter
202
Autotransformer starter
202
Resistance or reactance starter
203
Solid-state soft starting
204
Starting using a variable-frequency
inverter
206
Run-up and Stable Operating Regions
206
Harmonic effects – skewing
208
High inertia loads – overheating
209
Steady-state rotor losses and efficiency
209
Contents
ix
Steady-state stability – pullout torque
and stalling
210
Torque–Speed Curves – Influence of Rotor
Parameters
211
Cage rotor
211
Double cage rotors
213
Deep bar rotors
214
Starting and run-up of slipring motors
215
Influence of Supply Voltage on Torque–Speed Curve
217
Generating and Braking
218
Generating region – overhauling loads
219
Plug reversal and plug braking
220
Injection braking
221
Speed Control
221
Pole-changing motors
222
Voltage control of high-resistance cage motors
223
Speed control of wound-rotor motors
224
Power Factor Control and Energy Optimisation
225
Voltage control
225
Slip energy recovery (wound rotor motors)
227
Single-Phase Induction Motors
227
Principle of operation
227
Capacitor-run motors
229
Split-phase motors
230
Shaded-pole motors
231
Size Range
232
Scaling down – the excitation problem
232
Review Questions
233
7 INDUCTION MOTOR EQUIVALENT CIRCUIT
236
Introduction
236
Outline of approach
237
Similarity Between Induction Motor and Transformer
238
The Ideal Transformer
240
Ideal transformer – no-load condition,
flux and magnetising current
240
x
Contents
Ideal transformer – no-load condition,
voltage ratio
245
Ideal transformer on load
246
The Real Transformer
248
Real transformer – no-load condition,
flux and magnetising current
248
Real transformer – leakage reactance
251
Real transformer on load – exact
equivalent circuit
252
Real transformer – approximate
equivalent circuit
254
Measurement of parameters
256
Significance of equivalent circuit parameters
257
Development of the Induction Motor Equivalent Circuit
258
Stationary conditions
258
Modelling the electromechanical
energy conversion process
259
Properties of Induction Motors
261
Power balance
262
Torque
262
Performance Prediction – Example
263
Line current
264
Output power
264
Efficiency
265
Phasor diagram
266
Approximate Equivalent Circuits
267
Starting and full-load relationships
268
Dependence of pull out torque on
motor parameters
269
Analysis
270
Graphical interpretation via phasor diagram
271
Measurement of Parameters
274
Equivalent Circuit Under Variable-Frequency
Conditions
274
Review Questions
277
Contents
xi
8 INVERTER-FED INDUCTION MOTOR DRIVES
279
Introduction
279
Comparison with d.c. drive
280
Inverter waveforms
282
Steady-state operation – importance of
achieving full flux
284
Torque–Speed Characteristics – Constant
V/f Operation
286
Limitations imposed by the inverter – constant
power and constant torque regions
288
Limitations imposed by motor
289
Control Arrangements for Inverter-Fed Drives
290
Open-loop speed control
291
Closed-loop speed control
293
Vector (Field-Oriented) Control
296
Transient torque control
297
Cycloconverter Drives
300
Review Questions
303
9 STEPPING MOTORS
305
Introduction
305
Open-loop position control
306
Generation of step pulses and motor
response
307
High-speed running and ramping
308
Principle of Motor Operation
311
Variable reluctance motor
312
Hybrid motor
314
Summary
317
Motor Characteristics
318
Static torque–displacement curves
318
Single-stepping
319
Step position error and holding torque
320
Half stepping
321
Step division – mini-stepping
323
xii
Contents
Steady-State Characteristics – Ideal
(Constant-Current) Drive
324
Requirements of drive
324
Pull-out torque under constant-current
conditions
326
Drive Circuits and Pull-Out Torque–Speed Curves
328
Constant-voltage drive
328
Current-forced drive
330
Chopper drive
331
Resonances and instability
333
Transient Performance
335
Step response
335
Starting from rest
336
Optimum acceleration and
closed-loop control
337
Review Questions
338
10 SYNCHRONOUS, BRUSHLESS D.C. AND
SWITCHED RELUCTANCE DRIVES
340
Introduction
340
Synchronous Motors
341
Excited-rotor motors
343
Equivalent circuit of excited-rotor
synchronous motor
344
Phasor diagram and Power-factor control
347
Starting
349
Permanent magnet synchronous motors
350
Hysteresis motors
351
Reluctance motors
351
Controlled-Speed Synchronous Motor Drives
352
Open-loop inverter-fed synchronous
motor drives
353
Self-synchronous (closed-loop) operation
354
Operating characteristics and control
355
Brushless D.C. Motors
357
Contents
xiii
Switched Reluctance Motor Drives
358
Principle of operation
359
Torque prediction and control
360
Power converter and overall drive
characteristics
363
Review Questions
363
11 MOTOR/DRIVE SELECTION
366
Introduction
366
Power Range for Motors and Drives
366
Maximum speed and speed range
368
Load Requirements – Torque–Speed Characteristics
369
Constant-torque load
369
Inertia matching
374
Fan and pump loads
374
General Application Considerations
375
Regenerative operation and braking
375
Duty cycle and rating
376
Enclosures and cooling
377
Dimensional standards
378
Supply interaction and harmonics
378
Review Questions
379
APPENDIX – INTRODUCTION TO CLOSED–LOOP
CONTROL
381
Reasons for Adopting a Simplified Approach
381
Closed-Loop (Feedback) Systems
382
Error-activated feedback systems
383
Closed-loop systems
384
Steady-State Analysis of Closed-Loop Systems
386
Importance of Loop Gain – Example
390
Steady-State Error – Integral Control
392
PID Controller
394
xiv
Contents
Stability
396
Disturbance Rejection – Example Using D.C. Machine
397
Further Reading
400
Answers to Numerical Review Questions
401
Index
404
Contents
xv
PREFACE
Like its predecessors, the third edition of this book is intended primarily
for non-specialist users and students of electric motors and drives.
My original aim was to bridge the gap between specialist textbooks
(which are pitched at a level too academic for the average user) and
the more prosaic ‘handbooks’, which are full of useful detail but provide
little opportunity for the development of any real insight or understand-
ing. The fact that the second edition was reprinted ten times indicated
that there had indeed been a gap in the market, and that a third edition
would be worthwhile. It was also gratifying to learn that although the
original book was not intended as yet another undergraduate textbook,
teachers and students had welcomed the book as a gentle introduction to
the subject.
The aim throughout is to provide the reader with an understanding of
how each motor and drive system works, in the belief that it is only by
knowing what should happen that informed judgements and sound
comparisons can be made. Given that the book is aimed at readers
from a range of disciplines, introductory material on motors and
power electronics is clearly necessary, and this is presented in the first
two chapters. Many of these basic ideas crop up frequently throughout
the book, so unless the reader is well-versed in the fundamentals it
would be wise to absorb the first two chapters before tackling the later
material. In addition, an awareness of the basic ideas underlying
feedback and closed-loop control is necessary in order to follow the
sections dealing with drives, and this has now been provided as an
Appendix.
The book explores most of the widely used modern types of motors
and drives, including conventional and brushless d.c., induction motors
(mains and inverter-fed), stepping motors, synchronous motors (mains
and converter-fed) and reluctance motors. The d.c. motor drive and the
induction motor drive are given most importance, reflecting their dom-
inant position in terms of numbers. Understanding the d.c. drive is
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