IET Power and Energy Series 8
David Finney, B.Sc., CEng., FIEE, is division manager and chief engineer, responsible for large variable speed drive systems, at the G.E.C. Industrial Controls plant in Rugby, England. In this position he is responsible for the development, design and manufacture of large drive systems for use in mining, metals, paper, oil, and chemical industries throughout the world. He has been technically involved in the power semiconductor field since 1958, when thyristors were only just emerging, and during this time he has worked on all types of thyristor converters and inverter drives from a few kilowatts up to 10,000 kW using natural and forced commutation techniques and operating in square wave and pulse modulated modes. He has published a number of articles and given lectures around the world in his chosen subject.
Va
ria
ble
F
re
qu
en
cy
AC
M
oto
r D
riv
e S
ys
te
m
s
Fin
ne
y
Variable Frequency
AC Motor Drive
Systems
David Finney
Variable Frequency
AC Motor Drive Systems
This book is intended to explain the technical principles involved in the many AC variable speed drive systems available today. It deals with all the DC link inverter and direct AC to AC converter systems that are in commercial use. The principles of AC motors are considered specifically from the variable frequency point of view, and this chapter concentrates on the effects of harmonics. The different types of power semiconductor switches are considered separately from the drive systems in which they are used. A total of seven separate and technically different drive systems are considered in such a way that their principles can be fully understood and their performance capabilities explained. Square wave and pulse width modulated DC link inverter systems, cycloconverters and slip power recovery drives are all included in this comprehensive book.
This book has been written so that it can be understood by general engineers, not just by experts in the field. It should therefore be of great use to any engineer involved with variable speed drives in any capacity. It should also be of interest to university and college electrical engineering departments and students.
The Institution of Engineering and Technology www.theiet.org
0 86341 114 2 978-0-86341-114-4
IET PowEr and EnErgy SErIES 8
Series Editors: Prof. A.T. Johns G. Ratcliff J.R. Platts
Variable Frequency
AC Motor Drive
Other volumes in this series:
Volume 1 Power circuit breaker theory and design C.H. Flurscheim (Editor)
Volume 4 Industrial microwave heating A.C. Metaxas and R.J. Meredith
Volume 7 Insulators for high voltages J.S.T. Looms
Volume 8 Variable frequency AC motor drive systems D. Finney
Volume 10 SF6 switchgear H.M. Ryan and G.R. Jones
Volume 11 Conduction and induction heating E.J. Davies
Volume 13 Statistical techniques for high voltage engineering W. Hauschild and
W. Mosch
Volume 14 Uninterruptable power supplies J. Platts and J.D. St Aubyn (Editors)
Volume 15 Digital protection for power systems A.T. Johns and S.K. Salman
Volume 16 Electricity economics and planning T.W. Berrie
Volume 18 Vacuum switchgear A. Greenwood
Volume 19 Electrical safety: a guide to causes and prevention of hazards
J. Maxwell Adams
Volume 21 Electricity distribution network design, 2nd edition E. Lakervi and
E.J. Holmes
Volume 22 Artificial intelligence techniques in power systems K. Warwick, A.O. Ekwue
and R. Aggarwal (Editors)
Volume 24 Power system commissioning and maintenance practice K. Harker
Volume 25 Engineers’ handbook of industrial microwave heating R.J. Meredith
Volume 26 Small electric motors H. Moczala et al.
Volume 27 AC-DC power system analysis J. Arrill and B.C. Smith
Volume 29 High voltage direct current transmission, 2nd edition J. Arrillaga
Volume 30 Flexible AC Transmission Systems (FACTS) Y-H. Song (Editor)
Volume 31 Embedded generation N. Jenkins et al.
Volume 32 High voltage engineering and testing, 2nd edition H.M. Ryan (Editor)
Volume 33 Overvoltage protection of low-voltage systems, revised edition P. Hasse
Volume 34 The lightning flash V. Cooray
Volume 35 Control techniques drives and controls handbook W. Drury (Editor)
Volume 36 Voltage quality in electrical power systems J. Schlabbach et al.
Volume 37 Electrical steels for rotating machines P. Beckley
Volume 38 The electric car: development and future of battery, hybrid and fuel-cell cars M. Westbrook
Volume 39 Power systems electromagnetic transients simulation J. Arrillaga and
N. Watson
Volume 40 Advances in high voltage engineering M. Haddad and D. Warne
Volume 41 Electrical operation of electrostatic precipitators K. Parker
Volume 43 Thermal power plant simulation and control D. Flynn
Volume 44 Economic evaluation of projects in the electricity supply industry H. Khatib
Volume 45 Propulsion systems for hybrid vehicles J. Miller
Volume 46 Distribution switchgear S. Stewart
Volume 47 Protection of electricity distribution networks, 2nd edition J. Gers and
E. Holmes
Volume 48 Wood pole overhead lines B. Wareing
Volume 49 Electric fuses, 3rd edition A. Wright and G. Newbery
Volume 51 Short circuit currents J. Schlabbach
Variable Frequency
AC Motor Drive
Systems
David Finney
Published by The Institution of Engineering and Technology, London, United Kingdom First edition © 1988 Peter Peregrinus Ltd
Reprint with new cover © 2006 The Institution of Engineering and Technology First published 1988
Reprinted 1991, 2006
This publication is copyright under the Berne Convention and the Universal Copyright Convention. All rights reserved. Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act, 1988, this publication may be reproduced, stored or transmitted, in any form or by any means, only with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms of licences issued by the Copyright Licensing Agency. Inquiries concerning reproduction outside those terms should be sent to the publishers at the undermentioned address:
The Institution of Engineering and Technology Michael Faraday House
Six Hills Way, Stevenage Herts, SG1 2AY, United Kingdom www.theiet.org
While the author and the publishers believe that the information and guidance given in this work are correct, all parties must rely upon their own skill and judgement when making use of them. Neither the author nor the publishers assume any liability to anyone for any loss or damage caused by any error or omission in the work, whether such error or omission is the result of negligence or any other cause. Any and all such liability is disclaimed.
The moral rights of the author to be identified as author of this work have been asserted by him in accordance with the Copyright, Designs and Patents Act 1988.
British Library Cataloguing in Publication Data Finney, David
Variable frequency AC motor drive systems.
1. Alternating current electric motors 2. Variable speed drives I. Title II. Series
621.46’2
ISBN (10 digit) 0 86341 114 2 ISBN (13 digit) 978-0-86341-114-4
Printed in the UK by Short Run Press Ltd, Exeter
Contents
Page
Preface ix
1 AC motors 1
1.1 Introduction 1 1.2 The induction motor 2 1.2.1 Induction motor principles 3 1.2.2 The variable frequency induction motor 9 1.2.3 The equivalent circuit 14 1.2.4 The vector diagram 20 1.2.5 Equations and relationships 21 1.2.6 Examples of calculations 26 1.3 The synchronous motor 32 1.3.1 Synchronous motor principles 33 1.3.2 Equivalent circuits and vector diagrams 36 1.3.3 Equations and relationships 40 1.3.4 Examples of calculations 42 1.4 Harmonics in AC motors 45 1.4.1 Harmonic power losses 46 1.4.2 Torque pulsations 46 1.4.3 Harmonic equivalent circuits 47 1.5 Motor power losses 49 1.6 Motor voltages to earth 52
2 Power switching devices 54
2.1 Introduction 54
2.2 The thyristor 5 5
2.2.1 Capabilities and performance 57 2.2.2 The available thyristors 62 2.2.3 Thyristors in AC motor drive circuits 65 2.3 The transistor 73 2.3.1 Capabilities and performance 75 2.3.2 The available transistors 83 2.3.3 Transistors in AC motor drive circuits 86
vi Contents
2.4 The gate turn off thyristor 91 2.4.1 Capabilities and performance 93 2.4.2 The available GTO thyristors 98 2.4.3 GTO's in AC motor drive circuits 100
3 Power switching circuits 104
3.1 Introduction 104 3.2 The 3 phase, naturally commutated bridge 104 3.2.1 As a rectifier 104 3.2.2 As an inverter — regeneration 111 3.2.3 Switch voltages 113 3.2.4 DC voltage harmonics 115 3.2.5 AC current harmonics 115 3.3 The three phase bridge inverter 119 3.3.1 The voltage source bridge inverter 120 3.3.2 The current source bridge inverter 124 3.4 Isolation of electronics 126
4 The six step voltage source inverter for induction motors 131
4.1 Introduction 131 4.2 Principles of operation 131 4.3 Detailed analysis of the system 135 4.3.1 Circuit waveforms 136 4.3.2 Relationships and equations 142 4.3.3 Examples of calculations 148 4.4 Practical circuit design considerations 151 4.4.1 Overcurrent protection 152 4.4.2 Overvoltage protection 154 4.4.3 Factors affecting specifications 154 4.4.4 Circuit variations 156 4.5 Overall control methods 158 4.5.1 Supply convertor control 159 4.5.2 Inverter control 159 4.5.3 Typical control schemes 160 4.6 Performance and application 161 4.6.1 Torque/speed characteristics 162 4.6.2 Speed control accuracy 164 4.6.3 Supply power factor and harmonics 164
5 The Pulse Width Modulated voltage source inverter for induction
motors 166
5.1 Introduction 166 5.2 Principles of operation 166 5.2.1 Pulse width modulation 167 5.2.2 The PWM drive system 174
Contents vii
5.3 Detailed analysis of the system 177 5.3.1 Motor waveforms 177 5.3.2 Inverter circuit waveforms 182 5.3.3 Circuit relationships and equations 185 5.3.4 Examples of calculations 188 5.4 Practical circuit design considerations 192 5.4.1 Overcurrent protection 192 5.4.2 Regeneration 193 5.4.3 Factors affecting specifications 194 5.4.4 Typical circuit diagram 194 5.5 Overall control methods 196 5.6 Performance and application 198 5.6.1 Torque/speed characteristics 199 5.6.2 Efficiency 199 5.6.3 Supply power factor 200 5.6.4 Motor and supply harmonics 200 5.6.5 Accuracy and transient performance 201
6 The six step current source inverter drive 202
6.1 Introduction 202 6.2 Principles of operation 203 6.3. Detailed analysis of the system 206 6.3.1 Circuit waveforms 207 6.3.2 The motor vector diagram 213 6.3.3 Circuit relationships and equations 215 6.3.4 The standard current source inverter circuit 219 6.3.5 Examples of calculations 223 6.4 Practical circuit design considerations 229 6.4.1 Overcurrent protection 229 6.4.2 Overvoltage protection 230 6.4.3 Circuit variations 230 6.4.4 Factors affecting specifications 231 6.5 Overall control methods 232 6.6 Performance and application 235 6.6.1 Torque/speed characteristics 235 6.6.2 Efficiency 236 6.6.3 Supply power factor 236 6.6.4 Torque pulsations 236
7 The six step synchro-convertor system for synchronous motors 239
7.1 Introduction 239 7.2 Principles of operation 241 7.2.1 Starting and low speed operation 242 7.2.2 Normal running conditions 245
viii Contents
123 Reversing and regeneration 247 7.2.4 Motor excitation 248 7.3 Detailed analysis of the system 249 7.3.1 Convertor and motor waveforms 249 7.3.2 Armature reaction 251 7.3.3 Motor vector diagram 252 7.3.4 Relationships and equations 254 7.3.5 Examples of calculations 257 7.4 Practical circuit design considerations 261 7.4.1 Overcurrent protection 262 7.4.2 Factors affecting specifications 263 7.4.3 Circuit variations 265 7.5 Overall control methods 266 7.5.1 Supply convertor control 268 7.5.2 Motor convertor control 268 7.5.3 Excitation control 269 7.6 Performance and application 269 7.6.1 Torque/speed characteristic 270 7.6.2 Efficiency 271 7.6.3 Speed control accuracy 272 7.6.4 Stability and transient performance 272 7.6.5 Supply power factor 273 7.6.6 Torque pulsations 274
8 The current source inverter for the capacitor self-excited induction
motor 275
8.1 Introduction 275 8.2 Principles of operation 275 8.2.1 High speed running 278 8.2.2 Lower speed running 279 8.3 Detailed analysis of the system 281 8.3.1 Circuit waveforms 284 8.3.2 The motor vector diagram 286 8.3.3 Relationships and equations 288 8.3.4 Examples of calculations 292 8.4 Practical circuit design considerations 295 8.4.1 Protection 295 8.4.2 Commutation methods 296 8.4.3 Factors affecting specifications 299 8.5 Overall control methods 301 8.5.1 Supply convertor control 302 8.5.2 Motor convertor control 302 8.5.3 Motor magnetisation control 303 8.5.4 Typical overall control scheme 303
Contents ix
8.6 Performance and application 305 8.6.1 Motor current waveforms 305 8.6.2 Torque/speed capability 306 8.6.3 Supply power factor 307
9 The cycloconvertor 308
9.1 Introduction 308 9.2 Principles of operation 309 9.2.1 The fundamental principles 309 9.2.2 3 phase systems 312 9.2.3 Reversal and regeneration 313 9.2.4 Supply side conditions 315 9.3 Detailed analysis of the system 318 9.3.1 Circuit waveforms 318 9.3.2 Current reversal . 325 9.3.3 The motor vector diagram 326 9.3.4 Relationships and equations 327 9.3.5 Examples of calculations 331 9.4 Practical circuit design considerations 332 9.4.1 Overcurrent protection 333 9.4.2 Convertor polarity switching 334 9.4.3 Alternative power circuits 335 9.5 Overall control methods 338 9.5.1 Firing control 339 9.5.2 Typical control schemes 340 9.6 Performance and application 343 9.6.1 Speed range 343 9.6.2 Dynamic performance 344 9.6.3 Supply power factor 344 9.6.4 Harmonics 346
10 The slip energy recovery system for wound rotor induction motors 349
10.1 Introduction 349 10.2 Principles of operation 350 10.3 Detailed analysis of the system " 354 10.3.1 Circuit waveforms 355 10.3.2 The motor equivalent circuit 357 10.3.3 The motor vector diagram 360 10.3.4 Circuit equations and relationships 360 10.3.5 Examples of calculations 364 10.4 Practical circuit designs 367 10.4.1 Overcurrent protection 368 10.4.2 Overvoltage protection 369 10.4.3 Circuit variations 370
x Contents
10.5 Overall control methods 370 10.6 Performance and application 373 10.6.1 Efficiency 373 10.6.2 Power factor 374 10.6.3 Torque capability 376 10.6.4 Harmonics in the system 377 Bibliography 380 Index 390
Preface
During recent years there has been a surge of interest in the subject of AC Variable Frequency Motor Drives and this has been mainly due to the many technical and financial benefits which can be derived from being able to vary the speed of a process. The plant can be operated under its optimum condition whatever its loading and in many cases considerable energy savings can be made compared to other drive arrangements.
During the same period there has also been considerable technical advance in the capabilities of such drive systems due mainly to the emergence of high quality semiconductor power switches and control microprocessors. This has caused the cost of these drive systems to reduce so that the overall economics of their application can be favourable in an increasing range of potential uses. In writing this book my aim has been to explain the technicalities of these drives in such a way that they can be understood by as wide a range of people as possible so as to encourage the increasing use of these systems.
It has not been written just for the technical expert in this area of drives but also for the people who will use, apply and maintain such systems as well as those who only have a general interest in the subject. I have also included information which will be of particular interest to the college and university departments dealing with power electronic equipment and I hope this book helps them widen the scope of their curriculum to include variable speed drives. The preparation of this book was greatly assisted by my developing a set of computer programmes designed to model the individual drive systems. As a result I have decided to complete the development of these programmes and to make them available to others.
These programmes model the steady state behaviour of the drive systems and using them it is possible to:
a) Model any drive, of any size, of any speed range operating at any voltage level.
b) Operate the computer as though it were the drive, using the keyboard to input your requirements and observing the drive operation on the screen.
xii Preface
c) Establish all the variable parameters of the drive under any condition of operation. All the supply convertor, motor convertor and motor currents, voltages and power factors, etc., are available at any operating speed and torque.
d) Observe the switching sequences of the power circuits while con-trolling the drive model from the computer keyboard.
e) Obtain printed graph plots of the variation of all the drive parameters from a printer connected to the computer.
f) Carry out experiments on the drive model under a variety of con-ditions, as though it was a set of laboratory equipment. It is possible to start with a simplified system, e.g. neglecting power losses, etc. and to gradually increase the system complexity until a full practical drive is being modelled and studied.
These programmes are a very important aid to the full understanding of these drive systems. Further details can be obtained from ORANGE ENTER-PRIZES, 20, BADBY ROAD, DAVENTRY, NORTHANTS. NN11 4AP, ENGLAND.
I would like to thank all my colleagues at G.E.C. Industrial Controls, Rugby, for the help they have given me, this book would not have been possible without their help, specifically I would like to thank Mr. David Martin for much expert advice. Special thanks are due to my wife, Lesley, for being patient during the many hours of writing and for the time she spent transferring my untidy handwriting into our word processor and hence into the typed manuscript.
Acknowledgement is also given to The General Electric Company of England and to G.E.C. Industrial Controls, Ltd, for permission to publish this book, the contents of which I learned while in their employ.
May I hope that all readers find this book interesting, informative and readable.
DAVID FINNEY DAVENTRY 1987
