Power Electronics Thyristor Controlled Power for Electric Motors By G D Sims

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Power Electronics Thyristor Controlled Power for Electric Motors By G D Sims

Preface

The following pages are meant for those who wish to use thyristors. The
details of the physics of semiconductor materials or the design of thyristors
themselves are unnecessary here but a general description of the device may
help to avoid pitfalls during electric circuit design.
Thyristor is the internationally recognized name for a particular semiconductor
device. The name is derived from the Greek, the first part meaning
switch and the second part an association with the transistor family. It has a
trade name, viz. SCR (silicon controlled rectifier) and it got this name
principally because it is a silicon device and it is used as a rectifier which can
be controlled. As a controlled switch it forms a group together with the
electromagnetic relay, the thyratron and the mercury arc rectifier. The
advantages and disadvantages of the thyristor become apparent in the process
of describing the device and its range of application. However, the present
general interest, development and use of the thyristor, indicates that for many
cases its many advantages make it superior to other devices.
Control of rotating electric machines is a major interest of the author so
that in this book the applications of the thyristor are towards this end.
Thyristors are used so much in connection with the control of machines that
it is worthwhile to go into some details of both the electric drive to be
controlled and the possible thyristor control units.
One text cannot cover all aspects of power electronics in detail. The
important aspects of operation, protection and control are described carefully.
However, the manufacturers’ manuals must supplement this book, for
example, with respect to rating specifications.
Special mention is made of the thyristor’s place in technology. While much
emphasis is paid to the power circuits for the control of a.c. and d.c. drives,
the control circuits are not ignored. Care is taken in the practical and realistic
worked examples to provide the design details of the logic circuitry which
controls the behaviour of the electric drive.
Chapter 1 attempts to give an overall picture of electric drive control and
the part that power electronics plays is established. A brief description of
physical electronics in Chapter 2 establishes sufficient ‘feeling’ for the
thyristor. Then the operation of the thyristor as a device is described in detail.
The next three chapters deal exclusively with the thyristor as a part of the
many power circuits for the control of electric drives. Three of the most
conventional machines, d.c., induction and synchronous motors have been
chosen to illustrate the form of control. Special motors can also be controlled
by the same power electronic circuits. An appendix is used to record the
details of the logic circuitry so that there is no lengthy detraction from the
main theme in the text.
The book is suitable for degree and diploma courses embracing electrical
engineering as well as for practising electrical engineers in industry and
research.

Contents

Preface
1. Power Electronics and Rotating Electric Drives
1.1. Introduction
1.2. Power Electronics
1.2.1. The Thyristor
1.3. Rotating Electric Drives
1.3.1. The Direct Current Drive
1.3.2. The Alternating Current Drive
1.3.3. Choice of Drive and Control System
References and Bibliography
2. The Thyristor
2.1. Introduction
2.2. Semiconductors
2.2.1. The Diode
2.2.2. The Transistor
(a) Cut-off
(b) Linear region
( c) Saturation
2.2.3. The Thyristor
(a) The diode model of the thyristor
(b) The two transistor model of the thyristor
2.3. Thyristor Characteristics
2.3.1. The Thyristor Reverse Biased
2.3.2. The Thyristor Forward Biased and Blocking
2.3.3. The Thyristor Forward Biased and Conducting
(a) Light tum-on
(b) Gate tum-on
(c) Breakover voltage turn-on
(d) dv/dt turn-on
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viii CONTENTS
2.4. Thyristor Turn-Off 21
2.4.1. Ways of Turn-off 22
(a) Natural commutation 22
(b) Reverse bias turn-off 22
(c) Gate turn-off 22
2.4.2. Thyristor Turn-off Time 22
2.5. Thyristor Ratings 23
2.5.1. Voltage Ratings 23
2.5.2. Cu”ent Rating 24
2.5.3. Power Rating 25
(a) Load cu”ent forward conduction loss 25
(b) Forward leakage power loss 25
(c) Reverse leakage power loss and turn-off loss 25
(d) Gate power loss 25
(e) Turn-on loss 26
2.5.4. Intermittent Ratings 26
2.6. Thyristor Manufacture 26
2.7. Thyristors in Circuits 27
2. 7.1. Thyristors in Series 27
2.7.2. Thyristors in Parallel 28
2.7.3. Circuits to Turn on Thyrsitors 29
(a) Direct cu”ent firing signals 30
(b) Pulse firing signals 30
(c) Alternating cu”ent firing signals 31
2.7.4. Circuits to Turn-off Thyristors 33
(a) Self commutation by resonance 33
(b) Auxiliary resonant turn-off 34
(c) Parallel capacitance turn-off 35
(d) Series capacitance turn-off 36
2.8. Thyristor Protection Circuits 37
2.8.1. Overvoltage 37
2.8.2. Overcu”ent 37
2.8.3. Voltage Surges 38
2.9. Relative Merits of Thyristors 41
2.10. The BidirectionalTriode Thyristor (Triac) 41
2.11. Summary 45
Worked Examples 46
References and Bibliography 49
Problems 50
3. Induction Motor Control
3.1. Introduction 52
3.2. Induction Motor Starting 52
3.2.1. Thyristor Starting 55
CONTENTS ix
3.3. Induction Motor Speed Control 59
3.3.1. Thyristor Systems for Speed Control 60
(a) The alternating current switch 60
Worked example 64
(b) Inverters 65
(i) Inverter classification 67
(ii) A C1 inverter for a single-phase induction motor 68
Analysis of a C1 inverter with resistive
load 70
(iii) A class 4 inverter for a three-phase induction
motor 77
(c) Inverter commutation 81
(i) The McMu”ay inverter 81
Worked example 83
(ii) The McMurray-Bedford inverter 85
(iii) Auxiliary commutating supply 86
(d) Voltage proportional to frequency 87
(i) Variable ratio transformer 88
(ii) Variable voltage converter 88
(iii) Inverter voltage control 89
(e) Harmonic elimination 93
(i) Multiple pulse width control 93
(ii) Selected harmonic reduction 93
(iiI) Harmonic neutralization by wave synthesis 98
if) Appraisal of thyristor three-phase inverters 104
(g) Inverters in the induction motor rotor circuit 105
References 108
Problems 108
4. Direct Current Motor Control
4.1. Introduction
4.2. Starting Direct Current Motors
4.2.1. Thyristors and the Resistance Starter
4.2.2. Thyristor Starting without Resistance
4.3. Speed Control of Direct Current Motors
4.3.1. Thyristor Speed Control
4.3.2. Thyristor Controlled Rectifier Converters
(a) Single-phase half-wave converter
(b) Single-phase full-wave converter
Worked example
(c) Three-phase controlled converters
(d) Armature and field control
Worked example
(e) Converter voltage ripple
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x CONTENTS
4.3.3. Thyristor Voltage Choppers 139
(a) The Morgan chopper 140
(b) The Jones chopper 141
(c) The oscillation chopper 142
(i) Charging analysis 142
(ij) Commutation analysis 143
4.4. Position Control by Direct Current Motors 147
4.4.1. Thyristor Position Control 148
(a) Design study of a discontinuous servomechanism
for position control using thyristors 149
(i) Power circuit 151
(ij) Control circuit 153
Worked example 155
4.4.2. Alternative circuits 157
References and Bibliography 158
Problems 159
5. Synchronous Motor Control
5.1. Introduction 172
5.2. Synchronous Motor Starting 173
5.2.1. An1nverterforStarting 173
5.3. Speed Control 174
5.3.1. A Speed Control Problem 174
(a) A thyristor stepping motor 175
(b) A cycloconverter for low speeds 179
5.4. Synchronous Motor Excitation 182
5.4.1. Thyristor Automatic Excitation Control 183
(a) Brushless excitation protection during starting 185
5.5. A Synchronous or a Direct Current Motor? 186
References and Bibliography 189
Appendices
I. Logic Circuitry for Inverter Control 190
II. Logic Circuitry for Bidirectional Converter 196
III. Logic Circuitry for On-Off Servo 202
References 206
Additional Problems for Chapters One, Two, Three and Four 207
Index 217