THYRISTOR CONTROL OF SLIP RING INDUCTION MOTORS
by
NITIN Kuml,a JAIN
DEPARTMENT OF ELECTRICAL ENGINEERING
THESIS SUBMITTED
IN FULFILMENT OF THE REQUIREMENTS
OF THE DEGREE OF
DOCTOR OF PHILOSOPHY
TO THE
INDIAN INSTITUTE OF TECHNOLOGY DELHI
API:IL 1983
CERTIFICATE
Certified that the dissertation entitled
"THYRISTOR CONTROL OF SLIP RING INDUCTION MOTORS"
which is being submitted by Nitin Kumar Jain in
fulfilment for the award of the Degree of Doctor
of Philosophy in Electrical Engineering of the
Indian Institute of Technology, New Delhi, is a
record of the student's own work carried out by him
under our joint supervision and guidance. The matter
embodied in this dissertation has not been submitted
for the award of any other Degree or Diploma.
(B.P. Si. 41)
April, 1983.
(R. Arockiasamy)
ii
ACKNOWLEDGEMENT
My sincere thanks are due to Prof. R. Arockiasamy
for his invaluable guidance and encouragement throughout
the course of this work. I am also greatly indebted to
Dr. B.P. Singh for his unstinted help, guidance and
encouragement in the later stages of the research work.
They have always listened to whatever I had to say and
have always found time for discussions inspite of their
busy schedules.
I am grateful to Mr. A.R. Agarwala for his
invaluable help in development of the trigger controller,
trouble shooting at every stage and for going through the
manuscript. Thanks are also due to Mr. R. Mulchandani
for his help in analysing the trigger control problem and
in the development of the input stage. I am thankful
to Dr. Krishna Kant for his help in the modelling and
analysis. Thanks are due to Mr. N.K. Sharda for stimulating discussions and help. These colleagues have imensely
helped in keeping an academic environment, which I enjoyed
very much.
I am thankful to my friend Lt. Comdr. A. Anand
for his inspiring company and editorial help. My sincere
thanks are due to Mr. Bhim Singh for final checking of
the stensils.
Sincere thanks are due to the staff of Electrical
Engineering Department Senior Machines Lab and to the
staff of IDDC Electrical/Electronics Lab for their willing
help and cooperation.
Many thanks are due to Ms. Suresh for typing the
draft of the thesis and to Mr. N.D. Arora for final typing.
Thanks to Ms. Shashi for making the figures.
My profound gratitude is due to my affectionate
mother-in-law and rememberance to my late father-in-law.
Their loveaffection and inspiration has always given
me courage in the difficult times.
Finally, I gratefully acknowledge the contribution
of my wife Veena for her constant encouragement. She has
always "been there" providing renewed inspiration in these
black moments when it all seemed futile.
1983
(NITIN KUMAR JAIN)
iv
ABSTRACT
This investigation deals with the development of
a variable speed drive using slip ring induction motor
(SRIM) incororating thyristor converter rotor control
scheme. Study of available literature in this area highlights
various problems associated with the realisation of this
scheme. The main bottleneck in realising a wide speed
range of SRIM operation has been the presence of slot
ripple in the rotor voltage particularly at high speeds.
These ripples are responsible for spuribus zero-crossing
determination leading to unreliable motor operation at
high speeds with the trigger control schemes reported so
far. Moreover, obtaining a speed independent linear
relationship between the trigger angle of the thyrlstors
and control voltage is also a problem. Another aspect
needing attention has been the reauirement of a suitable
method of analysis for predetermination of the performance
of the SRIM with thyristor converter rotor control.
Different methods of analysis of the induction motors
employing thyristorised controllers, reported so far, do
not prove to be quite adequate for the present case.
Furthermore, the use of hybrid rectifier for SRIM rotor
control, which offers certain economic advantages, has not
been fully exploited. Yet another bottleneck in realising
a variable speed SRIM drive using thyristor converter
rotor control has been a poor load-speed regulation of the
SRIM.
The investigations reported in this thesis make
an attempt towards the solution of the problems discussed
above. The thesis reports about the development of a
reliable trigger control scheme for operation of the
thyristors in the rotor circuit of a BRIM. The trigger
controller uses phase-locked loop technique and the ripple
in the rotor reference voltage is eliminated by designing
a suitable interface circuit.
Direct throe phase model of the BRIM is modified to
incorporate the rotor current switching constraints imposed
by the thyristor converter. The model thus obtained is
simulated on a digital computer topY036termine the steady
state performance of the SRIM. The analytical results
thus obtained are compared with the experimental results
to ascertain the validity of the model. Study of the system
performance under typical converter fault conditions is
also carried out.
vi
The performance of the SRIM with hybrid rectifier
rotor control is experimentally investigated and a comparative
study of the SRIM performance with these two rotor control
schemes is presented.
Closed loop operation of the SRIM thyristor converter
system with speed and speed-current feedback is studied to
obtain a better load-speed regulation and modified torquespeed characteristics of the SRIM. Small signal linearised
model of the system is developed for analytical studies. The
feedback system is found to be stable in both the cases.
It is shown that the thyristor converter rotor control
using the trigger controller developed earlier in the investigation, permits the realisation of a SRIM based vafiable
speed drive which compares favourably with the dc motors
in terms of cost, maintenance, and load-speed regulation.
CONTENTS
Page
Certificate
ii
cknowl edgements
Abstract:
List of Figures
List of Principle Symbols
Chapter I INTRODUCTION
1.1 INTRODUCTION
1
1.2 ST d'Emzur OF THE PROBLEM
11
1.3 OUTLINE OF VARIOUS CHAPTERS
12
Chapter II TRIGGER . CONTROL SCHEME
2.1 GENERAL
2.2TRIGGER REQUIREM..iNTS
2.3 PROPOSED SCHEME
2.3.1 Introduction
2.3.2 Input Stage
2.3.3. PLL Ramp Generator
17
18
21
21
24
25
2.3.4 Three: Phase Ramp Generator 29
2.3.5 Phase Control
and Output
Stage
31
2.3.6 Processing
of the Speed
Signal
34
xvi
2.4 TRIGGER CIRCUIT DESIGN
38
2.4.1 Input Stage
38
2.4.2 PLL Ramp Generator
42
2.4.3 Three Phase Ramp Generator 46
2.4.4 Phase Control and Output
Stage 47
2.4.5 Speed Signal Processing
Circuit48
2.5 PERFORMANCE OF THE TRIGGER CIRCUIT 51
Chapter III
2.6 DISCUSSION
54
2.7 CONCLUSION
58
STEADY STA'TE ANALYSIS OF SLIP.ING
INDUCTION MOTOR-THYRISTORCONVERTER
SYSTEM
3.1 INTRODUCTION 60
3.2 ANALYSIS OF INDUCTION MOIORTHYRISTOR CONVERTER SYSTEM
61
3.2.1 Induction Motor Model
61
3.2.2 Induction Motor-Thyristor
Converter System
71
3.2.3 Modes of Operation75
3.2.4 Three Thyristor Conduction 83
3.2.5 Two Thyristor Conduction87
3.2.6 Open Circuited Rotor91
xv ii
92
3.3 DIGITAL COMPUTER SIMULATION
105
3.4EXPLIMENT,JJ SEAT UP
107
3.5 RESuLTS
3.5.1 Torque-Speed Characteristics
108
3.5.2 Stator Current-Speed
Characteristics 110
3.5.3 Input Power Factor-Speed
112
Characteritics
3.5.4 Efficiency-Speed Characteristics
114
3.5,5 Stator and Rotor Current
Waveforms
116
3.5.6 Variation in Performance
Quantities withcK122
3.5.7 Discussion
131
3.6 PERFORMANCE 0i THE SYSTEM UNDER
SUSTAINED CONVERTER FAULT CONDITIONS
3.6;1 System Operation
134
3.6.2 Performance of the system 135
3.7 CONCLUSION
140
Chapter IV PERFORMANCE OF INDUCTION MOTOR-HYBRID
VECTIFIER SYSTEM
4..1 INTRODUCTION
143
4.2 SYSTEM OPERTION
144
4.3 EXPEIMENTAL RESULTS
153
4.3.1 Torque-Speed Characteristics 154
4.3.2 Stator Current-Speed
Characteristics
156
4.3.3 'Input Power Factor-Speed
Characteristics
158
41r3.4 Effici ency-Speed Characteristics
160
4.3.5 Stator and IZotorCurrent
Waveforms
160
4.3.6 Variation of the Performance
Quantities with044,
167
4.3.7 Discussion
4,4 CONCLUSION
171
172
Chapter VCLOSED LOOP OPEiTION OF INDUCTION MOTORTHYRISTOR CONVERTER SYSTEM
5.1 IN'T'RODUCTION
174
5.2 PRINCIPLE OF OPEi- TION
176
5.2.1 Speed Feedback
176
5.2.2 Speed-Current Feedback• 178
5.3 CLOSED LOOP SYSTEM MODELLING180
5.3.1 Trigger Controller 181
5.3.2 Induction Motor
185
5.3.3 Control Voltage Processor189
5.3.4 Current Signal Processor193
xix
5.4 ANALYSIS OF THE SPEED FEEDBACK SCHEME196
5.4.1 Closed Loop Transfer Function196 •
5.4.2Stability Analysis196
5.4.3Step Response197
5.4.4 Steady State Analysis203
5,5 RESULTS
5.5.1 Stability
203
204
5.5.2 Step Response205
5.5.3 Steady State Performance208
5.6 DESIGN ASPECTS
210
5,7 SPEED-CURRENT FEEDBACK SCHEME212
5.7.1 Closed Loop Transfer Function213
5.7.2 Steady State Analysis214
5.7.3 Experimental Results219
5.8 CONCLUSION
220
Chapter VI CONCLUSIONS AND SUGGESTIONS FOR FURTHER WORK
6.1 MAIN CONCLUSIONS
6.2
SUGGESTIONS FOR FURTHER WORK
222
229
REFERENCES
232
APPENDIX I:DIGITAL PHASE DETECTOR
242
All:;NDIX II:TEST MACHINE SPECIFICATIONS
249