ELECTRIC MOTOR DRIVES Prof. M.S. BERDE Retd. Prof. Department of Electrical Engineering N.I.T. (formerly, MACT) Bhopal (MP) KP KHANNA PUBLISHERS 4575/15, ONKAR HOUSE, OPP. HAPPY SCHOOL DARYAGANJ, NEW DELHI-110002 Phone : 2324 30 42 ; Fax : 2324 30 43 e-mail : khannapublishers@yahoo.in Website : www.khannapublishers.in
Published by : Romesh Chander Khanna for KHANNA PUBLISHERS Nai Sarak, Delhi-110006 (India). This book or part thereof cannot be translated or reproduced in any form (except for review or criticism) without the written permission of the Authors and the Publishers.] ISBN NO. : 978-81-7409-312-7 Third Edition : 2012 Price : ` 165.00 Text Composition by : Excellent Graphics, Delhi. Printed at : Bright Printers, Turkman Gate, Delhi-110006.
Preface Electric motor drive is an evergreen subject and the variables are the type of motor, use of control method and the type of supply. Particularly, due to the use of power electronics and microelectronics, each application is unique in itself and challenging in nature. The objective of this book is to explain the various types of motors alongwith their characteristics and use of control circuits that may be applicable and methods of starting, stopping and calculating their power ratings. Chapters on preventive maintenance of motors & allied equipment and industrial applications are also given in the book. To illustrate the use of concept, formulae ; solved problems and university questions are also included with text. The recent advances in technical education require additionally special knowledge to educate the student to become a specialist. From this point of view, two more chapters have been added one on Methods of Protection of Motors and the other on Energy Saving as applied to motor loads. Chapter 4 is revised to explain in detail the starting methods of Induction Motors with special reference to control circuits. In fact mostly all the chapters have been revised and as per comments received many new concepts and refinements have been made throughout the book. Chapters on applications of electric motors and a separate chapter on worked examples, objective type of questions will benefit the students at large. It is hoped that the book will be useful for a course on Electric Motor Drives. ( iii )
( iv ) I am thankful to M/s Telemecanique for the information on starters. Special efforts have been made to use relevant standards ISS, BSS, ISO, IEC etc. My thanks are mainly due to our U.G. and P.G. students as this book has mainly evolved in the teaching of the subject and also due to my colleagues whom I involved in discussions from time to time. Prof. M.S. Berde A-1/30, Girijashankar, Karvenagar, Pune-411052
Contents Chap. Pages 1. Introduction to Electric Drives 1 22 1.1 Prologue... 1 1.2 Historical Background... 2 1.3 Drive Systems... 3 1.4 Choice of Drive System... 7 1.5 Selection of Drive for Individual Application... 8 1.6 Control Panel Requirements... 9 1.7 Motor Construction and Trends... 16 Questions... 21 2. Dynanics of Electrical Drives 23 47 2.1 Torque Production in Electrical Machines... 23 2.2 Mechanical System... 25 2.3 Choice of Gear Ratio... 29 2.4 Torque-Speed Characteristics of Industrial Loads... 31 2.5 Torque-Speed Characteristics of Electric Motors... 33 2.6 Traction or Transportation Drive... 37 2.7 Mechanics of Drives... 40 Questions... 47 3. D.C. Motor Characteristics, Control and Drives 48 101 3.1 Introduction... 48 3.2 Separately Excited D.C. Motor... 49 3.3 Armature Resistance Control... 51 3.4 Field Control of Separately Excited D.C. Motor... 52 3.5 Dual Control of Separately Excited D.C. Motor... 53 3.6 Series Wound D.C. Motor... 55 3.7 Transfer Function of a Separately Excited D.C. Motor... 56 3.8 D.C. Motor Feedback Control System... 62 3.9 Braking Methods for D.C. Motors... 71 3.10 SCR Converters... 74 ( v )
Chap. ( v ) Pages 3.11 D.C. Motors for Operation on Converters... 91 Questions... 100 4. Three Phase Induction Motors Characteristics, Control and Drives 102 204 4.1 Introduction... 102 4.2 Induction Motor Characteristics at Constant Frequency Supply... 104 4.3 Starting Methods for 3-Phase Induction Motors... 108 4.4. Variable Frequency Static Converters for Induction Motor Control... 136 4.5. Induction Motor Control at Power Frequency... 143 4.6. Variable Frequency Operation of Induction Motors... 149 4.7 Testing Methods for Induction Motors No Load and Blocked Rotor Tests... 156 4.8 Bee Test... 161 4.9 Method-2: Estimation of Motor Efficiency at Operating Load... 171 4.10 Method-2: Estimation of Motor Efficiency at Operating Load... 174 Questions... 204 5. Synchronous Motor Characteristics, Control and Drives 205 243 5.1 Introduction... 205 5.2 Excitation Methods for Synchronous Motors... 211 5.3 Speed Control of Synchronous Motors... 217 5.4 Reluctance Motor... 221 5.5 Permanent Magnet Synchronous Motors... 222 5.6 Stepper Motors... 222 5.7 Drive Circuits for Stepper Motors... 232 5.8 Servomotors and their Control... 237 6. Motor Power Ratings 244 256 6.1 Introduction... 244 6.2 Motor Loading and Standard Duty Classes... 245 6.3 Motor Heating and Cooling Under Continuous Duty... 247 6.4 Determination of Motor Ratings... 250
Chap. ( vii ) Pages 6.5 Short Time Rating of Motor... 252 6.6 Load Equalisation... 253 7. Selection of Motors 257 282 7.1 Introduction... 257 7.2 Squirrel-Cage Motors... 258 7.3 Wound Rotor Motors... 260 7.4 Applications of Induction Motors... 261 7.5 Fractional Horsepower Motors... 263 7.6 Single Phase Squirrel-Cage Motors... 264 7.7 Single-phase Wound-Rotor Motors... 266 7.8 Schrage Motor or Brush-Shifting Polyphase Shunt Motor... 268 7.9 Use of Standards in Choice of Drives... 269 7.9 Motor Terminology... 277 8. Preventive Maintenance of Electric Motors and Allied Equipment 283 313 8.1 Introduction... 283 8.2 Insulation Care... 285 8.3 Evaluation of Insulation... 286 8.4 Maintenance of Electrical Insulation... 290 8.5 Maintenance of Commutators, Slip Rings and Brushes... 293 8.6 Bearing Maintenance... 297 8.7 Maintenance of Transformers... 307 8.8 Maintenance of Contactors... 308 Questions... 312 9. Industrial Applications of Electric Motors 314 341 9.1 Introduction... 314 9.2 Steel Industry... 314 9.3 Textile Industry... 319 9.4 Control Systems for Elevators... 323 9.5 Mining... 325 9.6 Paper Industry... 329 9.7 Cement Mills... 331 9.8 Machine Tools... 335 9.9 Marine Applications... 339 9.10 Oil and Gas Applications... 339
Chap. ( viii ) Pages 9.11 Chemical, Petrochemical, Rubber Industries Applications... 340 9.12 Material Handling... 340 9.13 Food and Agricultural Applications... 341 10. Protection of A.C. Motors 342 362 10.1 Introduction... 342 10.2 Modes of Failures... 345 10.3 Protection Against Mechanical Faults... 346 10.4 Overload Protection... 349 10.5 Heating of Motor Windings... 352 10.6 Undervoltage Protection... 356 10.7 Unbalance Protection... 356 10.8 Overvoltage Protection... 357 10.9 Ground Fault Protection... 357 10.10 Differential Protection... 358 10.11 Stator RTD Protection... 359 10.12 Loss of Load Protection... 360 10.13 Negative Phase Sequence Protection... 361 11. Energy Conservation in Motor Drives 363 378 11.1 Introduction... 363 11.2 Running Costs... 363 11.3 Load Matching with the Motor... 364 11.4 Idle Running... 364 11.5 Drive Transmission... 365 11.6 Driven Equipment... 365 11.7 High Efficiency Motors... 365 11.8 Guidelines for Identifying Energy Saving Opportunities... 366 11.9 Reduced Voltage Operation... 370 11.10 Energy Saving with Fans... 370 11.11 Flow Control in Pumps... 374 12. Worked Problems on Electric Drives 379 411 Index 413 418
CHAPTER 1 Introduction to Electric Drives 1.1 PROLOGUE Every machine that is used in a modern factory, manufacturing unit or transportation unit consists of three parts: (a) Prime mover i.e., motor or engine, (b) transmission system such as belt, gears etc., and (c) proper machine. The prime mover with its system of control and transmission system is called as drive. Since the most convenient form of power that is available for use in home or industry or farm is electrical, in most places we have electric drive only and is used for converting electric power into mechanical power. Electric drive is therefore defined as: a form of machine equipment designed to convert electric energy into mechanical energy and provide electric control of this process. In view of this, fundamental elements of an electric drive are the electric motor, transmission system and electrical control. Since there is a variety of motors, a converting equipment that changes the characteristics of the electric supply, such as form (a.c., d.c. or chopped a.c. or d.c. and frequency and voltage) are changed to match the motor performance to the required load and is also a part of the drive. The main function of the electric drive is to impart motion to the working machine but a modern electric drive with complexity of feedback control can perform several other functions so that the production process is performed in the most efficient manner with high productivity and high quality of the drive. Historically, the development of different kinds of electric drives used in industry may be divided into three stages: (a) Group Drives, (b) Individual Drives, and (c) Multimotor Drives. 1
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