Preface

PREFACE

More than half of the total electrical energy produced in developed countries is converted into mechanical energy in electric motors, freeing the society from the tedious burden of physical labor. Among many types of the motors, three-phase induction machines still enjoy the same unparalleled popularity as they did a century ago. At least 90% of industrial drive systems employ induction motors. Most of the motors are uncontrolled, but the share of adjustable speed induction motor drives fed from power electronic converters is steadily increasing, phasing out dc drives. It is estimated that more than 50 billion dollars could be saved annually by replacing all "dumb" motors with controlled ones. However, control of induction machines is a much more challenging task than control of dc motors. Two major difficulties are the necessity of providing adjustable-frequency voltage (dc motors are controlled by adjusting the magnitude of supply voltage) and the nonlinearity and complexity of analytical model of the motor, aggrandized by parameter uncertainty. As indicated by the title, this book is devoted to various aspects of control of induction motors. In contrast to the several existing monographs

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CONTROL OF INDUCTION MOTORS

on adjustable speed drives, a great effort was made to make the covered topics easy to understand by nonspecialists. Although primarily addressed to practicing engineers, the book may well be used as a graduate textbook or an auxiliary reference source in undergraduate courses on electrical machinery, power electronics, or electric drives. Beginning with a general background, the book describes construction and steady-state operation of induction motors and outlines basic issues in uncontrolled drives. Power electronic converters, especially pulse width modulated inverters, constitute an important part of adjustable speed drives. Therefore, a whole chapter has been devoted to them. The part of the book dealing with control topics begins with scalar control methods used in low-performance drive systems. The dynamic model of the induction machine is introduced next, as a base for presentation of more advanced control concepts. Principles of the field orientation, a fundamental idea behind high-performance, vector controlled drives, are then elucidated. The book also shows in detail another common approach to induction motor control, the direct torque and flux control, and use of induction motors in speed and position control systems is illustrated. Finally, the important topic of sensorless control is covered, including a brief review of the commercial drives available on today's market. Certain topics encountered in the literature on induction motor drives have been left out. The issue of control of this machine is so intellectually challenging that some researchers attempt approaches fundamentally different from the established methods. As of now, such ideas as feedback linearization or passivity based control have not yet found their way to practical ASDs. Time will show whether these theoretical concepts represent a sufficient degree of improvement over the existing techniques to enter the domain of commercial drives. Selected literature, a glossary of symbols, and an index complete the book. Easy-to-follow examples illustrate the presented ideas. Numerous figures facilitate understanding of the text. Each chapter begins with a short abstract and ends with a summary, following the three tenets of good teaching philosophy: (1) Tell what you are going to tell, (2) tell, and (3) then tell what you just told. I want to thank Professor J. David Irwin of Auburn University for the encouragement to undertake this serious writing endeavor. My wife, Dorota, and children, Bart and Nicole, receive my deep gratitude for their sustained support.