Control Of Induction Motors
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PREFACE
More than half of the total electrical energy produced in developed coun-tries 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 nonlinear-ity 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 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 induc- tion machine is introduced next, as a base for presentation of more ad- vanced control concepts.
Principles of the field orientation, a fundamental idea behind high-performance, vector controlled drives, are then eluci-dated. The book also shows in detail another common approach to induc-tion motor control, the direct torque and flux control, and use of inductionmotors 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 dif-ferent 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 repre-sent 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.
INDUCTION MOTORS
Three-phase induction motors are so common in industry that in many plants no other type of electric machine can be found. The author remem- bers his conversation with a maintenance supervisor in a manufacturing facility who, when asked what types of motors they had on the factory floor, replied: ‘‘Electric motors, of course. What else?’’ As it turned out, all the motors, hundreds of them, were of the induction, squirrel-cage type. This simple and robust machine, an ingenious invention of the late nineteenth century, still maintains its unmatched popularity in industrial practice.
Induction motors employ a simple but clever scheme of electrome- chanical energy conversion. In the squirrel-cage motors, which constitute a vast majority of induction machines, the rotor is inaccessible. No moving contacts, such as the commutator and brushes in dc machines or slip rings and brushes in ac synchronous motors and generators, are needed.
This arrangement greatly increases reliability of induction motors and elimi- nates the danger of sparking, permitting squirrel-cage machines to be safely used in harsh environments, even in an explosive atmosphere. An additional degree of ruggedness is provided by the lack of wiring in the rotor, whose winding consists of uninsulated metal bars forming the ‘‘squirrel cage’’ that gives the name to the motor.
Such a robust rotor can run at high speeds and withstand heavy mechanical and electrical over- loads. In adjustable-speed drives (ASDs), the low electric time constant speeds up the dynamic response to control commands. Typically, induction motors have a significant torque reserve and a low dependence of speed on the load torque.
The less common wound-rotor induction motors are used in special applications, in which the existence and accessibility of the rotor winding is an advantage. The winding can be reached via brushes on the stator that ride atop slip rings on the rotor. In the simplest case, adjustable resistors (rheostats) are connected to the winding during startup of the drive system to reduce the motor current.
Terminals of the winding are shorted when the motor has reached the operating speed. In the more complicated so-called cascade systems, excess electric power is drawn from the rotor, conditioned, and returned to the supply line, allowing speed control. A price is paid for the extra possibilities offered by wound- rotor motors, as they are more expensive and less reliable than their squirrel-cage counterparts.
In today’s industry, wound-rotor motors are increasingly rare, having been phased out by controlled drives with squirrel-cage motors. Therefore, only the latter motors will be considered in this book.
Although operating principles of induction motors have remained unchanged, significant technological progress has been made over the years, particularly in the last few decades. In comparison with their ances- tors, today’s motors are smaller, lighter, more reliable, and more efficient.
The so-called high-efficiency motors, in which reduced-resistance wind-ings and low-loss ferromagnetic materials result in tangible savings of consumed energy, are widely available. High-efficiency motors are some- what more expensive than standard machines, but in most applications the simple payback period is short. Conservatively, the average life span of an induction motor can be assumed to be about 12 years (althoughproperly maintained motors can work for decades), so replacement of a worn standard motor with a high-efficiency one that would pay off for its higher price in, for instance, 2 years, is a simple matter of common sense.
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