Chapter 7 - Induction motors

Abstract

With the recent advances in microprocessor technology, it is now possible to develop commercially viable drives that allow alternating-current (a.c.) asynchronous induction motors to be controlled with the accuracy and the response times, which are necessary for servo applications. The importance of this development should not be underemphasised. Induction motors are perhaps the most rugged and best-understood motors that are presently available. Alternating-current asynchronous motors are considered to be the universal machine of the manufacturing industry. It has been estimated that they are used in 70 to 80% of all industrial drive applications, although the majority are in fixed-speed applications such as pump or fan drives. The main advantages of induction motors are their simple and rugged structure, their simple maintenance, and their economy of operation. In order to achieve the performance required by servo applications, induction motors have to be controlled using vector controllers. There are certain key features that differentiate between scalar and vector control. Vector control is designed to operate with a standard a.c., squirrel-cage, asynchronous induction motor of known characteristics. A vector controller and its associated induction motor form an integrated drive; the drive and the motor have to be matched to achieve satisfactory operation. A vector-controlled induction motor and drive is capable of control in all four quadrants through zero speed, without any discontinuity. The vector-controlled-induction-motor's supply currents are controlled, both in magnitude and phase in real time, in response to the demand and to external disturbances.