Chapter 15 AC Motor Speed Control

An important factor in industrial progress during the past five decades has been the increasing sophistication of factory automation which has improved productivity manyfold. Manufacturing lines typically involve a variety of variable speed motor drives which serve to power conveyor belts, robot arms, overhead cranes, steel process lines, paper mills, and plastic and fiber processing lines to name only a few. Prior to the 1950s all such applications required the use of a DC motor drive since AC motors were not capable of smoothly varying speed since they inherently operated synchronously or nearly synchronously with the frequency of electrical input. To a large extent, these applications are now serviced by what can be called general-purpose AC drives. In general, such AC drives often feature a cost advantage over their DC counterparts and, in addition , offer lower maintenance, smaller motor size, and improved reliability. However, the control flexibility available with these drives is limited and their application is, in the main, restricted to fan, pump, and compressor types of applications where the speed need be regulated only roughly and where transient response and low-speed performance are not critical. More demanding drives used in machine tools, spindles, high-speed elevators , dynamometers, mine winders, rolling mills, glass float lines, and the like have much more sophisticated requirements and must afford the flexibility to allow for regulation of a number of variables, such as speed, position, acceleration , and torque. Such high-performance applications typically require a high-speed holding accuracy better than 0.25%, a wide speed range of at least 20:1, and fast transient response, typically better than 50 rad/s, for the speed loop. Until recently, such drives were almost exclusively the domain of DC motors combined with various configurations of AC-to-DC converters depending upon the application. With suitable control, however, induction motor drives