Introduction to AC Drives

Abstract

Abstract An important factor in worldwide industrial progress during the past several decades has been the increasing sophistication of factory automation. Manufacturing lines in an industrial plant typically involve one or more 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 a few. Prior to the 1950s all such applications required the use of a de motor drive since ac motors were not capable of true adjustable or smoothly varying speed since they inherently operated synchronously or nearly synchronously with the frequency of electrical input. The inherent disadvantages of de drives, however, have prompted continual attempts to find better solutions to the problem. To a large extent, applications which require only a gradual change in speed are now being replaced by what can be called general purpose ac drives. In general, such ac drives often feature a cost advantage over their de counterparts and, in addition, offer lower maintenance, smaller motor size, and improved reliability. However, the control flexibility available with these drives is very limited and their application is, in the main, restricted to fan, pump and compressor types of application where the speed need be regulated only roughly and where transient response and low speed performance are not critical.