Variable-frequency invertor and its application to motor drives

Brushless a.c. machines have an inherent advantage compared with motors that have commutators, particularly in situations where the atmosphere is very dirty, where flameproof motors are required, or where high rotor speeds are needed. They can also be operated efficiently over their entire speed range. It is only recently, however, that the invertor-fed a.c. system has offered a serious challenge to alternative drive schemes for general industrial applications. This is most noticeable in the range from 1 to 20 kVA. Larger units are becoming increasingly competitive as full use is made of advances in power semiconductors and control techniques. Two methods of producing variable-frequency supplies from a static invertor are described, together with specific examples of equipment. The motorsupply requirements of the static supply are defined to allow the best invertor/motor combination to be produced. Applications are discussed which illustrate the unique features of the invertor fed induction motor