The need for variable speed of an ac induction motor arises in certain industrial applications and
this is often difficult to achieve because induction motors are of fixed speed. The available control techniques
such as variation of supply voltage, variation of number of poles, are characterized by low efficiency and high
maintenance cost. The improvement in power electronics technology through advancements in semi-conductor
electronic devices with increased reliability and reduced cost led to the development of modern solid-state
variable frequency motor drive which is an electronic device capable of controlling the speed of an induction
motor by varying the frequency with increased efficiency, reliability and low maintenance cost