The latest industrial trend is to use AC drives for variable speed application. As already discussed, to vary the speed of an AC motor and at the same time retain its torque producing capability a power source is required. This power source has to provide variable voltage and frequency output in such a way that, in most of the operating area the V / f ratio is maintained constant. This can be achieved through an AC drive which gives variable frequency and variable voltage as out put by taking fixed voltage as input.

The principle involved in this technique is first to convert the fixed frequency, fixed voltage AC supply into a variable or constant DC voltage. This is then reconverted into the AC supply of desired frequency & amplitude. The criteria for the selection of AC inverter drive are essentially the same as for a DC variable speed drive. The latest developments in technology and successful development of electronic drives (AC drives) for cage motors have resulted in the following benefits:

	Availability of full load torque from standstill to full speed.
	Absence of torque fluctuations at low speed.
	Ability to hold a set speed, regardless of load torque variations.
	Ability to control the rate of increase & decrease of speed.
	Dynamic response.