These are employed to control the speed of AC motors and torque by altering the voltage and frequency of the input source of the motor. It can be also regarded as adjustable speed drives, inverter driver, variable speed drives.

As we know that the frequency requirement is different for different plant and machinery so to achieve this thing Variable Frequency Drives are installed at either the input of AC Motor side or after the inverter output so as to get the desired frequency of the source.