Variable Frequency Drive

A variable-frequency drive (VFD) is a system for controlling the rotational speed of an alternating current (AC) electric motor by controlling the frequency of the electrical power supplied to the motor. A variable frequency drive is a specific type of adjustable-speed drive. Variable-frequency drives are also known as adjustable-frequency drives (AFD), variable-speed drives (VSD), AC drives, microdrives or inverter drives. Since the voltage is varied along with frequency, these are sometimes also called VVVF (variable voltage variable frequency) drives. (wikipedia.com).

It is possible to vary the speed of a motor by changing frequency, pole pairs or slip. However, many of these techniques have some problems associated with them and the most popular method is that of varying frequency.