A Variable Frequency Drive for AC motors, also known as AC drive, is an electronic device which receives energy from a fixed frequency fixed voltage AC source (either single phase or three phase) and converts it into a variable voltage (or current), variable frequency three phase supply. This controls and enables a three-phase motor to operate at multiple speeds. Hence it comes under the category of variable speed drives.

A motor drive is popularly known as a Variable Frequency Drive (VFD) or a Variable Speed Drive (VSD). Motor drive is a general term referring to the sub-system that controls speed of motor to be operated at multiple operating speeds as needed by the application. For a given application, ‘Drive’ is a portion of equipment that adjusts motor speed to change the flow of the pump or speed of a fan or controls the movement of an escalator.