A VFD (Variable Frequency Drive), also referred to as a VSD (Variable Speed Drive), inverter or drive, is a piece of hardware that is used to control the speed of an AC Induction motor. This is achieved by the VFD controlling the Frequency output (Hz) (in conjunction with the Voltage (V), being fed into the motor. Since the Frequency (Hz) output is directly proportional to the motor shaft rotational speed (RPM (Revolutions per minute)), the VFD is used to control the motor speed.