An electronic device that controls the speed and direction of a motor by changing the voltage and frequency of the power to the motor.

(sometimes abbreviated VFD) is electronic equipment that allows an electric motor to be run at varying rotational speeds. VFDs are frequently used to start large three-phase AC synchronous motors. These motors cannot be started simply by applying line frequency mains power; the rapidly rotating magnetic field would be unable to overcome the inertia of the rotor (and any connected load). Using a VFD provides one possible means to start these motors: the VFD will start using a low frequency that the rotor can follow, ramping up the frequency as the rotor accelerates. And unlike other starting methods, the VFD also allows very efficient speed control once the motor is running (simnply by varying the ultimate frequency of the supplied power).