A variable-frequency drive (VFD) or adjustable-frequency drive (AFD), variable-voltage/variable-frequency drive, variable speed drive (VSD), AC drive, microdrive or inverter drive is a type of adjustable-speed drive used in electro-mechanical drive systems to control AC motor speed and torque by varying motor input frequency and voltage. The basic principle is that it changes the frequency gradually to full load.

VFD in automation system :