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 VFD is a specific type of adjustable-speed drive and is also known by many other names such as the adjustable-frequency drives (AFD), variable-speed drive (VSD), AC drive, micro drive or the inverter drive. Since the voltage is varied along with frequency, these drives are also sometimes called as the VVVF (variable voltage variable frequency) drives.

VFDs are widely used in ventilation systems for large buildings, conveyor belt systems, escalators, pumps and even on the baggage handlng systems in airports. An example of the energy saving advantages of using a VFD - variable-frequency motors on fans save energy by allowing the volume of air moved to match the system demand in real time. They are also used on machine tool drives and have become increasingly an unavoidable component in any well designed energy-efficient automation system.