Frequency inverter (also termed variable frequency drive (VFD), adjustable-frequency drive, variable-speed drive (VSD), AC drive, frequency converter, motor speed controller etc.) 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.

Over the last four decades, power electronics technology has reduced VFD cost and size and has improved performance through advances in semiconductor switching devices, drive topologies, simulation and control techniques, and control hardware and software.

VFDs are available in a number of different low- and medium-voltage AC-AC and DC-AC topologies.