Motors that are designed for fixed-speed operation are often used. Some types of single-phase motors or synchronous motors can be advantageous in some situations, but generally three-phase induction motors are preferred as the most economical. The AC electric motor used in a VFD system is usually a three-phase induction motor. VFDs are made in a number of different low- and medium-voltage AC-AC and DC-AC topologies.Ī variable-frequency drive is a device used in a drive system consisting of the following three main sub-systems: AC motor, main drive controller assembly, and drive/operator interface. 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. However, the global market penetration for all applications of VFDs is relatively small.

Systems using VFDs can be more efficient than those using throttling control of fluid flow, such as in systems with pumps and damper control for fans. An increasing number of end users are showing greater interest in electric drive systems due to more stringent emission standards and demand for increased reliability and better availability. VFDs are used in applications ranging from small appliances to large compressors. VFDs may also be known as 'AFDs' (adjustable-frequency drives), 'ASDs' (adjustable-speed drives), 'VSDs' (variable-speed drives), 'AC drives', 'micro drives', 'inverter drives' or, simply, 'drives'. A variable-frequency drive (VFD) is a type of motor drive used in electro-mechanical drive systems to control AC motor speed and torque by varying motor input frequency and, depending on topology, to control associated voltage or current variation.

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