An AC motor drive is a unit that controls the rotational speed of an AC induction motor by controlling the frequency of the electrical power supplied to the motor. They are also called adjustable speed drives (ASD), variable frequency drives (VFD) or variable voltage variable frequency drives (VVVF). They are used, for example, in factories to control process motors, pumps, machine tools, and also fans in HVAC systems. They are becoming more prevalent also in household appliances as the demand for energy efficiency increases.