Variable speed drives (VSDs), also called variable frequency drives, are a valuable tool for the energy manager. Typically the VSD system consists of a three-phase AC induction motor and a variable frequency power supply. The variable frequency power supply uses solid state components to produce a pulse-width modulated current that varies the power and frequency supplied to the motor. This enables accurate control of the motor speed over a broad range. VSDs are used in connection with pump and fan applications to vary the pump or fan speed according to demand, often with large savings in energy use.