A Variable Frequency Drive (VFD) — also called an AC drive, variable speed drive (VSD), adjustable frequency drive (AFD), inverter drive or just drive — is used to run an AC motor at variable speeds.

Without a VFD, a motor can only run at full speed; however, this is not necessary for most applications. VFDs are frequently used in data centers cooling systems because they can control the frequency of starting and stopping the motor as well as the speed at which it runs, which saves on energy and thus reduces costs. Another advantage is that if the motor only operates when it is needed, rather than constantly, the motor will last longer.