A Variable Frequency drive is a device which provides AC power of varying frequency, generally used to control the speed of induction motors.
Variable frequency drives are widely used in ventilation systems for large buildings; variable-frequency motors on fans save energy by allowing the volume of air moved to match the system demand. They are also used on pumps, elevator, conveyor and machine tool drives. They are commonly used in data centers to vary the speed of fans, pumps, and chillers.
A variable frequency drive system usually consists of an AC motor, a controller and an operator interface. AC motor-driven applications that do not require full speed can save energy by controlling the motor with a variable speed drive. The motor will utilize only 25% as much energy at 63% speed than it will at 100% speed. With variable torque, energy cost savings can be significant, often paying for the price of variable frequency drives within a matter of months.