A Variable-Frequency drive is a device that provides air-conditioning power of varying frequency, generally by controlling the speed of induction motors.

Variable-frequency drives are widely used in ventilation systems for large buildings. These drives are motors on the fans of AC systems that 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 for efficient energy management.

A variable-frequency drive system usually consists of an AC motor, controller and operator interface. A variable-speed drive can save energy adjusting motor-driven applications that do not require full speed. As an example, the motor will use only 25% as much energy at 63% speed as 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.