A VFD controls the frequency of the power supplying AC motors, such as those found in the compressors of chilled water cooling systems. It controls the speed of the motor, adjusting the input frequency and voltage. The motors on compressors are sized for the maximum capacity of the system. However, they need to run at only a fraction of this capacity for the majority of the time. The VFD will match the motor speed to the demands being placed on the system.