Most motors operate at a nearly constant speed; however, much of the time the devices they drive may operate at less than the maximum design speed. A vfd controls the amount of electricity going to the motor, so energy is saved by matching the motor speed to the load.
In the hvac industry, vfd’s are usually found on motors that drive fans, blowers, pumps, cooling towers, etc. The Department of Energy reports that 64% of all electrical energy in the U.S. is consumed by electric motors. And you can bet that many of those electric motors are used in hvac applications.