The way we control the speed of fan motors or pump motors has tremendously changed in the past two decades with the introduction of variable-frequency drives (VFDs), aka variable-frequency controllers (VFCs). In the distant past, convincing an owner or a contractor to have VFDs control fan and pump motors required a detailed return-on-investment analysis. Today, thanks to their relatively low cost and to the code-mandated energy efficiency requirements of HVAC fan and pump systems, it would be unusual to not have VFDs control fan motors and pump motors. To ensure its safe operation, a VFD should also be listed and labeled by the Occupational Safety and Health Administration's Nationally Recognized Testing Laboratory (NRTL) as a complete unit, capable of providing self-protection and also capable of providing protection for the motor it controls.
When applied to the control of fans and pumps and when controlled properly by a building automation system (BAS), the VFDs have the potential of saving significant amounts of energy over the life of the motor.