Old electric motor drives work at one level: 100%. Variable Frequency Drives (VFDS) work at varying capacities proportional to the work required by the electric motor. An easy example to understand this principle is a fan in an HVAC system which delivers outside air to the inside of a building. Old drives could only operate the fan at 100% capacity. If 50% less air was needed, a damper inside the building would close half way. With variable speed drives, the fan can be operated at a lower speed and the damper is kept wide open. Many VFD applications can provide very attractive returns.