Variable speed drives can be used in conjunction with supply and return fans and cooling tower fans, as well as with virtually any type of centrifugal pump. In addition, centrifugal chillers are a prime candidate. Speed control is considered primarily for its energy savings benefits. There are several variable speed drive systems that are available that have been used for control of fan or pump speed. These include hydroviscous, eddy current clutch, direct current, and variable frequency converters. This chapter highlights alternating curent (AC) variable frequency drive (VFD) applications. This chapter illustrates the basic concept where the AC power input is converted into direct current by means of either a controlled or an uncontrolled rectifier. VFDs are the most efficient means of capacity control for centrifugal fans and pumps. They control a motor’s speed electronically rather than mechanically.