The synchronous speed of an induction motor is determined by the number of poles and the frequency, and is expressed as follows:
While the motor's actual running speed is just slightly lower than this, it is essentially fixed by the line frequency. However, significant energy cost reductions are available in fan and pump applications where the speed can be reduced for much of the operating time. For example, if a fan or pump operates at about half speed, the theoretical power required would be about 1/8th of the full speed power requirement. One way to achieve this is to use the two-speed motor options available from manufacturers. However, many pump and fan applications require greater speed variability and control. These wider speed control attributes are inherent in adjustable frequency drives. The recent rapid development in and lower costs of solid-state electronics has made them much more economical for variable speed induction motor applications.
