Induction motors are used, for example, to drive fans, blowers, pumps and compressors. It has been recognized that when these motors are operated at full speed they normally have excess capacity as compared to the load requirements, and this excess capacity is compounded when the load is variable. It has also been recognized that if the output of the motors could be adjusted to provide only the needed power, a significant reduction of energy usage would result. Hence, variable speed drives (VSD's) have been developed in the form of electronic devices which match motor speed to that required for a given application. A typical VSD rectifies incoming AC voltage and current into DC, then inverts the DC back to AC at a different voltage and frequency. The output voltage and frequency is determined by the actual power needs and is set automatically by a control system or by an operator.
Heretofore, VSD's have generally been so expensive that they have not been used extensively for energy savings. It has been reported that VSD's require the availability of highly trained maintenance personnel and shorten motor life.