Mechanical Drives - Electric Motors

The electric motor is far and away the most common chiller compressor drive. Most of these are fixed speed motors (typically 1,800 or 3,600 rpm). Since compressor power requirements are proportional to the difference between evaporator and condenser pressures and refrigerant flow requirements, motor loads vary accordingly. Load variations are handled by cylinder unloading or multiple compressor staging for reciprocating units, slide vane capacity control in screw compressors, and inlet guide vanes (and infrequently hot gas bypass) for centrifugal compressors.

In cases where the ability to change compressor speed may offer a better way to modulate compressor capacity and/or performance, a variable speed electric motor should be considered. This approach is seldom utilized in new chiller installations since chiller manufacturers can now build in excellent modulation control. Variable speed motors have been more often used in retrofit applications. One word of caution: always consult the chiller manufacturer for warranty and performance verification before accepting the claims of anyone wishing to modify an existing chiller in this way.