Variable–speed drives (VSDs) passed a milestone recently. As many facility executives know, VSDs reduce the amount of energy an electric motor uses by reducing the frequency of the electrical power delivered to the motor. With a chiller, for example, a VSD can slow the motor in part load conditions, reducing capacity to match the reduced load and saving energy in the process. Facility executives with long memories may be skeptical about VSDs. Older VSDs sometimes caused problems with harmonics, which can lead to overloaded conductors and transformers and overheated motors. But now VSDs with acceptable levels of total harmonic distortion are available. Just how mainstream are VSDs today? ASHRAE is proposing an addendum to standard 90.1 that all but calls for VSDs in water cooled chillers that will operate much of the time at part load. Although the proposal doesn’t specify VSDs, it creates tough new requirements for those chillers – requirements based on performance available in chillers with VSDs. For facility executives looking for energy-effficiency opportunities, the proposal is one more reason to consider VSDs in retrofit applications.