This column features helpful information, innovative equipment, systems and applications utilities around the nation can use to save energy and improve service.

Replacing motor-generator sets for efficiency
By Gil McCoy, P.E.

Motor-generator sets are used in many older industrial processes that require variable speed operation. Industries may want to calculate the energy savings potential of replacing an existing MG set that drives a direct current motor.

To accurately determine the total efficiency of the MG set, measure the input volts, amps and power factor to the alternate current motor and then compare this to the measured input kW or direct current power delivered to the DC motor terminals. The efficiency of an older MG set is generally between 72 and 81 percent.

Two energy-efficient alternatives offer significant energy savings. One is to replace the MG set with a solid-state DC motor drive (an electronic converter). The efficiency of a solid-state DC motor drive is 96 to 97 percent.

The other alternative is to replace the MG set with a new NEMA Premium efficiency AC induction motor controlled by a pulse-width modulated adjustable speed drive. An AC motor drive would have an expected efficiency of 97 percent.

Because older equipment is often significantly oversized, don’t select a replacement for your DC motor based on an identical horsepower rating. Instead, measure the pressure and flow provided by the centrifugal equipment at maximum speed, then use the fan or pump curve to obtain actual motor power requirements. By working backward from the system requirements, you can maximize your energy savings.