LEXINGTON, TN — At the recent ASHRAE 2001 Winter Meeting, a forum focusing on the utilization of high-efficiency electric motors and their potential impact on overall system characteristics was conducted. Participants explored the suggestion that higher transformer losses might well offset the energy savings of the motor, primarily due to lower system power factors. As is most often the case, there is no substitute for an informed buying decision, and that applies to electric motors as well.

High efficiency in an electric motor does not necessarily equate to low power factor. It is true, however, that the use of lower loss electrical-grade steels, larger operating air gaps, and the tendency toward as much locked rotor (inrush) current as possible do dominate motor design decisions at the potential sacrifice of power factor. The designer can compensate for these tendencies through the use of more active material in the motor, achieving power factors at or near Pre-EPAct 92 levels.

Understand The Application

Any motor-buying decision, replacement or new, must be based upon an understanding of the application and what attributes the high-efficiency motor has. A replacement motor with the same rating and footprint as the original is not necessarily identical to it.

Some general-purpose high-efficiency motors are marketed as National Electrical Manufacturers Association (NEMA) Design A in lieu of the more traditional NEMA Design B. Design A motors are permitted to have higher locked rotor currents than Design B motors — sometimes as much as 30% more. The locked rotor code letter on the motor nameplate or the locked rotor current information on the motor data sheet will provide this information. Motors with the same locked rotor code letter will have locked rotor (inrush) currents within 10% of each other.