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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 ApplicationAny 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.
Likewise, motors with the same design letter have important similarities in output performance characteristics potentially critical to existing applications.
Electric motors most often operate at a load less than the rating on the motor nameplate. An electric motor operating at partial load will have a power factor generally lower than that indicated by the full-load power factor shown on the motor nameplate.
For instance, a 20-hp motor operating at 15 hp will likely have a lower power factor than a 15-hp motor operating at full load. The 20-hp motor will draw 33% more locked rotor current at starting and thereby potentially require a larger starter. The 20-hp motor will cost more as well. Therefore, selecting the correct motor for the load requirement most often dominates system costs and energy consumption considerations.
Parkinson is section manager, Integral Horsepower Motor Engineering, A.O. Smith Electrical Products Co., Lexington, TN. He can be reached at firstname.lastname@example.org (e-mail).
Publication date: 03/19/2001