Machine Design

Explicating Efficiency

Motors with high electrical efficiency are still relatively new products, and terminology regarding "energy-efficient" and "premium-efficiency" motors remains somewhat unclear.

Electric motors consume about 60% of all energy produced in the U.S. Thus, improving motor efficiencies would have a significant effect on overall energy consumption. In 1992, Congress enacted the Energy Policy Act (EPAct) which went into effect in October of 1997.

Specifically, according to the DOE, the act applies to "generalpurpose, T-frame, single-speed, foot-mounted, continuous-rated, polyphase, squirrel-cage, induction motors of NEMA design A and B. The subject motors are designed to operate on 230/460 V and 60 Hz, and have both open and closed enclosures. EPAct applies to six-pole (1,200 rpm), four-pole (1,800 rpm), and two-pole (3,600 rpm) open and closed motors rated 1 through 200 hp. EPAct does not apply to definite-purpose motors or special-purpose motors."

Energy-efficient motors take advantage of a number of improvements to reduce operating losses, but their most important physical difference is that they incorporate a larger cross section of copper in their windings than old standard-efficiency motors.

Resistive losses in a motor, which show up as wasted heat, are governed by the equation

P = I 2R

where P = power consumed in watts, I is the current drawn in amps, and R is the resistance of the windings in ohms.

A decrease in resistance results in a reduction in losses. Increasing the cross-sectional area of the copper windings reduces losses and makes the motor more efficient.

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