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No Standards Yet for 'Small Motor' Efficiency

In both Europe and North America, full-load efficiencies of general-purpose three-phase induction motors rated from one to 500 hp (depending upon speed) are governed by industry standards, governmental regulations, or both. Smaller motors, however, are exempt. In the United States, the Energy Policy Act empowers the federal government to impose performance and test requirements on "small" motors if potential energy savings from higher efficiency appear significant. That has not happened.

That's because most small motors are either special purpose designs with performance requirements more important than higher efficiency, or are single-phase motors. Although some of the same design changes that raise polyphase motor efficiency (such as higher copper content) will do likewise for single-phase machines, the effects (and the testing needed to verify them) can differ among the numerous types of single-phase designs. Also, many such motors, like those in household appliances, operate only a few hours annually. Energy savings do not justify the investment in higher efficiency.

In the United States, the only single-phase motor test guide is IEEE 114, most recently re-published in 2001. That document supports an efficiency evaluation that is not equivalent to that in IEEE 112 for polyphase motor testing. A Canadian standard (CSA C747) has been published for small motors, both single-phase and polyphase. However, differences of opinion exist in North America as to the degree of efficiency measurement precision it provides. When efficiency is measured as the ratio of directly measured input and output powers, tolerances in those two measurements can produce large variation in the result. Defining efficiency as the ratio of measured output to the sum of output plus segregated losses tends to be significantly more accurate for larger motors. For either method, setting standard limits requires verifying the test facility's capability of producing consistent, precise results.

More important: the viewpoint of the National Electrical Manufacturers Association in the United States continues to be that the population of motors below one horsepower is too varied, too special in application, and too limited in operating hours to justify development of either design changes or new test procedures aimed at higher, more precisely verifiable efficiency.

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