Machinedesign 1658 Motor Efficiency997 0 0

Getting more from motor tests

Sept. 1, 2000
When comparing and specifying motors, designers must be aware that efficiency values can be misleading because manufacturers often use different test standards.

The Energy Policy Act of 1992, scheduled to take effect this fall, is forcing designers to take a fresh look at the fundamentals of motor efficiency. Of particular importance are test methods and terminology. Without an understanding of these issues, designers will have a tough time comparing motors from different manufacturers.

The problem is that not all efficiency tests are the same. And because motor ratings can vary dramatically depending on the type of test and vocabulary used to describe the results, motors often end up looking better than they actually are.

What’s the difference

So far, there are at least six motor test methods used throughout the world. In North America, “IEEE 112-1996, Method B” (Institute of Electrical and Electronic Engineers) and “CSA C-390” (Canadian Standards Association) are the most common. Also used is “ANSI C50.20,” which is essentially the same as IEEE 112.

Elsewhere, in the Pacific Rim, motor makers tend to use “JEC 37” (Japanese Electrotechnical Committee), while in Europe, the standard method is “IEC 34- 2” (International Electrotechnical Commission). The only exception is in the U.K., where motor makers typically use “BS 269” (British Standard).

The primary difference among test methods is how they treat stray load losses. CSA and IEEE methods determine stray load loss by indirect measurement. IEC 34-2 and BS 269, on the other hand, assume stray load losses to be fixed at 0.5% of the input, while JEC 37 ignores stray losses altogether. By minimizing losses, the latter methods tend to inflate efficiency levels and are not as precise as IEEE 112 and CSA C-390.

To get a better feel for how test methods can affect motor ratings, consider the results of a study conducted by R.E. Osterlei of Gould Inc. Osterlei’s study, which appears in the Proceedings of the 7th National Conference on Power Transmission, tests several motors using three methods. For a given motor, the efficiencies measured using JEC and IEC techniques are much rosier that those obtained through IEEE tests.

Going the extra mile

One reason IEEE and CSA standards are more accurate than the others is because they use dynamometers to load the motors. Dynamometers are mechanical devices which, when connected to motor shafts, place motors under measurable and repeatable loads.

In addition to dynamometer tests, the IEEE standard also includes a dynamometer correction procedure, special techniques to measure and isolate the four main motor losses, indirect measurements for calculating stray load loss, and linear regression methods to smooth data and improve accuracy.

What’s more, version 112-B was recently updated to better comply with EPACT efficiency standards. One modification, for example, mandates higher instrumentation accuracy. According to the new release, test instruments must now be calibrated to ±0.2% fullscale error or less for EPACT testing, and no more than ±0.5% error for general testing.

Another modification clarifies stator and rotor loss measurements and related temperature correction techniques. In the previous standard these issues were somewhat vague, forcing users into a lot of unnecessary guesswork. For the most part, these and other changes — including an extensive reorganization to streamline tests — make the new standard simpler, more accurate, and less prone to error.

Seeing is believing

Despite the lack of harmony in motor tests around the world, designers can still rely on efficiency data appearing on NEMA (National Electrical Manufacturers Association) nameplates. These values are standard across all polyphase induction motors in the 1 to 200- hp range, and they are obtained and specified using common test methods and definitions.

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One measure of efficiency, defined by NEMA standard MG-1, is nominal efficiency. It is based on the average full-load efficiency as measured across a sample population of a particular motor. In most cases, the calculated average doesn’t show up on the nameplate. What appears is a value from a NEMA table closest to, but not exceeding, the average.

Another NEMA efficiency, minimum efficiency, is the value below which no motor in the specific model range will test. This value is selected based on a 20% variance in motor losses due to manufacturing tolerances.

More about EPACT

EPACT92 sets a minimum nominal fullload efficiency for 1 to 200-hp motors manufactured separately or as part of another piece of equipment. It applies to T-frame, single speed, foot-mounted polyphase squirrel-cage induction motors of NEMA design A and B, as well as continuous rated motors operating on 230/460-V and constant 60-Hz line power.

As of October 24, 1997, these motors must meet the nominal full-load efficiency values as specified in NEMA standard MG1- 12.55A.

Jeff is a motor systems engineer at Advanced Energy Corp., a nonprofit organization in Raleigh, N.C.

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