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How consistent is motor efficiency?

Worldwide, governmental agencies and associations of electrical manufacturers and users now require full-load efficiencies that must be met throughout various ranges of standard a-c motors. Actions taken by the International Electrotechnical Commission and the U.S. Department of Energy have been well publicized.

Developing these requirements has been largely centered on the accuracy of efficiency tests. Standards have been revised to support more precise evaluation of stray load loss, for example. The accuracy of instrumentation and the need for periodic calibration have been stressed. Every effort has been made to minimize the variations that arise in testing a large number of machines.

However, far less attention has been paid to inherent efficiency differences from one motor to the next. These include: 1) variations in chemical or metallurgical properties of materials; 2) variations in manufacturing processes--such as dimensional tolerances in machining and coil winding.

For example, whatever grade of lamination steel is used, its silicon content and internal losses, its sheet thickness, and its surface insulation are all subject to tolerances. Wire for coil winding is subject to dimensional tolerances and therefore to resistivity variation. Machined fits between components cause air gap variation. Tolerances exist on overall core stack lengths and coil mean turn lengths.

All these conditions influence various motor losses, and therefore the efficiency. Ideally, then, a large number of supposedly identical motors will have a range of efficiencies from which a nominal or nameplate value must be selected. In a Gaussian distribution, many motors in the population will be more efficient than that, while an equally large number will be less efficient.

Whoever sets the required value to be published as applicable to the entire group must take account of these unavoidable variations, even if all units could be given perfectly accurate tests. The manufacturer has a choice between publishing an efficiency somewhat below what most motors will meet (which will please the users) or somewhat above--which risks non-compliance with applicable regulations. When only a few units have been built, setting a proper goal can be difficult.

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