Considerations about the Determination and Designation of the Efficiency of Electric Machines

Abstract

According to IEC 34-2 and the harmonized European standards derived from it, the additional losses are taken into account by an estimated 0.5% P₁ in the indirect determination of efficiency. In the USA, extensive measurements are made to this effect (IEEE/NEMA), whereas in Japan (JEC), the additional losses are ignored.

Depending on the frame size, the actual efficiencies of four-pole squirrel cage motors <200 kW may generally be slightly lower (up to 1 to 2 percentage points at ≈ 10 kW), those of large machines (≥ 700 kW) slightly higher than the nominal values obtained by indirect determination in accordance with IEC 34-2. Since, however, influences of the same order of magnitude due to temperature and measurement technology overlie this tendency, it becomes questionable whether the expenditure of precise determination of additional losses according to IEEE/NEMA is justified. On the other hand, the IEC method is easy to use and to reproduce and has proved in practice to be very worthwhile.

To realistically assess the behaviour of an electrical machine in terms of energy efficiency, one has to know, in addition to the nominal efficieny,the loss distribution which influences the efficiency at partial loads, as well as the dependence of this efficiency vs. load curve upon temperature and supply-voltage fluctuations.