A Comparison of direct and indirect measurement of induction motor efficiency

Induction machine efficiency is defined as the ratio of output power to input power. The efficiency determined from this ratio is known as the direct efficiency. Alternatively, the efficiency is often determined by first determining the losses and using either the input power or the output power to calculate the efficiency. This method, known in standards as the loss segregation method is an indirect method that is known to yield a more accurate result if the stray load losses are properly determined. However, the indirect method is complicated; it requires skilled operator and is time consuming. On the other hand, the direct method is relatively simple and less time consuming. The question is by how much the efficiency numbers produced by these two methods differ. This paper presents a comparison of the two methods from a collection of test data of about 1000 induction motors, rated 1-250hp, to show the relative accuracy of the direct method. The Bland-Altman procedure for comparing measurement methods is used in the analysis.