At an industry conference not long ago, a distinguished engineering educator presented a paper with this intriguing title: "Question: Can you provide your customers with the efficiency of your induction motor given only an input current value? The answer: yes." The author was concerned with polyphase a-c motors. In his paper, he proposed determining efficiency only by manipulation of the standard model equivalent circuit. Although it facilitates evaluation of three of the five losses in an induction motor, that circuit does not account for the other two: friction & windage, and stray load loss. The paper avoids the issue by simply assuming that the stray load loss is negligible. Unfortunately, stray loss is never negligible. And an efficiency determined by neglecting that loss cannot be termed reasonably accurate.
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