International standards for the induction motor efficiency evaluation: a critical analysis of the stray-loss determination

The motor efficiency has to be measured or calculated in accordance to International Standards. The most important world references are the IEEE 112-B, the IEC 34-2 and the JEC 37 Standards. In this paper, a comparison of the measurement procedures defined by these International Standards is reported, together with some comments to the prescribed methodologies. The comparison is based on experimental results obtained by testing four general purpose three-phase induction motors. The stray-load loss measurement represents a critical key for the correct evaluation of the motor efficiency. For this reason, a critical analysis of this type of losses has been performed. In particular, the stray-load loss sensitivity to the measurement errors is analyzed in order to understand which are the most critical quantities that influence their evaluation. In the final part of the paper the temperature influence on the conventional iron losses is experimentally analyzed. The performed tests show that the temperature difference between the no-load test and the motor real operative conditions is not negligible.