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This article proposes an enhanced procedure for executing the standard variable load test that allows obtaining high correlation factors for indirectly measured stray-load loss during efficiency tests of induction motors. This helps to achieve the minimum values required by the international standards to consider the variable load test well executed, avoiding unwelcome test repetitions and leading to significant amount of time saved. With the machine at steady-state temperature, the enhanced procedure consists of alternatively applying load levels higher and lower than the rated load, so that the stator winding temperature zigzags around the rated value. Hence, multiple readings can be performed for each load point, having the machine in isothermal conditions. This allows averaging many measurements of the same load point to mitigate the impact of instrumentation and reading errors. The article includes load tests conducted on different induction motor sizes and pole counts, applying both the standard variable load test procedure and the proposed approach. The experimental results show that the proposed technique allows achieving higher correlation factors on the stray-load losses than the standard procedure.

This paper proposes a procedure for executing the standard variable load test aimed at obtaining valid correlation factors in the stray-load loss calculation during efficiency measurement. This helps to avoid the tests being repeated, leading to significant amount of time saved, making the proposed methodology of potential high interest for industry. The proposed procedure allows for isothermal multiple readings for each load point of the variable load test; thereby, averaging many measurements of the same load point in order to mitigate the impact of instrumentation and reading errors. Experimental tests on different induction motors show that very high correlation factors for the stray-load losses can be achieved.