Stray-Load Loss Approximation for Partial Loads

For a given power rating of a three-phase induction motor, the American (IEEE Std 112) and European (IEC 600342-1) standards assume constant allowance for stray-load loss at full-load condition. This is important for the purpose of efficiency calculation when direct measurements are not available or not allowed in some uninterruptable operations. The assumed constant allowances of both standards exhibit different accuracies when compared to the measured values of stray-load loss. Moreover, when this type of loss is measured at partial loads, it demonstrates values that are significantly lower than the IEEE and IEC fixed allowances. Thus, in efficiency evaluation process, if the assumed stray-load loss is assigned for a specified load other than full-load, this may lead to an inaccurate evaluation results due to the exaggeration of the stray-load loss. Therefore, having a reasonably accurate value for partial-load stray-load loss is highly important especially in the efficiency evaluation of in-service (online) machines, when they are being part of critical uninterruptable industry processes, given the fact that those machines are not always operated at their full-load capacity. The availability of a partial-load stray-load loss estimation method is also important for offline efficiency evaluation at specified load points. This paper introduces a new approach for the approximation of stray-load loss in three-phase induction motors that operate at any specified load point. The proposed method is validated via its application on induction machines of different power rating and rated frequency. The results demonstrate high accuracy. The technique may help produce reliable efficiency evaluation for online and offline applications.