Evaluation of additional loss in induction motors consequent on repair and rewinding

Abstract

The effect on the efficiency, and particularly the effect on the stray load loss of induction motors due to the process of repair and rewinding, are considered. To assess these changes, a range of induction motors covering powers between 5.5 and 225 kW has been very carefully tested both before and after rewinding. The sources of loss in induction motors are several, and all apart from stray load loss can be determined experimentally with reasonable accuracy. Stray load loss, however, is more challenging. The standard experimental approach of input-output power measurement, using the IEEE 112-B or C390 technique, is frequently used to evaluate stray load loss but has limitations. A limited set of results obtained by calorimetric methods has been used to validate the standard test methods. The most significant changes to the loss in induction motors caused by the repair process would be expected to be to stator copper loss, to core loss and to stray load loss. This is shown to be the case but the overall effect is normally less than 0.5% loss of efficiency. Occasionally efficiency loss up to 0.7% occurs for motors with core faults from new. With random-wire-wound machines, it is possible to compensate somewhat for this increase in core and stray load loss by using a 'tight' winding to reduce the copper loss. The trade associations of machine repairers EASA (USA) and AEMT (UK) sponsor the work as part of research into the influence of electric motor repair on machine losses.