Interbar current losses in cage induction motors due to harmonic flux

Abstract

The interbar current losses have been studied through measurements and calculations. However, the quantitative effect of the contact resistance between the rotor bar and the iron core inter-bar current losses is not fully understood because of difficulties both in measurement of contact resistance and its modeling in analysis. This paper describes a study of the inter-bar current losses in induction motors due to harmonic flux and compares the measured and calculated results. The interbar current losses due to harmonic flux are measured on the machine of the rotor with end rings removed. The slot insulation and core heating are applied to make differences in the interbar resistances. The losses decrease as the interbar resistances increase. The increase in interbar resistances caused by the slot insulation treatment is ascertained to decrease the losses effectively. The inter-bar current losses are calculated with three-dimensional magnetic field analysis. The finite element model contains the iron core portion with corrected conductivity reflecting the measured inter-bar resistance. The calculated losses agree well with the dependency of the measured losses on the interbar resistances. Moreover, additional measurements of high-frequency impedances of rotors reveal the interbar loss reduction effects of core heating caused by increasing the high-frequency impedances.