Origins of load losses in induction motors with cast aluminium rotors

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Abstract
The theory presented in the paper attempts to explain the origins of load losses in detail. It includes an analysis of the losses produced, at any given value of slip, by current flow through the rotor iron between two adjacent bars, owing to the imperfect insulation of the squirrel cage. The theory also includes a calculation of the eddy-current and hysteresis losses in the rotor teeth, caused by the high-frequency harmonic fluxes that penetrate them. It shows how eddy currents in the iron surface can reduce the value of the differential harmonic leakage of a bar, thus allowing more harmonic bar current to flow, and hence more I2R losses to be produced. It discusses how the effective skew differs from its geometrical value, depending upon the width of the rotor bars. The theoretical formulas developed are used to predict the torque/slip characteristics of three 7½hp squirrel-cage induction motors, and the results show good agreement with the experimental curves, thus verifying the validity of the theoretical argument. A simplified theory of interbar currents is finally given, and the effect of skew and slot combination on the magnitude of the load losses, under normal operating conditions, is discussed.

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