Derating of Synchronous Motors Fed from Polluted Supply

Abstract

Synchronous motors when fed from a power system polluted with harmonic voltages will exhibit harmonic rotating fluxes. The speed of each harmonic flux will correspond to the harmonic voltage frequency, and the direction of rotation will depend on the harmonic voltages sequence. The backward rotating fluxes (produced by the negative sequence harmonic voltages) produce braking torques. Harmonic voltages and the associated currents and fluxes result in additional copper and iron losses. Therefore, the motors, which are fed from polluted-voltage supply, should be derated otherwise the motor temperature would exceed its rated value. This endangers the windings, insulation and bearings of the motor, hence shortening its life span. In the present paper, the proper derating factors for the synchronous motors are determined when fed from polluted supply to avoid thermal stresses.