Enhanced Thermal Model for Motors Fed with Distorted Waveforms

Electrical motors are designed on the basis of balanced three-phase sinusoidal input voltage. Non-sinusoidal voltage has detrimental effects on induction motors performance; hence derating of the machine is required. Thermal models used in protective relays are based on pure sinusoidal waveform, which could not effectively protect the motor if the supply voltage waveform is distorted. In this paper a new thermal model is proposed that not only protects the motor from overload under normal power supply condition, but also applies a proper derating factor under waveform distortion. The modified model evaluates the thermal limitations of the stator and rotor together and protects the motor in the presence of excess overheat. Under distorted waveforms, the trip threshold is indicated by the level of distortion in addition to the service factor. The simulation results indicate the effectiveness of the model in the presence of different harmonics.