Thermal Overload Protection of Induction Motors Under Waveform Distortion

Induction motors require thermal protection to prevent overheating due to cyclic as well as steady state overloads. A protective relay uses the thermal model to continuously calculate the temperature in real time. The virtual temperature is monitored and trips to prevent overheating. Although the thermal models are relatively suitable for protecting the motors in the overload condition but they are not quite effective under distorted waveforms. Due to the harmonics in the system, the total losses in the motor will be more resulting in higher temperature rise of the motor. Under distorted supply system the motors cannot be operated at full load condition. To utilize the motor most effectively under distorted supply condition, a proper derating of the motor is required which essentially depends on an accurate determination of the motor winding temperature. 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 simulation results indicate the effectiveness of the model in the presence of different harmonics.