Introduction of a modified thermal model for squirrel cage induction motor based on calculation of rotor bars' current

Thermal stresses on the induction machine are the main cause of reliability reduction, which can lead to further faults in the machine. Using the thermal sensors to determine the temperature of the electric machine is costly and difficult; therefore, indirect methods have been considered to this end. One of these methods is utilizing the thermal model of the induction machine, which can determine the temperature of various parts of the motor without requiring any additional circuits for signal injection or the use of rotor resistance estimation techniques. This paper presents a modified thermal model of the induction machine. In this model, each rotor bar's current is designated as a heat source. Therefore, to solve the new model, it is required to calculate the rotor bars' current. The currents of the rotor bars of a squirrel cage induction motor are calculated by using the motor terminals data measured with an experimental test setup and both winding function and finite element methods. Then, the losses of the rotor cage are obtained by these currents. By inserting these losses into the modified lumped parameter thermal model of the machine, the rotor temperature will be determined. The results have been validated by analyzing the motor temperature with MotorCAD software, and it has been observed that the new proposed model achieves more accuracy compared to the conventional model. In addition, the accuracy of MotorCAD simulation was confirmed by implementing a temperature sensor in stator winding and comparing the achieved temperature with MotorCAD simulation results.