Thermal-Loss Coupling Analysis of an Electrical Machine Using the Improved Temperature-Dependent Iron Loss Model

Abstract

Iron loss prediction is very important for the evaluation of efficiency, temperature and demagnetization of electrical machines. The models developed in [1]–[4] are most commonly used iron loss models for electrical machine analyses. However, none of these iron loss models considers the influence of temperature, which has been experimentally confirmed in [5]. In [6], an improved iron loss model which can consider temperature dependencies of hysteresis and eddy current losses separately is developed. By applying the improved iron loss model, the temperature influence on the iron loss can be fully considered. It is then possible to couple the thermal and loss analyses with each other by utilizing the improved iron loss model, which will be the subject of this paper. The iron loss model developed in [4] is one of the most accurate iron loss models when the temperature is constant with the help of variable coefficients. The iron loss p Fe can be expressed as: Equation (1) where f is the frequency. B m is the peak value of alternating flux density. k h (f,B m ) and k e (f,B m ) are the hysteresis loss and the eddy current loss coefficients, respectively. It should be noted that the iron loss model (1) cannot consider the influence of temperature on iron loss while the temperature influences iron loss significantly. According to the iron loss model developed in [5], both the hysteresis loss k h (f,B m ) coefficient and the eddy current loss k e (f,B m ) coefficient vary not only with frequency and flux density but also with temperature. Therefore, the improved iron loss model is then developed and can be expressed as: Equation (2) where p Fe,T is the iron loss density at the actual temperature T. k h (T,f,B m ) and k e (T,f, B m ) are the hysteresis loss and eddy current loss coefficients, respectively. In order to evaluate the iron loss models in electrical machines, thermal tests and analyses are carried out in a 12-slot/10-pole IPM machine. The schematic diagram of the test system is shown in Fig. 1(a). The 12-slot/10-pole IPM machine is connected a three phase AC power source. The magnets are removed and the rotor is locked in order to eliminate magnet eddy current and mechanical losses. As shown in Fig. 1(b), four thermal couples are equipped in the electrical machine to measure the temperature at different positions, i.e. the stator tooth, the stator yoke, the rotor magnet slot and the rotor yoke. The temperatures are measured when the electrical machine is powered by the AC power source. For the temperature prediction, the iron losses are calculated by the existing model and the improved model, respectively. The thermal model of the IPM machine is also built in Motor-CAD. The thermal model is then analysed with calculated copper loss and iron losses. The predicted temperatures of the electrical machines can be obtained. Fig. 2 compares the measured and predicted results by the existing iron loss model and the improved model. It can be seen that the predicted temperatures by the existing iron loss model become inaccurate when the temperature is high. This is due to the fact that the existing iron loss model cannot consider the temperature dependency of the iron loss. The input iron loss for the thermal analysis keeps constant while the actual iron loss decreases significantly with the temperature rise. On the other hand, the predicted temperatures keep good accuracy when the temperature reaches 100 degrees Celsius or even higher. This is due to the fact that the improved iron loss model considers the temperature dependency of the iron loss. Input iron losses for the thermal analysis vary with the temperature rise. In other words, the thermal and loss analysis can be coupled with each other by utilising the improved iron loss model, which is more close to the actual condition in electrical machines. The details will be investigated and described in the full paper.