Abstract
Thermal loading can induce mechanical stresses in the windings of electrical machines, especially those impregnated with epoxy resins, which is mostly the case in modern traction motors. Although designers look for cooling techniques that give better performance in terms of the power density and efficiency of the machine, several thermal cycles can lead to fatigue and the degradation of the copper insulation, epoxy and consequently the windings. In this paper, the performance of different cooling techniques has been compared based on the temperature distribution and the mechanical stress induced in the windings. Three-dimensional finite element thermo-mechanical models were built to perform the study. Two different variants of water jacket cooling, two configurations of direct coil cooling and two cases of combined water jacket and direct coil cooling methods have been considered in the paper. The results show that the combined water jacket and direct coil cooling perform the best in terms of the temperature and also the mechanical stress induced in the windings. An experimental set-up is built and tested to validate the numerical results.
Highlights
The steep development in the field of electric automotive and other modern traction applications has led to the demand for electrical machines with a high power density and high efficiency
The results show that the combined water-jacket and direct coil cooling seem to deliver much maximum temperature obtained is about 80 °C while the average temperature is the stator core is lower temperature levels
Since the heat is removed via two different cooling paths, the temperature level is much lower compared to cooling with water-jacket or direct coil cooling separately
Summary
The steep development in the field of electric automotive and other modern traction applications has led to the demand for electrical machines with a high power density and high efficiency. The impregnation of the windings with epoxy have relatively much more significant consequences due to the difference in the expansion coefficients, as epoxy typically has a high coefficient of thermal expansion compared to copper and coatings This presents the need for a comprehensive study of such stresses. The results showed that an increased copper fill factor and the use of epoxy with a low thermal expansion coefficient helps to prevent high stresses Another way to reduce the mechanical stress in the winding is to limit the temperature in the winding. We assess the water jacket cooling and direct coil cooling techniques with respect to the temperature distribution over the volume, and with respect to the mechanical stress induced due to the thermal loading of the machine, especially in the windings of the machine. The numerical results are validated by measurements on an experimental set-up identical to that used in the simulations at the end of the paper
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