Average Heat Transfer Coefficient Measurements on Radial Oil Jet Cooled Stranded End Windings

Compact and high power density electrical machines are of high relevance in current trends of electrification. Direct oil cooling of the end windings can lower hot spots and considerably increase the power density of electrical machines. However, calculating the heat transfer coefficient is challenging due to the difficulty in predicting the fluid distribution on the end windings. This paper presents experimentally determined average heat transfer coefficients of radial oil jet cooled stranded handwound end windings. The findings show that, despite the oil jets spraying from the housing side onto the end winding, the shaft speed has an influence on the heat transfer coefficient. This is due to fluid passing through the hand-wound end winding and being reflected by the spinning shaft onto the inside of the end winding. The influence of the jet diameter on the heat transfer coefficient is also shown.