Local Heat Transfer Coefficient Measurements on Shaft Spray Cooled End Windings

High power density electrical machines are mainly limited by their cooling system. Oil spray cooled end windings are one possibility to significantly increase the power density. However, foundations for the estimation of the cooling capability in the design process are rather scarce. While approaches to estimate the global heat transfer coefficients for spray cooled end windings exist in literature, they are missing for local heat transfer coefficients, which are essential for the prediction of hot spots. Therefore, in this contribution, we present local heat transfer coefficient measurements for shaft spray cooled end windings using a model fluid concept to apply the results to various coolants. It is shown that the existence of a sump remarkably lowers the heat transfer on the end windings, whereas it is almost constant over the remaining circumference. The heat transfer exhibits a systematic dependency on the rotational speed, where it is increasing for very low as wells as very high speeds and decreases in between. A modeling approach of the dimensionless heat transfer coefficient, the Nusselt number, is developed based on existing approaches to apply it to various coolants and changing operating conditions. The achieved mean absolute percentage error between experimental and modeled Nusselt number is given by 10.8%.