Improved Thermal Models for Predicting End Winding Heat Transfer

Sammanfattning: The thermal analysis is of primary importance for the design of any kind of electrical machine. The cooling has a significant impact on the electromagnetic performances and on the durability and reliability of the machine. One of the main thermal issues for AC machines is the cooling of the end windings, which usually appear to be the most important hotspots.The thermal analysis of the end space region of an electrical machine is mainly linked to the characterization of the convective heat transfers between the solid surfaces and the cooling fluid, normally air. This results in a very complex analysis, mainly because of the unpredictable turbulence which is generated inside the end space by the rotating surfaces, such as the rotor, the shaft and the wafters. Different relations have been proposed in the literature by several authors but the calculation of the heat transfer coefficients (HTC) is still mainly related to empirical correction factors, based on the experience.The aim of this work is to investigate the convection inside the end space region with the double objective of studying how the geometry influences the HTCs and of developing a new improved correlation for the computation of the HTCs.The study is conducted with the support of an advanced CFD software, ANSYS Fluent CFD. The Nissan Leaf motor is modelled and the volume of air inside the end-space region is extracted and simulated in different conditions. These includes the study of the channels/ducts underneath the end winding toroids, the variation in the wafters length, the evaluation of the impact of the housing and endcap distance from the end-windings and the study of the HTCs response to a variation in the rotor mechanical speed.The investigation leads, as a main result, to a dimensionless equation that links the Nusselt number to the rotational Reynolds number in the end-space region. This equation is supported by a table of coefficients to be selected depending on the wafters length and the considered surface of the end-space. The new relation still has to be validated with the support of experimental data. However, it is suitable of a direct application in commercial software that uses a lumped-parameter thermal network (LPTN) for the thermal analysis of electrical machines, such as Motor-CAD.