Thermal Analysis and Management of High-Performance Electrical Machines
Abstract: This thesis deals with thermal management aspects of electric machinery used in high-performance applications with particular focus put on electric machines designed for hybrid electric vehicle applications.In the first part of this thesis, new thermal models of liquid (water and oil) cooled electric machines are proposed. The proposed thermal models are based on a combination of lumped parameter (LP) and numerical methods. As a first case study, a permanent-magnet assisted synchronous reluctance machine (PMaSRM) equipped with a housing water jacket is considered. Particular focus is put on the stator winding and a thermal model is proposed that divides the stator slot into a number of elliptical copper and impregna- tion layers. Additionally, an analysis, using results from a proposed simplified thermal finite element (FE) model representing only a single slot of the sta- tor and its corresponding end winding, is presented in which the number of layers and the proper connection between the parts of the LP thermal model representing the end winding and the active part of winding are determined. The approach is attractive due to its simplicity and the fact that it closely models the actual temperature distribution for common slot geometries. An oil-cooled induction machine where the oil is in direct contact with the stator laminations is also considered. Here, a multi-segment structure is proposed that divides the stator, winding and cooling system into a number of an- gular segments. Thereby, the circumferential temperature variation due to the nonuniform distribution of the coolant in the cooling channels can be predicted.In the second part of this thesis, the thermal impact of using different winding impregnation and steel lamination materials is studied. Conven- tional varnish, epoxy and a silicone based thermally conductive impregnation material are investigated and the resulting temperature distributions in three small induction machines are compared. The thermal impact of using different steel lamination materials is investigated by simulations using the developed thermal model of the water cooled PMaSRM. The differences in alloy con- tents and steel lamination thickness are studied separately and a comparison between the produced iron losses and the resulting hot-spot temperatures is presented.Finally, FE-based approaches for estimating the induced magnet eddycurrent losses in the rotor of the considered PMaSRM are reviewed and compared in the form of a case study based on simulations. A simplified three-dimensional FE model and an analytical model, both combined with time-domain 2D FE analysis, are shown to predict the induced eddy current losses with a relatively good accuracy compared to a complete 3D FE based model. Hence, the two simplified approaches are promising which motivates a possible future experimental verification.
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