Thermal Design of Electrical Machines - Investigation and Evaluation of Cooling Performances

Abstract: This Thesis focuses on thermal design of permanent magnet synchronous electrical machines (PMSM) used in hybrid vehicles (HEVs) or zero emission vehicles (ZEVs). Electrical machines in such applications are usually designed with high power density requirements but within limited spaces, which lead to high rate of heat generation but low rate of heat dissipation. Therefore, highly efficient thermal design or cooling methods are needed Firstly, the thesis discusses different possible approaches to improve the thermal design of electrical machines, which by either decreasing heat generations or increasing heat dissipations. Among different options, direct cooling system is studied further and compared to indirect cooling system. In this study, three simulation approaches namely Computational Fluid Dynamics (CFD), Finite Element Analysis (FEA) and Lumped Parameter Method (LPM) are applied to different calculation do mains with different purposes. The calculation domain of CFD simulation is selected for coolant, sometimes with limited volume of solid of electrical machines. The aims of CFD simulations in this study are: 1) rough comparisons on direct and indirect cooling systems 2) study of individual cooling channel and forming heat transfer coefficient maps. On the other hand, the calculated domain of FEA simulation is the whole machine, besides the coolant. The interfaces between the coolant and solid are set with boundary conditions which are heat transfer coefficients obtained by CFD simulations. The aim of FEA simulation in this study is to compare the heat transfer and differences between the direct and indirect cooling systems especially at the hot spots. Moreover, the calculated domain of LPM is the complete machine which also uses the heat transfer coefficients calculated by CFD at the boundaries between coolant and solid. The aim of using LPM in this study is to predict the transient distribution of machine temperatures and furthermore to evaluate the machine thermal performances with different driving cycles. This thesis also proposes Hydraulic Equivalent Circuit (HEC) method in order to evaluate the flow network for the cooling system, which uses the analogy between the electric circuit and hydraulic circuit In addition, this thesis also shows the in-house built experimental system in order to test the cooling system and temperatures for electrical machines. The comparison between measurement results and simulation results are discussed.