Modeling and Testing of Insulation Degradation due to Dynamic Thermal Loading of Electrical Machines
Abstract: Electrical machines in electrified vehicles are subjected to dynamic loadings at different driving conditions, which results in dynamic temperatures. The aging of the Electrical Insulation System (EIS) in electrical machines is caused by these dynamic temperatures, namely high average temperatures and temperature cycles. In addition, the degradation of EIS affects the lifetime of the electrical machine. In this thesis, three cornerstones for lifetime estimation of electrical machines in electrified vehicles are identified and studied, which are the usage, the degradation mechanisms and the lifetime model. A combination of computational simulation and lab testing is required to design a comprehensive model. Furthermore, the indicators of EIS degradations and the diagnostic methods of stator segments (or motorettes) and electrical machines with aged insulations are studied. A system thermal model, including a drivetrain model of vehicles, a loss and cooling model and a thermal model of electrical machines, is proposed to predict the temperature distribution inside the electrical machine of an electrified vehicle. The estimated dynamic temperature at the hotspot is one of the inputs to a lifetime model of the electrical machines. To identify the degradation mechanisms of the EIS under the dynamic temperatures, both enameled wires and motorette specimens are tested with accelerated degradation tests. It is found that the aging of the EIS of an electrical machine subjected to the dynamic temperature is not only caused by oxidation of insulations with high average temperature, but also caused by the fatigue of insulations due to thermal-mechanical stress induced by the temperature or thermal cycles. A revised lifetime model of electrical machines is proposed, which covers both aging mechanisms mentioned above. Another input to the lifetime model, the thermal-mechanical stress is estimated by Finite Element Analysis (FEA) using Ansys Structure simulation. The condition monitoring approaches are simulated by both electrostatic FEA model and analytical model and implemented during the accelerated degradation testings. These approaches assess the State of Health of the EIS of motorette specimens . Insulation capacitance shows more consistent trends during aging at different stress levels compared to insulation resistance. Insulation capacitance reduction of 4 to 6% and 11 to 12% are found between winding and winding and between winding and ground, respectively. A diagnostic method is proposed for measuring the high frequency current with a voltage pulse simply set by the drive of an electrical machine. The migration of both amplitude and frequency of the current detected are indications of aging of the insulation system of an electrical machine due to the decrease of the insulation capacitance.
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