Assessment of the Dual-Temperature Aging Method for the Electrical Insulation System of Direct Oil-Cooled Motors in Electric Vehicles

It is very essential to qualify the impact of the automatic transmission fluid (ATF) to the electrical insulation system (EIS) of direct oil-cooled motors, with the development of electrification trends in the automotive industry and prevalence of direct oil cooling in electric powertrains. This study performed the dual-temperature aging test to evaluate the ATF impacts on the motorettes: one motorette was submerged in the ATF, and the other was exposed to ATF vapor. Both of them flowed through the same current, and generated heat via the Joule effect to simulate the direct oil-cooled motor operation. The temperature of the ATF was controlled at 100°C during the high-temperature (200°C) aging process. The results show that the ATF vapor significantly reduced the reliability of the EIS. Additionally, the properties of the ATF were analyzed before and after aging. A possible aging mechanism of the motorettes under ATF conditions was discussed. The result also suggests that the dual-temperature aging method provides more comprehensive information than the traditional oven tests, enhancing a better understanding of the effects of the lubricating oil, additives, etc., on the EIS of direct oil-cooled motors.