Temperature Dependence on Dielectric Properties of Automatic Transmission Fluid

The development of direct oil cooling motors is gaining popularity in electric vehicles owing to the growing demand for electric motors with higher energy efficiency and heat dissipation in insulating systems. As a coolant and lubricant, the automatic transmission fluid (ATF) comes into direct contact with the components of the integrated electric motor in this cooling architecture. Therefore, the dielectric properties of ATF are critical for the durable operation of vehicles. In this study, the volume resistivity, permittivity, and dielectric dissipation factor of fresh ATF, as well as preliminary compatibility between ATF and impregnated resin, are tested at different temperatures. The results show that ATF exhibits certain dielectric properties and its breakdown voltage is sufficient to meet the existing rated voltage level of electric vehicles; however, its dielectric dissipation factor is much larger, mainly due to conduction loss. Meanwhile, ATF should achieve a trade-off between electrical conduction and insulation; its resistivity and permittivity cannot be too high or too low. Finally, favorable compatibility between the ATF and impregnated resin is observed after prolonged exposure experiments at 155°C with a water content of 0.5% in the ATF.