505 / 2022-03-24 16:05:06

Temperature Dependence on Dielectric Properties of Automatic Transmission Fluid

electric vehicle,dielectric pro

Direct oil cooling with ATF(automatic transmission fluid) as the coolant is increasingly applied to the drive motor of new energy vehicles. The automatic transmission fluid indirectly becomes a part of the drive motor insulation system. Meanwhile, the thermal evaluation of the solid-liquid composite system composed of ATF and solid insulating materials also needs to be evaluated and verified.

In this article, first of all, for the unused ATF, the relative permittivity, dielectric dissipation factor and volume resistivity at different temperatures, as well as the dielectric spectra characteristics of the ATF at different temperatures were tested in the frequency range of 10^-1~10⁶ Hz to know the dielectric properties of the ATF. Secondly, a self-designed ageing platform was used to conduct dual-temperature thermal ageing experiments on the GPM(general-purpose model) of the drive motor stator winding with an expected thermal class of 155 (one GPM is immersed in ATF, and the other is surrounded with vapor formed by ATF evaporation, thus forming dual-temperature). So far, only the thermal ageing experiment at 200℃ has been completed, and the water content, acidity, kinematic viscosity, relative permittivity, dielectric dissipation factor and breakdown voltage of ATF before and after thermal ageing have been tested respectively.

The preliminary conclusions are as follows:

1. The relative permittivity and volume resistivity of ATF decrease with the increase of temperature, while the dielectric dissipation factor increases with the increase of temperature.


2. At different temperatures, the real part of the permittivity(ε′) of ATF crosses around 100Hz, and the imaginary part of the permittivity(ε″) crosses around 200kHz.


3. After the single point ageing at 200℃, the variation of acidity, kinematic viscosity, dielectric permittivity and dielectric dissipation factor of ATF before and after ageing is no more than 6%, that is, the change is not obvious before and after ageing, however, the water content and breakdown voltage decrease dramatically.