Breakdown Characteristics of Streaming Lubricating Oil toward Higher Power Density Motors for Electric Powered Aircraft

The authors aim to apply a lubricating oil used for cooling motor windings to an insulating material to improve the dielectric strength of motors for electric powered aircraft. There exists an oil flow and temperature rise in the lubricating oil to cool the motor windings. Therefore, dielectric strength of the streaming lubricating oil is investigated in this paper. First, ac breakdown voltage (BDV) of stationary lubricating oil with controlled moisture content of 8-52% was measured, and the moisture content dependence of breakdown electric field strength (BDE) was discussed in consideration of the volume effect <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(10^{0}-10^{5}\text{mm}^{3})$</tex> of the BDE. The results suggest that the decrease in BDE due to the volume effect would be more dominant than that due to the moisture effect. Next, ac BDV of streaming lubricating oil was measured at the flow rate of 0–38 L/min and the temperature of 23–65 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\circ \mathrm{c}$</tex> for the coaxial cylindrical electrodes. As a result, the BDV of streaming lubricating oil at the flow rate of 30 L/min was 24-42% higher than that of stationary oil at different temperatures. Also, the BDV at low flow rates decreased as temperature increased.