Applicability of Lubricating Oil Insulation in Higher Power Density Motors for Electric Powered Aircraft

Higher power density motors for electric powered aircraft require higher voltage. The authors aim to apply a lubricating oil used for cooling motor windings to an insulating material to improve the dielectric strength of motors. In motor insulation, it is required to prevent the occurrence of partial discharge (PD) and understand the insulating characteristics of the lubricating oil. In this paper, we first measured and compared the breakdown voltage (BDV) of the lubricating oil and mineral oil in a fundamental sphere-sphere electrode system, and discussed the basic breakdown characteristics. The results suggest that the lubricating oil has superior dielectric characteristics to the mineral oil. Next, PD inception voltage (PDIV) of an arrow pair of enamel-coated rectangular wires in the lubricating oil was measured and compared with the estimated PDIV by electric field analysis. As a result, the measured PDIV was generally consistent with the estimated PDIV. Therefore, the lubricating oil insulation is expected to improve the dielectric strength in the motor windings for electric powered aircraft.