Transition of partial discharge inception voltage under high humidity for enameled rectangular wires

In recent years, the miniaturization and greater power output of motors for EVs and HEVs have led to the higher drive voltages and the increased adoption of rectangular wire in coils (higher volume ratio). In order to ensure that the motor is electrically insulated, it is necessary to restrict partial discharges between coil wires, and to do this, it is important to reduce the potential difference between wires resulting from the operation of the inverter and to ensure that the coil wire has the proper coating thickness. However, the measurement of partial discharges between wires results in variability in the measured values depending on the environment and methods, which is a problem for the stringently controlled design of motor insulation. To clarify the factors affecting the partial discharge inception voltage (PDIV) between wires as well as the mechanism, the effect of ambient humidity, pre-discharge conditions on the PDIV between wires was analyzed using rectangular wire with a coating thickness of 40 μm and 160 μm. As a result, it was confirmed that when a pre-discharge is performed in a high-humidity environment, transient characteristics were displayed in which PDIV momentarily decreased and then increased. We performed discharge observations and electric field analysis. The results showed that in high-humidity environments, the pre-discharge causes moisture to adhere to the coating surface, resulting in the increase in surface conductivity, which influences the electric field strength in the air gap.