Modelling Partial Discharge Inception in magnet wires at different altitudes

The design of the insulation for inverter-fed induction motors is chiefly empirical. In general, a commercial conductor is selected based on experience. Qualification of motor insulation is carried out according to the IEC Std. 60034-18-41. However, qualification might be time consuming. Therefore, an approach based on a preliminary screening of alternative conductors would be useful. Goal of this screening would be to determine which geometries ensure that partial discharges are not incepted under operation. Therefore, we investigate the feasibility of using streamer inception criteria in combination with FEM modelling to determine PDIV. Emphasis is given to the possibility of extrapolating results at different altitudes, to deal with aircraft actuators. PDIV measurement were carried out on twisted pairs. Pressures were in the range from 100 kPa to 20kPa, i.e. that typical of aircraft. Frequencies from 50Hz to 10 kHz were investigated. The results were compared to those provided by streamer inception criteria. A slight dependence on supply frequency was observed, although it could not be explained in terms of the models employed (based on the electrical field lines and the ionization characteristics of air). This dependence seems to be related to phenomena not explicitly considered in the models (e.g., secondary emission from the cathode, although covered by a dielectric, partial discharges occurring below detection sensitivity threshold). Streamer inception criteria failed to predict correctly these phenomena, especially al low pressures, and need further research work to become applicable.