Degradation Model of Hairpin Winding in Inverter-fed Motors Considering Thermal and Electrical Stress

This paper presents the findings of a study on the degradation of hairpin winding insulation. Hairpins are commonly used in inverter-fed motors in electrical vehicles. In order to achieve an ageing or lifetime model, the geometry and the electric field stress inside a stator-slot of an electric motor was analyzed and a suitable test setup was developed. A group of hairpin-in-slot specimens have been subjected to superimposed thermal and electrical load conditions at pulse-width modulated (PWM) voltage. The levels of stress on the hairpin windings have been defined based upon Design of Experiments (DoE) principles. The tests have been performed in several stages, taking into account that the physics of failure are unknown. In order to get the data for the lifetime analysis, partial discharge (PD) measurements were used. PD inception voltage (PDIV) and PD extinction voltage (PDEV) were recorded during aging tests. Based on the data from the PD measurements, several degradation path models were fitted and compared. From the varied stress levels, the parameter influence on the degradation rate was evaluated. Based upon the plausibility of the found results, the validity of the obtained degradation model was assessed.