Early-onset Degradation of Thin-film Magnet Wire Insulation for Electromechanical Energy Converters

One of the predominant fault modes of electric machines is insulation failure which may lead to short circuits and catastrophic failure. Electrical insulation materials provide the vital function of turn-to-turn, phase to phase, and phase-to-ground electrical isolation for the electromagnetic coils and circuits. This paper investigates the characterisation of early-onset degradation of thin-film magnet wire insulation at elevated temperatures from 200 to 275 °C. Sample specimens were analysed after ageing for 100 hours in terms of their physical properties (surface roughness, mass), chemical properties (Fourier Transform Infra-Red (FTIR) spectroscopy), dielectric properties (capacitance, dissipation factor and impedance) and electrical properties (voltage breakdown strength and resistance). The roughness and mass increase and decrease fairly uniformly, respectively, as might be expected, with increased ageing temperature. The dielectric and electrical properties, however, do not change uniformly with ageing temperature and the results here appear not to conform to the commonly accepted Arrhenius law for insulation lifetime versus temperature. Instead we find that for slightly elevated temperatures (200 and 215 °C) the breakdown voltage performance is significantly worse than the unaged insulation, but this improves at 230-260 °C, and then finally drops to the lowest value at 275 °C. It is unclear exactly why this is the case, but we hypothesize that it could be related to build up of thermo-mechanical stress in the polymer layers which is not relaxed at the lower ageing temperatures.