Life models of polyimide film under combined thermal and electrical stresses used in inverter-fed traction motor

Ageing under multifactor stresses is an area that has recently challenged considerable interest. Failure in electrical components is largely attributed to the breakdown of electrical insulation subjected to presence of degrading and deteriorating stresses in which they operate. The single stress approach in design stage and also in ageing studies offers relative simplicity, but it is inapplicable to real life operating conditions which mostly are of multi-stress type. In this paper, a novel test apparatus of insulation failure time was designed for accelerating aging test. Meanwhile, electrical breakdown experiment on magnet wire insulating polyimide film (FCR 100) used in the inter-turn insulation of JD117 inverter-fed motor was conducted. The effects of multi-stress including impulse amplitude-temperature and impulse frequency-temperature were investigated respectively. The test results under different conditions were analyzed based on statistic analysis of Weibull distribution. Finally, insulation life models under combined thermal and electrical stresses were proposed using the multi-stress approach. A fitted formula for ageing life was presented which have two independent variables of voltage and temperature. The generality of the model lies in the fact that it can be used for designing of insulation system of all electrical components containing solid insulating materials surrounded by gas. Since the model was based on the probability of insulation deterioration under the action of the stresses applied, that gave us the opportunity to calculate the life time of insulation system for given system of stresses, the materials selected and the requested probability of failureless system operation.