Lifetime determination of polyimide under square voltage waveforms

Nowadays, rapid development of both new types of inverters and new electronic switches permits adjustable control of AC machines. These adjustable speed drives generate square voltage waveforms. These specific stresses should not, as a side effect, introduce defects during the operation of rotating machines, particularly with regard to their electrical insulation. Indeed, this insulation was initially designed for 50 Hz sine wave applications and square wave voltages used could be the cause of a higher failure rate. Therefore, the study of such stresses on the dielectric integrity is of crucial interest, in order both to develop an appropriate material choice and a maintenance policy. The application of these voltage shapes on a model using Polyimide (Kapton^(TM)) for turn to turn insulation, permits a study of this stress in terms of life time determination. Life time measurements were performed on samples (25 μm thick) for square voltage waveforms with a magnitude between 500 and 700 V peak at two frequencies (3 and 10 kHz) and for the same voltage front (around 1 kV/μs). In this paper, the results of such a study are given. Two different aging phenomena are detected: one, directly related to the voltage magnitude, which is practically not voltage waveform dependent (except through its frequency), and the other, directly related to the voltage waveform and to its characteristics. These results were confirmed using another material: PET