Influence of rise time on the dielectric behavior of stator insulation materials

New rotating machine power supplies (inverters) produce voltage shapes whose characteristics are very different from the classical a.c ones. Up to now, their impacts' on stator insulation materials are not clearly understood. The most harmful parameters have been identified. These are the waveform (square or pulse like), the rise time (>kv//spl mu/s) and the high frequency solicitation (kHz). These stresses have been recreated in laboratory on two kind of samples representative of turn to ground insulation: PET and PEN. In a first step, these materials have been studied before ageing using dielectric spectroscopy from 20 Hz to 1 MHz and for temperature ranging between LN2 and 200/spl deg/C. Different relaxation phenomena are identified. Aging is then undertaken for 1000 hours in different conditions. After aging, different characterization have been undertaken to understand the induced changes (dielectric spectroscopy measurements, surface potential measurements, breakdown measurements...). The main conclusion of this work is that aging induced changes are directly related to the voltage rise time values. Reasons of such a behavior will be discussed and a physical interpretation will be given.