Electrical aging of stator insulation of low voltage rotating machines supplied by inverters

Nowadays, the use of inverters for speed variation of low voltage motors has become more and more common. The most common technology for this kind of apparatus uses pulse width modulation (PWM). It is characterized by a square voltage shape with short rise and fall times. In order to insure the quality of turn to ground insulation submitted to these constraints, a study on the aging of polyethylene terephthalate (Mylar^TM) has been undertaken. Influence of rise and fall times, frequency and voltage magnitude has been investigated for time duration up to 1000 hours. To realize a good voltage application, samples were previously metalized. Two different behaviors according to the rise time and magnitude are observed. For long rise and fall times, changes in the capacity values are obtained although, for smaller rise or fall times, a degradation of the metalized electrode appears. A surface charge generation process is discussed to explain this phenomena