Study of the ageing of different dielectrics under specific electrical constraints

Development of new power apparatus for rotating machine control using adjustable frequency inverters (such as PWM inverters) leads to new constraint shapes for the dielectrics involved in the insulation. As a matter of fact, large dV/dt and frequency may be encountered. In order to understand what occurs in dielectrics when they are submitted to these stresses, a study on different materials used in class F insulation is undertaken. They are characterised prior and after ageing by dielectric spectroscopy. Then, they are submitted to different waveforms in a new set-up built to simulate the real ones (parameters include: sinusoidal shapes at different frequency, f=50, 500, 1000, 2500 Hz; square form, dV/dt=1 KV/μs, f=10 kHz). The role of the different parameters and the influence of the wave shape may thus be demonstrated. If the complex permittivity ε^* appears to be a good marker in this study of ageing, a correlation between nondestructive and destructive tests has to be undertaken to find the threshold of irreversible phenomena