Dielectric Characterization of Impregnating Varnishes for Inverter-Fed Motors

Fast switching converters cause increased electrical stresses on the insulation of rotating machines. Within this context one of the roles of impregnating varnishes/resins is to guarantee high partial discharge inception voltages (PDIV). Most of the resins operate in the plasto-viscous region (above their glass-transition temperature) where large permittivity values can be expected. Field enhancement in the air surrunding such materials may be expected, possibily reducing the PDIV level. Here the value of the resin permittivity and its impact on the PDIV of impregnated samples of turn-turn insulation have been investigated as a function of temperature. The results indicate that PDIV depends mostly on the coating properties of the resin. Permittivity seems to affect PDIV to a lesser extent, but results are entangled with the thermal expansion of the resin, making definitive evaluations very difficult.