**Abstract**

This paper shows the behavior of several dielectric properties of coronaresistant polyimide tapes as a function of thermal (high temperatures) and ambient stress (high humidity coupled with high temperature). The main goal of the investigation is to understand and explain the evolution of the partial discharge endurance of the tapes as a function of nonelectrical aging. The results indicate that polyimide tapes are very stable up to temperatures of 320°C. However, the most elevated temperatures reduce the partial discharge endurance to 50% of the original value. This is probably due to morphological changes in the (outer) nanostructured layers of the tapes.

**Keywords:** electrical drives, insulation reliability, partial discharges, corona-resistant insulation, thermal aging
