Degradation and Dielectrics measurements of rotating machines insulation at high temperature (200-400°C)

Due to a continuous power density increase, the thermal stresses endured by the electrical insulation of rotating machines are also increasing. However, up to now, most of the characterizations (breakdown voltage, permittivity, loss tangent) are usually carried out in a temperature range which rarely exceeds the thermal class. Consequently, there is a lack of knowledge on the behaviour of these materials when they are submitted to temperatures which may reach 50°C above their thermal class. In this paper, experimental results regarding the electrical behaviour of enamels are presented and discussed. Hence, permittivity and loss tangent are measured from room temperature to 400°C in the frequency range of 50Hz-100 kHz. The samples consist in 50¿m films of various polymers (polyester-imide, polyamide-imide and polyimide) deposited on different conductive plates (copper or stainless steel). To find the impact of the interfaces, some samples are composed of two insulating layers (same or different varnishes). The differences observed between the different polymers are correlated to their chemical structures obtained thanks to chemicals measurements like ATG and FTIR.