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

Due to a power density increase, the thermal stresses endured by the electrical insulation of rotating machines are also increasing. However, up to now, most of the characterization (breakdown voltage, permittivity, loss tangent) are carried out in a temperature range which rarely exceeds the thermal class. Consequently, there is a lack of knowledge on the behavior of these materials when they are submitted to temperatures which may reach 50 degC above their thermal class. In this paper, experimental results regarding the electrical behavior of enamels (varnished or not) are presented and discussed. Hence, permittivity and loss tangent are measured from room temperature to 350 degC in the frequency range of 50 Hz-100 kHz. The samples consist in 50 mum films of various polymers (polyester-imide, polyamide-imide) deposited on different conductive plates (copper, 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