High Frequency Dielectric Properties and Losses in Partial Discharge Resistant Rotating Machine Insulation

The complex permittivity of glass-fiber and polyimide backed mica tape for winding insulation in rotating machines have been investigated using a dielectric spectrometer. A frequency ranging from 10
^‐2
Hz to 10
⁶
Hz and a temperature ranging from 20 °C to 150 °C was applied to both pristine samples and samples aged for one week at 150 °C and 200 °C, respectively. The real part of permittivity, real part of conductivity and loss factor increased for decreasing frequencies and increasing temperatures. For both tape materials, a significant decrease was seen after one week at high temperature, but for the polyimide tape the change was less significant at 150 °C. This temperature is below the thermal class of the insulation. The observed decrease in the real part of permittivity, and hence decreased loss factor is possibly due to a continuing curing of the resin in the insulation system.