Dielectric response during curing of a resin-rich insulation system for rotating machines

Pre-impregnated (resin-rich) mica tape based insulation is used as electrical insulation in rotating machines. The dielectric properties of this system change drastically during the curing of the resin. We report on measurements of capacitance and loss, in the frequency range of 10 mHz to 1 MHz, on insulation models during the curing process. In order to relate the degree of curing to the dielectric response and to understand more about the curing reaction, differential scanning calorimetry (DSC) measurements were made. The parameters in a simple thermal activated model for the degree of cross-linking were estimated from DSC data. From shifting the dielectric data measured at different temperatures it is concluded that there are different activation energies for the uncured and the ready cured samples. Two simple equivalent circuit models are proposed to describe the dielectric behaviour. The conclusion is that dielectric response measurements can be used to monitor the curing process and evaluate the final degree of curing.