Thermal deterioration diagnosis by optical fiber sensors for mica-epoxy insulation of HV induction motors

Optical diagnosis for aged mica-epoxy insulation of HV (3.0 to 6.6 kV), enclosed induction motors used in compressors has been studied. The developed optical fiber sensor consists of two optical fiber cables (one for light transmission, the other for light reception; each ~3 m long), two types of laser diodes as light sources, and an optical power meter as a detector. It was used to measure the change ΔA_R in the reflective absorbance of insulation at two wavelengths for model pieces and samples obtained from three aged induction motors. This optical sensor could detect only deterioration on the surface layer as a ΔA_R change, and thus deterioration of the inner layer was evaluated by the peak temperature of tan δ (T_g,DMA) in dynamic mechanical analysis (DMA). The T_g,DMA of insulation is one of the most important parameters which can be used to evaluate material lifetimes, because it relates to elasticity. The ΔA_R correlated with the T_g,DMA of DMA, and the correlation coefficient of an accelerated experiment using model pieces was ~0.79. Thus, this optical diagnosis can be applied to evaluate the deterioration of mica-epoxy insulation of HV induction motors. The extent of insulation deterioration can be calculated quantitatively using the diagnostic master curve, which is obtained from accelerated heating experiments based on chemical kinetics