DEVELOPMENT OF A PORTABLE DIAGNOSTIC APPARATUS FOR COIL INSULATORS INLOW-VOLTAGE INDUCTION-MOTORS

When most organic insulators are exposed to thermal stress, they turn darker in color. The increase of electronic transition absorption due to thermal oxidation of the resin is responsible for this darker color ation. A correlation between the degree of aging of these insulators a nd their reflective absorbances in the near IR (infrared) wavelength r ange has been established previously. In this paper, a novel non-destr uctive diagnostic apparatus using an optical fiber sensor developed to evaluate the aging of thermally deteriorated insulators is reported. The key point of this technique is measuring the change of reflective absorbance ratio at two different wavelengths. The sensor consists of two plastic optical fibers (for light transmitting and receiving), a p lastic optical coupler, two kinds of near-IR light emitting diodes as light sources, and an optical power meter as a detector, and hence is very compact and lightweight (< 2 kg). The aging of an insulator can b e estimated using the diagnostic curve, which is obtained by an accele rated heating experiment using model pieces. We applied this measureme nt technique to predict the residual life of low-voltage induction mot ors used in elevators.