FAAA8C90d01 - 290 IEEE Transactions on Dielectrics and...

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290 IEEE Transactions on Dielectrics and Electrical Insulation Vol. 5 No. 2, April 1998 evelopment of a Portable Diagnostic Induction Motors Apparatus for Coil Insulators in Low-volta Y. Takezawa, Y. ltoh Hitachi Research Laboratory, Hitachi, Lid, Ibaraki, Japan M. Shimodera, H. Miya Hitachi Building Systems Engineering Co., Ltd., Tokyo, Japan. ABSTRACT When most organic insulators are exposed to thermal stress, they turn darker in color. The in- crease of electronic transition absorption due to thermal oxidation of the resin is responsible for this darker coloration. A correlation between the degree of aging of these insulators and their reflective absorbances in the near IR (infrared) wavelength range has been established previ- ously. In this paper, a novel non-destructive 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 wave- lengths. The sensor consists of two plastic optical fibers (for light transmitting and receiving), a plastic optical coupler, two kinds of near-IR light emitting diodes as light sources, and an op- tical power meter as a detector, and hence is very compact and lightweight (< 2 kg). The aging of an insulator can be estimated using the diagnostic curve, which is obtained by an accelerated heating experiment using model pieces. We applied this measurement technique to predict the residual life of low-voltage induction motors used in elevators. 1 INTRODUCTION N the field of low-voltage induction motors, conventional electric I diagnosis such as the corona discharge method is not applicable, because there is hardly any change before and after aging. We have worked from the viewpoint of chemical structure change of aged in- sulating resins, and have studied a new measuring method to diag- nose deterioration of the insulator nondestructively [l-31. Generally, when most organic insulators are exposed to thermal stress, they turn darker in color. The increase of electronic transition absorption a, due to thermal oxidation of the resin is responsible for the darker coloration [l]. The spectrum has a peak in the uv (ultraviolet) range, and de- creases exponentially with an increase in wavelength. For aged resins, the strength of cy, in the uv and visible range is too large for an ordinary spectrophotometer to measure. We derived a relationship between the extent of aging of these resins used in low-voltage induction motors and their optical absorption losses in the near-IR wavelength range. Then, we devised a method to evaluate aging by measuring the tail absorp- tion of the spectrum peaking in the UV range. For transparent in- sulating resins, the extent of aging can be estimated through the optical absorption loss per unit optical path length [2]. However, most electri- cal insulators are not transparent but opaque, containing pigment, alu- mina fillers, etc.
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FAAA8C90d01 - 290 IEEE Transactions on Dielectrics and...

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