Long-term aging experiments conducted to examine mechanisms which could degrade a stator conductor slot stress control system are described. Models consisting of two conductor bars in a short length of core, single conductor bars, and other arrangements were used to investigate the development of slot discharge and compare the performances of various materials. Damage was found to start in the junction region between the slot stress control and endwinding stress control layer. Removal of the low-conductivity paint at the junction region led to surface discharge, which caused the consequent further erosion of the paint and led to complete removal of the slot stress control layer. The cause of the initial removal of paint is thought to be the action of ozone on the binding resin used in the paint. The ozone arose from discharge on bars in the test vessel or on an endwinding stress control layer. An improved version of the paint was shown to perform well, but the best results have been obtained with a low-conductivity molded tape layer. Such damage did not appear to develop in a hydrogen atmosphere.
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