Thermal Endurance of Polyester Enameled Wires Using Twisted Wire Specimens: Statistical Analysis of Final Test Results

Thermal endurance tests to AIEE Standard 57 [1] have been performed in six laboratories each using self-prepared varnished specimens made from a common sample of polyester enameled wire and polyester varnish. The results have been analyzed to test the hypothesis of the departure from linearity of the relationship between the logarithm of life and the inverse of the absolute temperature. A comparison is made between the experimental results at the lowest test temperature and the corresponding extrapolated values based on the experimental results at the higher temperatures. The variation between the results of different laboratories is considered. Comparison is made with previous work [2] undertaken in six laboratories on PVA enameled wire specimens, prepared and impregnated with an oil-modified phenolic varnish by a single laboratory, and with published AIEE results. Comparison is also made with the results of a previous interim statistical analysis, [3] applied to the test results at the three higher temperatures only. The main statistical analysis is performed both for the complete set of results, and for the median results only. The extrapolated thermal lives based on the two methods are shown, with one exception, to differ by less than 6 percent at 20°C below the lowest test temperature. The importance of the median method is that generally not all of the specimens are required to be tested to failure, with a consequent saving of a substantial amount of testing time, particularly at the lowest test temperature.