Some relations of thermal cycling to thermal life temperature classification of enameled wire determined by AIEE 57 test procedures

The fact that thermal cycling rate has a definite effect on life values obtained on enameled wires using the AIEE No. 57 test procedure has long been recognized. For each thermal classification of enameled wires AIEE No. 57 has a proposed schedule of cycling times for each test temperature. These cycling times are designed to give each set of samples an average of ten thermal cycles before failure. These are only approximate and assume a certain life-temperature slope. In section 2-2 of AIEE No. 57 the following is suggested concerning cycles: "Thermal life values obtained from test specimens subjected to an average of less than 8 or to more than 20 cycles at the exposure temperature may not be reliable and should not be used to predict the temperature rating of the enameled wire. Therefore, the cycle times may be increased or decreased during the aging, to assure that the average number of cycles to failure fall within this range. For example, if a set of test specimens has been exposed for 8 cycles and less than half have failed, the exposure time should be increased to the next longer time given in the vertical columns of Table II." Experience of people using this test has indicated that a geometric progression of cycling time in cases such as this example, is better insurance against over-cycling samples, i.e., doubling the cycling time with each test. A similar decrease in cycling times can be used when samples are failing too rapidly. It is considered important to standardize on approximately 10 cycles to failure at each temperature for the following reasons: 1. So that data obtained at different laboratories can be compared. 2. So that the slope of the life-temperature curve is not affected by varying number of cycles to failure at the different temperatures tested.