A method for evaluation of thermal stability of magnet wire enamel

A method for evaluating the thermal stability of magnet wire enamel has been developed using the dielectric strength of the enamel film as a criterion. This test method was designed for accuracy, simplicity, and economy in time, labor, materials, and equipment. Standard National Electrical Manufacturers Association (NEMA) dielectric strength twists1 are dipped in an electrical impregnating varnish and used as test samples. To achieve sufficient accuracy, a large number of these twists are included in an enamel evaluation program. The twists are placed in aging ovens at various temperatures. Periodically the samples are removed and tested by applying a high voltage. The average aging time required to reduce the dielectric strength of the twists to a preselected value is considered the life of the enamel. The relative life of the enameled wire insulation system over a wide temperature range may then be predicted from the life of the samples determined at several temperatures. Studies have been made of this test method under different sample preparation methods, failure criteria, and aging conditions. Results determined by this test are compared with motor-aging test results on the same enamel-impregnating varnish insulation systems. This comparison shows that this is a convenient functional test method that can be correlated with the results of a more complicated motor test program.