Specific heats of a magnet wire varnish-enamel combination as a measure of thermal stability of the insulation system

摘要

In magnet wires the thermal stability of the insulated wire is gauged by tests of twisted-paired conductors (ASTM D2307-64T). This determines the thermal life rating of the wire, based upon the Arrhenius equation and the assumption of a first order degradation. S. D. Bruck determined the activation energies for decomposition of a polyimide film to be 33 kcal/mole in air and 72 kcal/mole in vacuum. It is seen that thermal stability is also a function of the environment (air, vacuum). If the polyimide magnet wire is exposed to a steam environment (120°C, 15 psig), the life of the film as determined by an 1100 volt breakdown test is of the order of 103 hours. The life of the polyimide in air is of the order of 20,000 hours at 220 °C. It is evident that the activation energy of the polymide film in a steam environment would thus be far lower than its value of 33 kcal/mole in air.