Apparatus and Procedures for Evaluating Dielectric Properties of Wire Enamels in Controlled Environments

Methods are described for measuring intrinsic dielectric properties of wire enamels and wire enamel-varnish combinations in controlled environments. Random wound bifilar coils serve as test specimens and environment control is achieved by suspending these coils in a pressure tube connected to a gas handling system. Dielectric properties have been measured at pressures ranging from vacuum to 32 atmospheres and at temperatures from -40° to 350°C. Typical results are reported which illustrate utility of the apparatus and procedures. The importance of measuring dielectric properties at elevated temperatures is stressed and a method for establishing a practical thermal limit tmax is described. It is demonstrated that in a controlled environment, tmax, is a characteristic intrinsic property of an insulating material. Interpretation of typical conductance-temperature data according to the theory of rate processes, suggests that tmax corresponds to a temperature at which the polymer undergoes significant changes in viscoelastic behavior. There is evidence that the mechanism of ac conductance below tmax is by polarization, while above tmax conductance results from ion movement. The influence on tmax of degree of cure, frequency of the applied voltage, and nature of the atmosphere surrounding the insulating material, is discussed.