The dielectric strength of air films entrapped in solid insulation and a practical application of the problem for alternator coils and cables

It is of theoretical importance, as well as of practical importance in design, to be able to predetermine the dielectric strength of air films or bubbles entrapped in solid insulation. If such films are over-stressed the solid insulation may be damaged by chemical action, local heating, mechanical action etc., due to corona. An experimental investigation was made of the strength of air films of various thicknesses between glass plates. In the arrangement used, the breakdown of the air or the starting point of corona could be readily observed. Tests were also made of the dielectric strength of air films between other solid insulations. It was found that the dielectric strength of air films between insulations was practically the same as the dielectric strength of air films between conductors. The second part of the paper is devoted to the practical application of the data to the design of armature coils and cables, and several specific examples are given.