Gaseous Ionization in Built up Insulation

A study of a series of 6600-volt armature bars with different types of insulation, with special reference to the influence of entrapped air spaces on the dielectric losses, power factor, and life of th. e insulation. The variation of the losses between the green and dried states was studied. Gaseous ionization losses begin at about 4000-volts, above which the losses increase more rapidly than as the square of the voltage. In well wrapped mica foliun insulation from 10 per cent to 40 per cent of the total dielectric losses are due to internal ionization. Ionization losses, while not being of sufficient amount to materially increase the temperature of the coil, do, however, cause progressive deterioration of the insulation. The total dielectric losses in mica folium and paper insulation increases with decreasing mica content. Reducing the mica folium by 75 per cent and substituting shellaced paper approximately doubles the loss. Omission of the final 25 per cent of mica increases the original loss ten times. The loss in an all paper bar is probably almost entirely due to conductivity. The principal function of mica in this type of insulation is the reduction of the conductivity, and withstanding the action of internal ionization.