Abstract :
It has been suggested frequently that the failure of high-voltage armature bars might be due to deterioration caused by gaseous ionization in entrapped air spaces. In an earlier paper a series of tests on a number of 6600-vollage mica folium armature bars made up with different degrees of mica content was described. The variations of the dielectric losses with voltage and with temper lure were studied and by means of the application of pressure it was shown to what extent losses due to internal ionization were present. The influence of these ionization losses on the life of the bars was also studied. The tests of the foregoing paper are continued in the present paper, extending to a wider range of type of armature insulation. The general results are as follows: 1. The absolute values of loss due to internal ionization in well constructed armature bars are small compared with dielectric losses of other types. 2. The losses due to internal ionization do, however, cause a progressive deterioration of the insulation. This is shown by a gradual increase in the loss and power factor of the insulation. 3. It is indicated that the principal function of mica in this type of insulation is in the reduction of the conductivity of the insulation and the withstanding of the action of internal ionization. The indications are that full mica folium content can be safely reduced only by use of the best grade of mica folium and the best conditions of application without variation. Several bars having relatively low mica content have compared favorably with those having the maximum mica content, indicating reliable insulation over long periods. It appears that high mica content is necessary in order to maintain a high factor of safety to cover the variations in factory processes. 4. In the drying out period this type of insulation is subject to great danger from relatively brief periods of application of voltage. During these periods the losses are high, internal temperature is raised- with further increase of loss, leading to breakdown. The greatest increase of loss in this danger period is in the range 25 deg. to 50 deg. cent. 5. The drying out period at 125 deg. cent. and during which normal voltage may not be applied to the insulation for more than a minute or two, varied in the specimens studied from fifteen days to three months.
