The effect of heat on the electric strength of some commercial insulating materials

Of the three principal constituents of electrical machinery the greatest improvements in the future are to be looked for in the insulating materials. A knowledge of the effect of heat upon the electrical and mechanical characteristics of insulating materials is of the greatest practical importance. An investigation has been carried out to ascertain the variation in electric strength between temperatures of 30° C. and 100° C. of many of the solid insulating materials at present employed in the manufacture of electrical machinery and apparatus. These tests show that in the case of some of the materials used at present the breakdown voltage at 100° C. may be only one-half that at 30° C. Except in the case of certain mica products the electric strength of all the materials was found to decrease appreciably with increase in temperature. Consequently, all insulating materials should be judged and used in accordance with the breakdown voltage at 100° C. and not on the results of tests carried out at air temperature. Further research work would appear desirable: (a) to investigate other materials on the lines of these tests, and also the effect of heat on the mechanical properties; and (b) to develop new insulating materials with better electrical and mechanical properties at temperatures likely to be attained in modern electrical machinery.