Insulation Resistance and Dielectric-Absorption Characteristics of Large A-C Stator Windings

Abstract
Insulation-resistance and dielectric-absorption data have been presented which give a comprehensive picture of average and normal spread in insulation characteristics of new large a-c synchronous and induction machines having mica insulation. Dielectric-absorption characteristics have been analyzed. Average curves and normal variations are given. Ratio of insulation resistance at 25 degrees centigrade as measured after ten minutes and after one minute of voltage application has been found to increase from one, for relatively moist windings, to approximately four, for moderately dry windings, and then to remain substantially constant over a wide range in insulation resistivity. Ratio for well-dried insulation is of the same order. Average temperature-insulation resistance characteristics for machines based on data available are given. Temperature-insulation resistance characteristics of well-dried asphalt-bonded mica insulation are given as a function of various times of voltage applications. Indications are that variation of absorptive current with temperature for asphaltic-bonded mica insulation does not even approximate a simple logarithmic function. Considerations involved in the proper selectin of a safe minimum value of insulation resistance are discussed. A linear formula which takes into account effects of speed, kilovolt-ampere rating, and voltage rating, with adequate precision when consideration is given to other variations involved, is suggested for large class B a-c induction and synchronous machines. Work is being continued on the study of variations involved and significance which may be attached to insulation-resistance measurements.

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