Short term voltage withstand for testing of uncured high voltage form wound coil insulation

Abstract

With the current trend away from fully cured insulation materials to uncured low binder mica tapes, a need to develop adequate proof testing methods during taping and stator winding has become increasingly evident. The motor/generator manufacturer or repair facility wants to test at as high a voltage as possible to detect faults but without risk of creating damage to the insulation. Too many end users of mica ground wall insulation continue to proof test using methods adaptable to material construction prior to the introduction of VPI processing. Since then (circa 1960) mica tape has been engineered by several manufacturers specifically for VPI. Several changes have been introduced to optimize resin impregnation and retention post cure. Lower dielectric breakdown values are initially obtained in the “green” or unprocessed state precure. Post cure the breakdown minimum frequently exceed 10 X rated voltage DC. It is necessary to proof test in the uncured state at lower values. This paper will report reasons for this necessity. Also included will be possible alternatives. Work will be continued to establish a maximum safe limit for uncured short term voltage withstand.