Electric field enhancement studies on stator windings with defect sites using finite-element computation

Several dissections performed on stator coils and cars following an accelerated voltage endurance test reveal failure initiation sites to be predominant at defect sites within the insulation. The types of defects observed include occlusions, particulate contamination, strand deformity, and the presence of tape folds. Such defects, when present at the corners of the stator coil, seriously affect the aging time (i.e., time-to-failure) in the voltage endurance test. Preliminary studies that use a finite-element technique to quantify the levels of stress enhancement on a coil when such sites are present in the system are reported. Three examples are described. The results show that the level of field enhancement could help explain the postulated failure modes that occur prematurely during accelerated voltage endurance tests.