Aging Characteristics of Inter-turn Insulation of Form-wound Stator Windings of Inverter-fed AC Motors

Phase-resolved partial discharge patterns of an inter-turn insulation specimen for a stator winding are investigated at different aging times under continuous square-wave pulse voltage. Comparing phase-resolved partial discharge patterns after square-wave pulse-voltage aging to those after sinusoidal-voltage aging, obvious independent clusters of discharge numbers occur with increases in the aging time of pulse voltage, whereas no similar phenomenon is observed after sinusoidal-voltage aging. Based on detailed analysis, partial discharge activity in a larger void is more intense, because the larger void has a larger memory effect, which is related to the number of residual charges. The enhanced electric field of the larger void accelerates the increase in void size. Furthermore, when the phase-resolved partial discharge patterns are measured under 50 Hz of sinusoidal voltage, since sinusoidal voltage creates less memory effect due to less residual charges, the corresponding partial discharges of different-sized voids occur at distinctly different phases of the sinusoidal voltage. For this reason, the independent clusters occur and become obvious with pulse-voltage aging. As a result, for pulse-width-modulated motors in industry, it is possible to evaluate the insulation condition through phase-resolved partial discharge patterns using 50 Hz partial discharge measurement.