Polarization and Depolarization Current Characteristics of Thermally Stressed Stator Coils

When the motor is exposed to thermal stress more than its limit, its insulation characteristic degrades and/or deteriorate. This paper presents the characteristic of stator coils under thermal stress by analyzing the polarization and depolarization current (PDC) characteristics. This research used a slot part of stator coils, which were obtained from a synchronous motor rated 6.6 kV with class F (155°C) insulation. Twelve test samples of motor stator coils were divided into four groups depending on the stress temperature levels, 100°C, 140°C, 180°C, and 220°C. Each specimen was heated at the specified temperature for four cycles (6 hours per cycle). To analyze the insulation characteristic of the tested specimen, PDC measurement was performed. It was found that the PDC magnitude decreased related to the number of thermal cycles and temperature levels, as described in this paper.