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The thermal and electrical characteristics of a stress grading system are studied by simulations and measurements of the electric field and temperature distributions in the overhang region of a form wound medium voltage motor coil. Simulations show that a highly conductive stress grading tape (SGT) reduces the maximum electric field in this region but increases heat production and temperature. In contrast, a highly conductive armor tape (CAT) decreases the maximum electric field in this region and also reduces the heat production and temperature in the stress grading system but also produces a slight increase in the electric field in the SGT region. For the simulation studies, accurate measurements of the electrical and thermal conductivities of materials are made, taking into account the effects of the vacuum pressure impregnation (VPI) process, temperature, and tape builds. The electrical conductivity of single and double half-lap layers of CAT and SGT are measured from 30 to 100 °C. For the evaluation of the simulation model, the temperature profile along the stress grading system was measured and simulated under pulsed voltage at room and near typical operating temperatures. [ABSTRACT FROM AUTHOR]

The lifespans of enamelled turn insulation samples are studied when stressed with square waves characterized by different rise times, duty cycles, switching frequencies, and applied voltages. Partial discharge activities under these test conditions have also been recorded using an RF antenna in an effort to correlate the time to breakdown with material degradation under fast repetitive pulse aging. The aging and PD features were independent of rise time for a 50 % duty cycle; however, slightly faster times to failure were observed at a 15 % duty cycle for the faster rise times, for the considered range. The higher duty cycle and the higher number of applied cycles accelerated the aging. [ABSTRACT FROM AUTHOR]

The paper addresses the temperature and electric field dependence on the conductivity of the conductive armor and stress grading tapes on form-wound motor coils. The temperature and electric field dependence is measured in the laboratory, and the nonlinear dependency is used in transient FEM simulations of the potential distribution on the form-wound coils. The surface potential distribution, for various temperatures and applied voltages, is measured in the laboratory from which the surface electric stress is calculated. Good agreement between the simulated and measured stresses is obtained. A temperature of 155 °C is shown to have little influence on the stress grading, but the electric stress dependence on conductivity is shown to have a major influence on the stress grading of the motor coil examined in this study. [ABSTRACT FROM AUTHOR]

In this paper, the degradation processes in stator-bar groundwall insulation that is subjected to steep-front voltage pulses, at different temperatures, are studied. The test conditions include pulse voltage amplitudes up to 6 kVp and pulse switching frequencies up to 3 kHz in ambient temperatures of 23°C, 120 °C, and 155 ° C. During the aging process, the surface temperature is monitored with an infrared camera to determine the regions with highest temperatures and the partial discharge (PD) activity. The visual examination of failed bars confirmed that the groundwall insulation in these regions is degraded. [ABSTRACT FROM AUTHOR]

The paper presents the experimental results on accelerated aging of enameled wires with and without nanofiller in the coating under partial discharge (PD) activity. The residual life of the wires, and the surface roughness as measured by a scanning electron microscope (SEM), are used to evaluate the effects of the nanofihler. The surface erosion in the specimens without nanofiller is considerably greater, which is 1120-1270 nm, under pulse aging; whereas, in specimens with ~1%, by wt, fumed silica (SiO2) nanofiller it was measures to be 230-250 nm. An evaluation of the residual life of the wires, aged under high frequency ac waveforms, reveal that the wires with SiO2 achieved a life that is twice the life of conventional wires. The two-parameter Weibull distribution of dielectric strength data show that the value of the shape parameter, β, increases from 2.6 to 11.4 for the coatings with nanofillers indicating that the material becomes more homogeneous and exhibits fewer defects. Finally, the paper discusses possible reasons for the reduction in surface erosion and improvements in breakdown strength of enameled wires filled with various types and concentrations of nanofillers. [ABSTRACT FROM AUTHOR]