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In this article, the impact of the waveform switching frequency (SWF), overshoot (OS), and rise time on the time-to-failure and the endurance of Type II machine insulation is investigated. Reference life curves at 1 and 4 kHz SWF are derived utilizing a unipolar repetitive square-impulse of 15% OS and 300 ns rise time. The effects of the impulse OS and rise time in the ranges of 0%–30% and 400–600 ns are also evaluated and reflected on the established reference life curves. Based on the endurance test results and the reported time-to-failure data, it is evident that the turn insulation is subjected to additional stress factors when energized by an inverter supply as opposed to the stresses exhibited by sinusoidal waves. This indicates an incomparable aging rate between the two types of waveforms, even at similar peak voltages applied, resulting in inadequate use of sinusoidal supply to qualify inverter-fed machines. Moreover, the OS component of the waveform is found to have a substantial impact on the endurance of turn insulation apart from the jump voltage impact, and therefore, is identified as a significant factor influencing the life of turn insulation independent of other factors. Moreover, the reported results of the time-to-failure analysis advocate a nonlinear relationship between the SWF and the life of turn insulation, whereas the waveform rise time is found to have a negligible impact within the considered range in this study. [ABSTRACT FROM AUTHOR]

In this article, the effect of repetitive square wave impulse voltage, switching frequency, rise time, and overshoot (OS), on partial discharges (PDs) within Type II turn-to-turn insulation is reported. Two types of samples are used in this study: a back-to-back turn insulation sample and a single cavity layered insulation. This work aims to analyze the influence of the square wave impulse voltage parameters on the characteristics of PDs during aging while relating their effect on the life of turn insulation. The experimental setup used in this study separates the effect of the waveform rise time from the OS and highlights the role of the OS rather than the jump voltage in aging. The results, within the range of parameters considered in the study, indicate that the number and magnitude of PDs are lower at a higher switching frequency while shorter rise time triggers fewer discharges of larger magnitudes. It is also found that PDs, both occurrences and magnitudes, are in favor of waveforms of higher OSs even when the waveforms have a similar jump voltage. This highlights the role of the OS, rather than the jump voltage, as a main and additional factor affecting the life of turn insulation which is contrary to what has been described in the International Electrotechnical Commission (IEC) 60034-18-42. Based on the average magnitude of PDs per unit time, the results show, in the reported range of parameters, that the switching frequency and OS of the voltage waveform are among the main parameters affecting turn insulation life, whereas the effect of the rise time is found to be minimal. [ABSTRACT FROM AUTHOR]

This paper investigates the tracking and erosion of silicone rubber and EPDM insulation in the DC inclined plane tracking and erosion test. Silicone rubber filled with alumina tri-hydrate fails due to combustion in the gas phase or erosion paths, but with no evidence of tracking. EPDM fails due to tracking and surface capturing fire, from dry-band arcing leaving a black residue while impinging the surface. Carbonaceous residue on EPDM gives rise to leakage current magnitude, whereas fused silica-based residue on silicone rubber is shown to promote combustion. A high amount of alumina tri-hydrate replacing the polymeric fuel in the composite is shown as a primary factor preventing failure and changing the nature of the residue in the DC inclined plane tracking and erosion test. Depolymerization is the main degradation mechanism reported for silicone rubber and EPDM, with notable degrees of crosslinking reported only for silicone rubber. Crosslinking reduces the amount of silicone rubber undergoing depolymerization, thus, the amount of fuel prone to combustion is reported to promote a coherent residue shield against the eroding dry-band arcing. [ABSTRACT FROM AUTHOR]

The paper presents the results of surface charge measurements on insulation film samples with different surface resistivities. Polytetrafluoroethylene (PTFE) and Nylon 6 (PA6) as well as mica paper subjected to unipolar and bipolar square waves at different switching frequencies. The experimental data are presented to explain the differences observed in the times-to-failure of turn insulation when aged using unipolar and bipolar square waves. Time-to-failure experiments and COMSOL? simulations are used to estimate the accumulated charge on the surface of insulation films with different surface resistivities. [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 provides an analysis of the thermal stresses in medium-voltage form-wound motor coils that arise from pulsed width modulated, repetitive square-wave, and impulse voltages. The effects of the shape, repetition rate, and rise and fall time of the pulses are studied through experimental and computational work. Based on the frequency spectra of the pulse trains and their corresponding signal powers, it is shown that a set of sinusoidal high-frequency voltages of predetermined amplitudes can effectively produce consistent thermal stresses, and both the experimental and simulation results confirm the validity of the proposed scheme. [ABSTRACT FROM PUBLISHER]