Inverter-fed drives of form-wound motors become essential for the high efficiency operation in the industrial field. It has been revealed that the high frequency repetitive pulses cause the local heating and electric field enhancement at the end-turn stress grading (SG) system, however, few paper deals with the stresses under the pulse width modulation (PWM) waveforms. In this research, potential distributions, electric field and time-averaged power densities under the PWM waveforms are numerically investigated, as functions of the carrier and the fundamental frequency. As a result, the power densities increase with the fundamental and the carrier frequency, whereas the field at the SG system is almost constant, regardless of the both frequency component. The estimation method of the power density and resulting temperature rise under the PWM is also proposed.
For focusing electrodes of X-ray tube, a high voltage performance in vacuum is required from an initial conditioning process. In order to obtain a high voltage performance in a vacuum, it is necessary to clarify the breakdown factor and its mechanism for a vacuum electrode gap in the initial conditioning process. In this study, a vacuum breakdown test was performed using pure nickel electrodes that had been polished and heat-treated at various temperatures, and we investigated the relationship between the local contamination point on the electrode surface and the dielectric breakdown point. As a result, it was clarified that the high voltage performance was improved by smoothing the electrode surface using both magnetic polishing and electro polishing. With carefully observing the electrode surface by SEM, it was found that the trace of breakdown events appeared in the local contaminated point after several breakdown shots of the initial stage of conditioning. Moreover, it was found that the vacuum breakdown appeared in the edge of local contaminated area. It was suggested that the interface between the electrode, vacuum, and the local contaminated point was one of the weak point of electrical insulation.
In general, cathode triple junctions (CTJ) are regarded as electrical weakness points because of the occurrence of electron emission. It's believed that vacuum surface flashover starts from the initiation of electron emission at CTJ. Therefore, it is important to investigate electron emission phenomenon around the CTJ region. However, such a study which is direct measurement of the electron emission from CTJ has not been almost done because it is difficult to measure it directly. So the purpose of this study is to elucidate the electron emission phenomenon from CTJ by investigating field electron emission current characteristics and charge distribution at CTJ region. Thus, we measured the electron emission current and the surface potential for the samples which CTJ was formed artificially. In addition, we simulated the electric lines of force near CTJ region. The results show that electron emission current increases due to the effect of the insulator surface being charged by the electrons emitted from the CTJ.
To apply dissolved gas analysis (DGA) for ester insulating oils, the gas generation characteristics of various ester insulating oils were studied under the partial discharges (PDs) and arc discharges. The test vessel was made for qualification of dissolved gas, and both PD and arc energy measuring devices were developed to estimate the discharge energy. The oil samples containing the dissolved gases were sampled and compared with before and after the discharge test, namely, these samples were analyzed by gas chromatograph. The analysis revealed the type and amount of dissolved gases caused by the discharges. In the paper, new diagnosis methods based on DGA are proposed to find abnormal conditions such as PDs and arc discharges in ester oil filled transformers.
Ti-doped vanadium dioxide (V0.75Ti0.25O2) thin films were fabricated by metal-organic decomposition (MOD). Precursor films were fabricated at a temperature of Tp = 300 ºC for 15 min in a N2 atmosphere. Then, these precursor films were fired at Tf = 520~600 ºC for tf = 15~45 min in a N2 atmosphere. From X-ray diffraction (XRD) patterns of the fired films, diffraction peaks indicating VO2(M) phase were observed with a wide range of Tf = 540~600 ºC. Furthermore, the peaks indicating VO2(M) phase were also observed with a wide range of tf = 15~45 min. Regarding the surface morphology of the films, the nanoparticles of 100~200 nm were densely packed. From R-T characteristics of the films, the phase transition with a rapid resistance change and hysteresis loop disappeared completely at Tf = 560~600 ºC. Relatively flat temperature dependence of the temperature coefficients of resistance (TCR) as high as -3.2~-6.2 %/K at the temperature range from 20 ºC to 80 ºC was obtained for the film fired at Tf = 580 ºC. From these experimental results, it was found that wide windows of firing conditions for fabricating V0.75Ti0.25O2 thin film can be obtained in the MOD process.
The air discharge currents from two humans having different physique with charging voltages of ±4 kV and ±8 kV are measured and compared to reveal that the same relationship as our previously published result holds between the peaks, spark lengths and potential gradients. The relationship is quantitatively shown by analyzing a simplified equivalent circuit for human air discharge based on the IEC standard.
The air discharge currents from an ESD generator were measured at charging voltages from 2 kV to 15 kV in an environment with different temperature and humidity. Dependence of the current peaks on breakdown potential gradients in addition to spark lengths is shown, and is calculated using a simplified equivalent circuit based on the IEC standard for the ESD generator.