Yuichi Tsuboi

Study on electric motors with bulk superconductors in the rotor

Abstract We have studied electric motors using YBCO bulk superconductors in the rotor. The superconductor can be used as a screening material, a magnetic hysteresis material, a flux source, or a secondary conductor, and form several types of motors. This paper describes experimental results of torque characteristics obtained with a small electric motor using a superconducting bulk rotor, which was operated in the synchronous and slip modes. Dependence of the torque on the stator three-phase current (gap flux density) and the slip frequency will be analyzed.

Torque characteristics of a motor using bulk superconductors in the rotor in the transient phase

A motor using bulk superconductors in the rotor (bulk superconducting motor) is considered to achieve higher power density than conventional machines. When the bulk superconducting motor is operated with slip, for example at startup or during overload operations, traveling electrical fields are applied to the bulk superconductors. In slip operation, the characteristics of the motor are complex because, under the traveling field, flux flow occurs in the bulk superconductors, which causes the increase in the current but at the same time generates heat, which decreases the critical current density of the bulk superconductors. In order to analyze the characteristics of the bulk superconducting motor in the transient phase and those of a bulk superconductor in traveling fields, pull out tests and locked rotor tests were performed using an experimental apparatus for a hysteresis motor with bulk superconductors in the rotor. Numerical analysis when operated with slip was also performed to know the electromagnetic phenomena in the bulk superconductor used in the motor when the effect of the heat is ignored.

The potential distribution measurement on the stress grading system of high-voltage rotating machines by using pockels effect

A surface potential measuring system was developed by utilizing Pockels sensor for measuring the potential distribution on the stress grading (SG) system of form wound motor coils. A Bi4Ge3O12 (BGO) Pockels crystal is put close to a measuring object and the capacitively-induced voltage on the end surface of the BGO crystal is measured by using Pockels effect of the crystal itself. The spatial resolution of the measuring system is 5 mm, and the frequency response reaches up to GHz-range. With this system, the surface potential distributions on a model bar coil were measured. According to the measuring results, it is shown that the newly developed potential measuring system here is very useful for evaluating the reliability of the SG system of motors.

Potential distribution on the stress grading system of high-voltage rotating machines –I measuring system

A surface potential measuring system has been developed utilizing a Pockels sensor to measure the potential distribution of the stress grading (SG) system of form wound motor coils. A Pockels BGO crystal is positioned just above (<;1 mm) the SG, and the capacitively-induced voltage on the surface of the BGO crystal adjacent to the SG is measured by using Pockels effect of the BGO crystal. The sensing system is applicable to surface potential measurement over wide frequency region from 0.001 Hz to 100 MHz. The spatial resolution of the measuring system after enhancement through deconvolution is discussed with the result that the -3 dB spatial frequency of the measuring system is 0.22 cycle/mm which implies a resolution in the range of 4 mm.

Electromagnetic phenomena in bulk superconductors applied in a rotating machine during transient state

Rotating machines using bulk superconductors in the rotor could achieve higher energy densities than conventional machines. There are several configurations of these motors. In order to understand the characteristics of these motors during the transient state, it is important to study the electromagnetic characteristics of bulk superconductors in transient electromagnetic fields. In this study, in order to better understand the influence of transientmagnetic fields in a rotating machine, torque characteristics of the motor using bulk superconductors in the rotor were examined.

Transient potential measurement on stress grading under multi-level PWM

In this paper, the influence of lower-frequency voltage components in PWM voltage on the performance of stress grading system of motor insulation is investigated by observing the light emission of partial discharge and by measuring potential distribution under 2- and 3-level PWM and sinusoidal ac voltages. Computations by finite element method are conducted to determine electrical field on coil surface. According to the results, the potential gradient under PWM voltage is largest during the first pulse after polarity reversal, as space charge of reversed polarity accumulates at the interface between corona armor tape (CAT) and stress grading tape (SGT) due to a fundamental frequency component in PWM voltage. Therefore, the intensity of light emission during the first pulse is the largest. From the computed results, it has become cleared the maximum field at the interface between the CAT and the SGT becomes higher under PWM voltage than that under rectangular ac voltage whose jump voltage is the same due to such charge accumulation.

Effects of ions and waveforms on water tree initiation and propagation

In this paper, we describe an investigation of the effects of ions and waveforms on water tree initiation and propagation utilizing a new water electrode method. The results indicate that ferric ions contribute to water tree initiation and propagation. Moreover, a water tree was initiated when the waveform of a pulse simulating an inverter waveform was applied for 2 weeks even at room temperature, in spite of no ac voltage application.

Fundamental study of a generator using bulk superconductors in the rotor

We have investigated a superconducting generator using bulk superconductors as strong flux sources. Bulk superconductors have flux conservation characteristics, and therefore the superconducting generator has specific and different features from a conventional one. In this study, three-phase sudden short-circuit tests are performed for analyzing fundamental characteristics of the bulk superconducting generator. The distribution of flux density in the gap is also measured for evaluating the influence of the transient currents to the trapped field on the surface of the rotor. In addition, numerical analysis is carried out for comparing the electromagnetic characteristics of superconducting and conventional generators.

Optimal resistance of stress grading system in converter-fed rotating machines

Excessive heat generation and occurrence of partial discharge have been observed in end-turn stress grading (SG) system in form-wound machines under PWM voltage. In this paper, multi-winding stress grading (SG) system is proposed as a method to change resistance of SG per length. Although the maximum field at the edge of stator and CAT are in a trade-off relationship, analytical results suggest that we can suppress field and excessive heat generation at both stator and CAT edges by multi-winding of SG and setting the length of CAT appropriately. This is also experimentally confirmed by measuring potential distribution of model bar-coil and observing partial discharge and temperature rise.

Surface corona inception on stress grading system in end-turn regions of rotating machines

The transients generated by drives based on pulse width modulation (PWM) have adverse effects on the insulation system of rotating machines, especially the stress grading (SG) system used in the endturn region. The surface corona inception voltage on SG system under repetitive impulse, 1kHz sinusoidal, and 1kHz square voltages was measured using a photomultiplier connected to optical fiber bundle, and the evolution of corona was observed using a ICCD camera. Four samples were evaluated to assess process-induced variations in performance. The macroscopic repetitive corona inception field was computed for each waveform from the measured surface corona initiation voltage on the SG system and was 25% to 50% of critical field for gas ionization.