Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

DOI： 10.1109/tasc.2021.3136801

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Publishing type：Research paper (scientific journal)  

A novel magnetizing technique for permanent magnets (PMs) embedded in interior permanent magnet (IPM) motors has been proposed as a safer and more effective process than the conventional processes using copper pulse-coils. In the new process, the high temperature superconducting (HTS) bulk magnet, emitting an intense magnetic field of over 3 T, was employed to activate the rotor and PMs inside. A magnetic pole containing the HTS bulk magnet was scanned on the surface of the IPM rotor, resisting against the extremely strong attractive force between the magnet and rotor. Following experiments in the past, numerical simulation studies have been conducted to evaluate the magnetic field-trapping performances of the PMs. In this study, the numerical analysis of the magnetization behaviours was conducted for the PMs embedded in the rotor of the commercial air-conditioner motor in a hybrid car. The simulation results showed us that the demagnetized Nd-Fe-B plates in the rotor were fully magnetized in the intense static magnetic fields on the magnetic pole containing the HTS bulk magnet. The magnetizing behaviour in this peculiar magnetizing operation was made understandable by estimating the anisotropic easy-direction of the PM plates embedded in the rotor. The novel activation method for PM is characterized as a no-shock and no-heating method, which would promote the degree of freedom in the IPM motor design.

DOI： 10.1088/1361-6668/ab9543

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Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

Abstract

The magnetic flux capturing of MgB₂ bulk magnets made by spark plasma sintering process has been precisely investigated to clarify the mechanism of flux motions during the pulse-field magnetization processes. The field trapping ratio B_T/B_P was evaluated as a key parameter of field trapping ability which strongly relates to the heat generation due to the rapid flux motion in the samples. The time dependence of magnetic flux density revealed the actual flux motion which penetrated the samples. The trapped fields B_T and field trapping ratios B_T/B_P of various samples were classified into three regions of ‘no flux flow’, ‘fast flux flow’ and ‘flux jump’ according to the generation of heat and its propagation. A flux jump was observed late at 280 ms from the beginning of PFM process, while the field penetration B_P showed its peak at 10 ms. Considering the heat propagation speed, the long-delayed flux jump should be attributed to the macroscopic barriers against the heat propagation to the surface centre of bulk magnet.

DOI： 10.1088/1742-6596/1559/1/012024

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Other Link： https://iopscience.iop.org/article/10.1088/1742-6596/1559/1/012024

Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

DOI： 10.1109/tasc.2020.2966421

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Publishing type：Research paper (scientific journal)  

A unique activation technique for permanent Nd-Fe-B magnets which were embedded in the rotors of interior permanent magnet (IPM) motors has been developed as a magnetizing tool with use of high temperature superconducting (HTS) bulk magnet. The experimental and numerical simulation studies were conducted to evaluate the magnetic field-trapping performances of the rotor of air-conditioner compressor in the hybrid-type automobiles. The sample rotor containing six demagnetized permanent magnet plates were exposed in the intense static magnetic fields above the magnetic pole which contained the bulk magnet generating over 4 T. As a result, the sample magnets were perfectly magnetized in the static magnetic field. The precise simulation on the flux distribution in the rotor clarified that it is important to make the flux direction in the rotor core and the easy magnetization axis of PM identical. We convinced this activation technique should enable us to promote the degrees of freedom of motor designing and its processing.

DOI： 10.1109/TASC.2020.2989181

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Publishing type：Research paper (scientific journal)  

© 2020 The Author(s). Published by IOP Publishing Ltd. Nuclear magnetic resonance (NMR) magnets generally need an extremely uniform magnetic field in the space where the specimens will be placed. Although high temperature superconducting bulk magnets can obtain an intense magnetic field of more than 3 T, the field distribution is intrinsically characterized to be inhomogeneous, showing steep gradient. Aiming at the practical application of compact and portable NMR magnets, the authors have developed magnet systems capable of generating uniform magnetic fields in a narrow space between face-to-face settled magnetic poles, targeting to obtain a uniform magnetic field between the poles. Here, the authors modified the shape of magnetic field distribution from convex to concave by attaching a ferromagnetic iron plate on one of the pole surfaces, and then settled them face to face with a gap of 70 mm. The uniformity of the magnetic field in the x-y plane was experimentally measured and estimated along various z-axis directions. The best uniformity of 463 ppm at 1.25 T was obtained in the experimental evaluation in the x-y plane of 4 4 mm2 at z = 0.5 mm from the iron plate surface. The value of uniformity along the z-direction range was extended from 0.8 mm to a depth of 1.1 mm. These results show that the possible space of NMR signal detection or the available sample size can be enlarged.

DOI： 10.1088/2053-1591/ab8e6d

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Language：English   Publisher：Japanese Society for Engineering Education  

The technology of superconductivity is regarded as the most developed in the world due to the efforts of Japanese scientists who invented multiple novel and efficient superconductors. A characteristic research activity titled international collaboration aiming at reducing ac energy loss of superconducting materials has been carried on through the international network with respect to the extreme technologies such as large current, intense magnetic field and super-high pressure. To nurture young researchers, the international collaboration project has been conducted with an assistance of Japanese government. A characteristic international collaboration project has been carried on for three years in cooperation with the European forefront institutes and promoted the research activities of young Japanese researchers to the world top level.

DOI： 10.20549/jseeen.2020.0_34

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers Inc.  

This paper reports the design, fabrication, and test results of a 50-kW-class fully superconducting induction/synchronous motor for transportation equipment. Its maximum speed is 1800 rpm, and both rotor and stator windings were fabricated using Bismuth strontium calcium copper oxide tapes. The so-called ring-winding structure was used for the stator windings to improve the critical current of the stator windings in the core. The fabricated motor was immersed in liquid nitrogen and tested using a commercial pulse width modulation inverter. We successfully carried out a 3-kW load test at a synchronous speed of 1800 rpm. Furthermore, a variable speed test under light load conditions was also completed.

DOI： 10.1109/TASC.2019.2902504

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1109/TASC.2019.2902692

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1109/TASC.2019.2908752

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1109/TASC.2019.2898237

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1109/TASC.2019.2901213

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1109/TASC.2018.2867784

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1109/TASC.2018.2792530

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1109/TASC.2017.2776923

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1109/TASC.2017.2786708

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Publishing type：Part of collection (book)  

Various types of strong magnetic field generators with use of HTS bulk magnets and the small-scale refrigerators have been actually proposed and experimentally estimated for the feasible applications. The most characteristic feature of bulk magnets is regarded as compact and strong field generators with respect to their magnetic field spaces or their machine structures of themselves. Among them, the magnetic flux density of the face-toface magnetic poles reached 3.2 T and 4.4 T when they were activated by the pulsed field magnetization and the field cooling method, respectively. Furthermore, a very small bulk magnet generating 2.8 T and the system featured by the wide magnetic poles containing seven magnets have been constructed with use of ST pulse tube coolers in order to expand the feasible application areas. On the other hand, although the bulk magnets are characterized by their steep field distributions, we attempted to gain the uniform magnetic field space, and successfully achieved the uniformity of 385 ppm in 4 mm range by combining the deformed magnetic field profiles which generated between the face-to-face magnetic poles. The performances of these unique systems have exceeded those of the conventional electromagnet or any permanent magnets by far, demonstrating the superiority to other strong magnetic field generators.

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1109/TASC.2016.2639298

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1109/TASC.2017.2651099

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Language：Japanese   Publisher：CRYOGENICS AND SUPERCONDUCTIVITY SOCIETY OF JAPAN  

<p>Various experimental attempts have been made to obtain a uniform magnetic field in the space between face-to-face HTS bulk magnets that could possibly be utilized as NMR magnets. In general, the magnetic fields emitted from the magnetic pole surfaces containing HTS bulk magnets are characterized as non-uniform field distributions. Since the NMR magnets require highly uniform magnetic-field spaces, it has been assumed to be difficult to form uniform magnetic-field spaces between magnetic poles placed face-to-face. The authors modified the shapes of the magnetic-field distribution from convex to concave by attaching ferromagnetic iron plates to the pole surfaces. The magnets were then set face-to-face with various gaps of 30-70 mm, and the experimental data on magnetic-field uniformity was precisely measured in the space. In order to detect the NMR signals, the target performance for uniformity was set as 1,500 ppm throughout the 4-mm span on the x-axis, which is equivalent to performance in the past when the world's first detection of NMR signals was observed in the bore of hollow-type HTS bulk magnets. When we combined the concave and convex field distributions to compensate the uneven field distributions, the data of the best uniformity reached 358 ppm and 493 ppm in the 30 mm and 50 mm gaps, respectively, which exceeded the target value for the purpose of detecting the NMR signals within the space. Furthermore, it was shown that the field distributions change from concave to convex shape without any change at 1.1 T in the range from 7 to 11 mm in the 30-mm gap, indicating that the distributions are uniform. This suggests the possibility that the uniform magnetic-field space between the HTS bulk magnets set face-to-face expands.</p>

DOI： 10.2221/jcsj.52.38

CiNii Article

CiNii Books

J-GLOBAL

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1109/TASC.2016.2535208

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1109/TASC.2016.2516349

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DOI： 10.1109/TASC.2016.2543504

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DOI： 10.1109/TASC.2016.2529425

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

The billet heating technique by rotating conductive material in a magnetic field for aluminum extrusion has been reported, but metal melting technology by the induction heating with the dc magnetic field has not. Further complex geometries of aluminum profiles are formed by the casting that requires highly efficient metal melting technology. In order to verify the capability of induction heating using dc superconductor coils for metalmelting in the casting process, we performed a metalmelting experiment with the small examination equipment in a magnetic field. The small examination equipment consists of a motor, a heat insulation structure, a rotating shaft, a specimen holder, a frame, and others. An aluminum pipe of about 0.77 kg, where its outer diameter was 180 mm and its inner diameter was 170 mm, was put into the examination equipment. A dc magnetic field, ranging from about 1 T at the nearest point to 0.4 T at the furthest point, was applied to the pipe by the superconductor magnet. The temperature of aluminum exceeded 500 degrees C within 2 min by the rotation of the pipe at 800 r/min. Aluminum melting in the sample holder was observed after the rotation of 1200 r/min in 90 s, and some amounts of melted aluminum rose over the sample holder and sputtered out by centrifugal force. These results indicate that induction heating with the superconductor coils can supply melting metal of required amount in a short time.

DOI： 10.1109/TASC.2016.2524651

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1088/0953-2048/29/2/024003

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：ELSEVIER SCIENCE BV  

The authors have been attempting to obtain the uniform magnetic field distribution in the space between the face-to-face HTS bulk magnets. The magnetic poles containing the HTS bulk magnets are usually characterized as non -uniform magnetic field distribution. Since the distributions show the conical or convex shapes, it is difficult to obtain the uniform magnetic field spaces even when the magnetic poles would be placed face-to-face. The authors have modified the shape of the distribution of one-side magnetic pole by attaching an iron plate on the surface, and formed the concave magnetic field distribution on the pole surface. The steep concave or convex distributions at each pole surface change to be flat with increasing distance from the pole surface. After the experimental result recording the best uniformity of 358 ppm by combining the concave and convex field distributions face-to-face, we attempted to simulate the feasible performance in this configuration. In the numerical simulation, the concave field distribution modified by attaching an imaginary spiral coil on the pole surface was coupled with the original convex field. We succeeded in obtaining the best uniformity of 30 ppm at 1.1 T in 4 x 4 mm(2) x-y plane at 7 mm distant from the pole surface in the gap of 30 mm. This result suggests that the concave and convex magnetic field distributions compensate the field uniformity with each other with keeping the magnetic field strength in the gap, and also suggests the novel compact NMR/MRI devices in the future. (C) 2016 The Authors. Published by Elsevier B.V.

DOI： 10.1016/j.phpro.2016.04.019

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers Inc.  

We have fabricated a racetrack-shaped spliceless double-pancake test coil with GdBCO coated conductors and investigated its performance in air and magnetic cores. We have been developing a high-temperature superconductor induction/synchronous machine (HTS-ISM) for electric vehicles. To improve the specific power and torque density of the fully superconducting HTS-ISM, an increase in the current capacity of the HTS stator windings is crucial. The fabricated test coil was wound with two GdBCO conductors in parallel, which were transposed at a coil end. Thus, the self-inductances of the two conductors were equal, and the coil was free from splice resistances due to transposition. We confirmed the soundness of the transposition method by critical current measurements. We also investigated the critical currents of the test coil by placing it in a magnetic core and observed a decrease in the critical current compared with that in the air core. This is caused by the increase in the component of the magnetic field perpendicular to the coated conductors. To overcome this issue, magnetic materials were used for magnetic-field correction. We methodically compared the results of the critical current measurements and evaluated the results by numerical magnetic-field analyses.

DOI： 10.1109/TASC.2014.2363041

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1109/tasc.2014.2382337

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DOI： 10.1109/TASC.2014.2373653

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DOI： 10.1109/TASC.2014.2366236

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DOI： 10.1109/TASC.2013.2284859

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DOI： 10.1109/TASC.2013.2282174

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DOI： 10.1109/TASC.2013.2280730

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

Single-pancake coils having different winding turns are fabricated. The bobbin material used for the coils is a Dyneema fiber-reinforced plastic (DFRP), which has a thermal expansion property during cooling. The influence of the DFRP's expansion on thermal strains of the coil during cooling is experimentally estimated. In the experiment, the thermal strains on the superconducting tape of the outermost turns in the coils are measured. The dependence of epoxy impregnation on the thermal strain of the coil is also experimentally estimated. According to the experimental data, the combination of impregnation and the bobbin's expansion effectively decreases thermal contraction of the outermost tape.

DOI： 10.1109/TASC.2013.2285709

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.physc.2013.01.018

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Publishing type：Research paper (international conference proceedings)  

© 2014 The Authors. The flux-invasion behavior into the melt-processed Y-Ba-Cu-O bulk magnets were precisely measured and analyzed during and after their pulsed-field magnetization processes operated at 30.6 K. The materials were fabricated as the bulk monoliths grown by adopting two seed-crystals, or shifting the seed-crystal positions from the centre of the sample surface, which exhibited the magnetically single-domain distributions. Although the performances of the trapped flux density after activations showed no obvious differences, the flux started invading into the sample bearing two seeds obviously at lower fields than those of normally-grown isotropic crystal. Since the flux penetration behavior were thus clearly different between the samples with the structure grown from two seeds and uniformly grown samples with a seed crystal, it is suggested that the structure results in an effective magnetizing method with less heating than those of conventional samples, which results in the higher performance of field trapping in the bulk magnets than usual.

DOI： 10.1016/j.phpro.2014.09.021

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Publishing type：Research paper (international conference proceedings)  

The magnetic flux penetration into the melt-textured Y-Ba-Cu-O high temperature superconducting bulk magnets were precisely evaluated during and after the pulsed field magnetization processes operated at 30 K. The bulk magnets were carefully fabricated by the cold seeding method with use of a single and a pair of seed crystals composed of the Nd-Ba-Cu-O thin films. These seed crystals were put on the top surfaces of the precursors to let the large grains grow during the heat treatments. We observed the flux penetrations which occurred in the lower applied-field regions at around 3.1 T for the samples bearing the twin seeds than those of the single-seeded crystals at around 3.8 T. This means that the magnetic fluxes are capable of invading into the twin-seeded samples more easily than the single-seeds. It suggests that the anisotropic grain growths of parallel and normal to the rows of seed crystals affects the variations of Jc values with different distributions of the pinning centers, results in the preferential paths for the invading magnetic fluxes. © Published under licence by IOP Publishing Ltd.

DOI： 10.1088/1742-6596/507/1/012039

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.physc.2013.04.022

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DOI： 10.1109/TASC.2013.2238982

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.physc.2012.03.008

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

We fabricated nonimpregnated HTS superconducting coils with a BSCCO tape. Bobbin materials in the coils are a Dyneema(R) fiber reinforced plastic (DFRP) and a glass fiber reinforced plastic (GFRP). We excited those coils with ac currents, and estimated mechanical losses. According to the measured data, the mechanical loss decreased with increase of winding tension of the coils, because strong winding tension fixed the coils tightly. Also, the mechanical loss occurred in the DFRP coil was smaller than that occurred in the GFRP coil. A thermal expansion coefficient of the DFRP is a negative value, that is, the DFRP expands with cooling down from room temperature to cryogenic temperature. The expansion of the DFRP bobbin made the winding of the coil fix tightly, and the mechanical loss decreased. From those experimental results, we think that the DFRP bobbin is useful for decreasing of mechanical losses of the ac coils.

DOI： 10.1109/TASC.2012.2236372

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1007/s10948-012-2048-5

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DOI： 10.1016/j.physc.2012.03.010

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Language：English   Publisher：The Materials Research Society of Japan  

The magnetic flux invasion into the Y-Ba-Cu-O-based high temperature superconducting bulk magnets were precisely evaluated during the pulsed field magnetization processes. Various bulk magnets with different contents of Y₂BaCuO₅ (Y211) particles were fabricated so as to examine how the Y211 concentrations affect the flux invasion behaviors. The flux motions and the resultant trapped field densities were monitored at the surface of each bulk sample at 30.6 K. As a result, the applied fields at which the magnetic flux began to invade the sample increased with increasing Y211 contents. This means that the pinning centers are promoted with increasing nominal Y211 addition. According to the evolutional profiles during the pulsed-field applications, the flux-penetration ratio gradually promoted with increasing applied fields, whereas the flux-trapping ratio decreased. This implies that the heat generation due to the flux motion seriously affects the field-trapping ability. The anomalous behaviors for the sample, which has displaced position of seed crystal, showed us more effective flux-trapping than usual, exhibiting the higher flux-penetration and flux-trapping ratios at lower applied-field than those of other samples. This suggests a possible high-field activation method with preferential flux invasion due to the different microstructures in the bulk magnet.

DOI： 10.14723/tmrsj.38.273

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Language：English   Publishing type：Research paper (scientific journal)  

By the accident of the Fukushima Daiichi Nuclear Plant, radioactive substances have been scattered in the environment. The decontamination of radioactive substances in the soils and the wreckage, and the volume reduction of contaminated waste are urgent issues. In this study, we examined the possibility for the soil decontamination of Cs-137, the main radioactive species with longer half-life, by means of the magnetic separation with a high-temperature superconducting magnet. We investigated the desorption behavior of the nonradioactive cesium from several kinds of clay minerals, and the removal rate by the ferromagnetic adsorbent from the aqueous phase. The results showed the possibility of the removal of cesium from the clay. Based on the results, we proposed a magnetic separation system of radioactive cesium by using high gradient magnetic separation with a superconducting magnet. © 2013 IEEE.

DOI： 10.1109/TASC.2013.2255952

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DOI： 10.1109/TASC.2011.2174580

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DOI： 10.1109/TASC.2011.2179510

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DOI： 10.1109/TASC.2012.2183331

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DOI： 10.1109/TASC.2011.2175684

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DOI： 10.1109/TASC.2011.2177792

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DOI： 10.1016/j.cryogenics.2011.10.005

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：ELSEVIER SCIENCE BV  

A demagnetized Nd-Fe-B permanent magnet was scanned in the strong magnetic field space just above the magnetic pole containing a HTS bulk magnet which generates the magnetic field 3.4 T. The magnet sample was subsequently found to be fully magnetized in the open space of the static magnetic fields. The finite element method was carried out for the static field magnetization of a permanent magnet using a HTS bulk magnet. Previously, our research group experimentally demonstrated the possibility of full magnetization of rare earth permanent magnets with high-performance magnetic properties with use of the static field of HTS bulk magnets. In the present study, however, we succeeded for the first time in visualizing the behavior of the magnetizing field of the bulk magnet during the magnetization process and the shape of the magnetic field inside the body being magnetized. By applying this kind of numerical analysis to the magnetization for planned motor rotors which incorporate rare-earth permanent magnets, we hope to study the fully magnetized regions for the new magnetizing method using bulk magnets and to give motor designing a high degree of freedom. (C) 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of ISS Program Committee

DOI： 10.1016/j.phpro.2012.03.442

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DOI： 10.1016/j.physc.2011.05.226

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We made small superconducting coils and measured the total AC loss under the AC and DC external magnetic fields. The coil winding conditions are that 1, 5, and 10 N of the winding tensions and bobbin materials are glass fiber reinforced plastic (GFRP) and Dyneema fiber reinforced plastic (DFRP). The GFRP and the DFRP have thermal strain properties of contraction and expansion respectively with cooldown to cryogenic temperature. Mechanical losses were estimated from the measured data. According to the estimated mechanical losses, the loss of the coil whose bobbin material is the DFRP was small because the DFRP bobbin expanded and hence the winding was tightly fixed. Approximate 10 to 13% of the AC loss can be reduced by only changing the bobbin material from the GFRP to the DFRP. © 2010 IEEE.

DOI： 10.1109/TASC.2010.2091240

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DOI： 10.1109/TASC.2010.2090115

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DOI： 10.1109/TASC.2010.2081655

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In order to electrically investigate the AC losses of the conductor and the shield in an actual HTS cable with shield, we attached two voltage-leads to the conductor and shield of a short cable which is the same as the Korea Electric Power Corporation (KEPCO) HTS cable under the field test, and examined it. The result shows that the polarities of the loss voltages measured from both voltage-leads are opposite. The ratio (1.6) between the losses for different transport periods measured from the conductor-lead is quite large. It is considered from these results that the conductor loss is the sum of the losses measured from the conductor-lead and shield-lead. And this conductor loss corresponds well with the numerical result too. Moreover, the total loss of the short cable measured from the proposed electrical method at 77 K was compared with that of the KEPCO cable by calorimetry at 66.6 K through our numerical model. The difference between the measured losses and numerical ones is almost the same. This means that the shield loss is measured from the shield-lead, and the conductor loss from both voltage-leads. © 2010 IEEE.

DOI： 10.1109/TASC.2010.2084064

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DOI： 10.1109/TASC.2010.2080659

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DOI： 10.1109/TASC.2010.2088099

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DOI： 10.1109/TASC.2010.2091236

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DOI： 10.1016/j.physc.2010.05.210

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DOI： 10.1109/TASC.2010.2044161

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DOI： 10.1109/TASC.2010.2041766

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DOI： 10.1109/TASC.2010.2041641

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Language：English   Publishing type：Research paper (international conference proceedings)  

We measured mechanical loss in an AC magnetic field for a short Bi-2223 superconducting tape. Experimental results indicated that mechanical loss decreased as tension applied to the sample tape increased. Furthermore, when a sample holder with a negative coefficient of thermal expansion was used, mechanical loss reduced. The origin of mechanical loss was confirmed to be the vibration of the sample tape. The expansion holder was the most effective means to reduce the vibration. © 2006 IEEE.

DOI： 10.1109/TASC.2010.2041338

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DOI： 10.1109/TASC.2010.2043661

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DOI： 10.1016/j.physc.2009.05.123

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.3769/radioisotopes.58.469

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DOI： 10.1109/TASC.2009.2019546

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DOI： 10.1109/TASC.2009.2018050

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We have studied the vibration of superconducting windings during AC excitation of coils. Our previous study revealed that mechanical loss due to the vibration of a coil that had an expansion bobbin during cool-down was smaller than that of a coil having a contraction bobbin. However, critical currents of the coil with the expansion bobbin decreased as the number of thermal cycles increased between room and cryogenic temperatures. Hence, excessive expansion of the bobbins causes degradation of coil performance. In this study, we fabricated superconducting coils whose bobbins expanded a little, and measured the mechanical losses of those coils. We present here the expansion properties of the bobbins, test results of the critical currents and mechanical losses, and the dependence of the thermal cycles on those results. © 2009 IEEE.

DOI： 10.1109/TASC.2009.2019242

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DOI： 10.1088/1468-6996/10/1/014610

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DOI： 10.1016/j.physc.2008.05.234

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DOI： 10.1016/j.physc.2008.05.206

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For designing inexpensive high-temperature superconducting (HTS) coils, it is essential to obtain large magnetic fields and stored energy with shorter lengths of HTS tape. To improve the performance of a coil, it is necessary to improve its transport-current performance. The critical current and n-value of an HTS tape were measured at various magnetic field magnitudes and angles at 77 K. The HTS tape employed in the coil was Bi-2223/Ag tape. From this measured data, fitting equations of the critical current and n-values were obtained. With these fitting equations, coil critical currents were analyzed according to our analytical model. The analysis showed that relatively large electric fields are generated at coil edges, inhibiting improvement of the transport-current performance of the coil. To solve this problem, we propose an HTS coil, which is produced by cutting and displacing the central portion of the rectangular cross section. By this rearrangement, the magnetic field distribution changes, resulting in an improvement in the coil critical current. We calculated performances of the proposed coil by varying the magnitude of displaced coils while maintaining a constant total HTS tape length. We found that there is an optimum cross section shape of the proposed coil, which provides improvements in the stored energy and the central magnetic field. In particular, the stored energy improves by approximately 43% compared with a rectangular cross section coil employing the same HTS tape length. © 2008 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2008.05.148

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1109/TASC.2008.920817

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DOI： 10.1109/TASC.2008.920696

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DOI： 10.1109/TASC.2008.921248

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Language：English   Publishing type：Research paper (international conference proceedings)  

We investigated the effect of thermal cycles between room and liquid-nitrogen temperatures on the critical current (Ic) and AC loss of two superconducting coils: one with a bobbin that expands during cooling from room temperature to cryogenic temperature and one with a bobbin that contracts during the cooling. After 100 cycles, neither bobbin suffered degradation. The Ic of the contraction-bobbin coil did not decrease, and that of the expansion-bobbin coil decreased due to the repeated thermal strain. The expansion coil's AC loss was significantly smaller than the contraction coil's loss at the first cycle
however, it increased with the thermal cycles and eventually surpassed the contraction coil's loss because the Ic of the expansion coil decreased due to the thermal fatigue. These results indicate that a moderate expansion in the size of the bobbin effectively decreases AC loss and that excessive expansion reduces the Ic and increases the AC loss. © 2008 IEEE.

DOI： 10.1109/TASC.2008.920586

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1109/TASC.2008.920823

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The authors have constructed various types of strong magnetic field generators using superconducting bulk magnets in conjunction with compact refrigerators. The magnetic field in the open space outside the vacuum chamber that contains a bulk magnet has been estimated as over 3 T when activated by the static fields of a 5 T superconducting solenoid magnet. The authors have tried to extend the variation of such strong field generators which are promised to develop in the future. In the study, a novel and compact bulk magnet system with use of a compact pulse tube cryocooler has recorded the maximum trapped field of 2.78 T on the magnetic pole surface. The characteristic feature of superconducting bulk magnet is defined as a compact and strong magnetic field generator. This implies that various kinds of equipments must be successively proposed even in the very early stage of industrialization of strong magnetic field generators. © 2008 IOP Publishing Ltd.

DOI： 10.1088/1742-6596/97/1/012102

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1088/0953-2048/20/12/025

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DOI： 10.1016/j.physc.2007.04.300

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DOI： 10.1016/j.physc.2007.02.022

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DOI： 10.1016/j.physc.2007.04.306

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DOI： 10.1109/TASC.2007.897249

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DOI： 10.1109/TASC.2007.899884

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DOI： 10.1109/TASC.2007.898426

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DOI： 10.1109/TASC.2007.899000

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DOI： 10.1109/TASC.2007.897243

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DOI： 10.1109/TASC.2007.898425

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

We fabricated small superconducting coils whose bobbins were made of a Dyneema fiber reinforced plastic (DFRP), a Dyneema and glass fiber reinforced plastic (DGFRP), and a glass fiber reinforced plastic (GFRP). Because Dyneema fibers in the FRP expand, we are able to control thermal expansion/contraction property in a circumferential direction of the DFRP and the DGFRP pipes during a cooling process from room temperature to low temperature. We fabricated two kinds of the DFRP and the DGFRP coils whose properties were expansion and contraction. The GFRP has always a characteristic of contraction during cool-down. We measured a mechanical loss that occurred in the coil during AC excitation of those coils. The mechanical losses decreased with increasing winding tension of the conductor in the coils according to the experimental data. And the mechanical loss of the coils whose bobbins had characteristic of expansion was considerably smaller than that of contraction. It was experimentally shown that the bobbins with expansion property during the cooling down were effective to decrease the mechanical loss of the AC coils.

DOI： 10.1109/TASC.2007.899879

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DOI： 10.1016/j.cryogenics.2006.09.003

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DOI： 10.1016/j.physc.2006.05.038

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DOI： 10.1109/TASC.2005.869673

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DOI： 10.1109/TASC.2006.870018

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DOI： 10.1109/TASC.2006.870818

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DOI： 10.1109/TASC.2006.871213

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DOI： 10.1109/TASC.2006.870817

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DOI： 10.1109/TASC.2006.871336

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

We have experimentally studied relation between mechanical losses and thermal expansion properties in superconducting coils. For the experiments, three kinds of structural materials (GFRP, DGFR-P, and ZFRP) were used. A glass fiber reinforced plastic (GFRP) contracts with decreasing temperature; on the contrary, a Dyneema and grass fiber reinforced plastic (DGFRP) and a Zylon fiber reinforced plastic (ZFRP) expand with the decrease of temperature. We used sample holders made of those plastics and measured AC current losses of a Bi-2223 tape in liquid nitrogen. Even if the magnetic field applied to the. tape and an AC transport current of the tape were same, the loss depended on the sample holder materials. When the sample holder is the GFRP, the loss was largest in the three kinds of plastics, and in case of the ZFRP, the loss was smallest. The ZFRP has the largest expansion property of the three materials, that is, the loss decreased with expansion of the sample holder's materials.

DOI： 10.1109/TASC.2005.869690

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DOI： 10.1016/j.physc.2005.03.052

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DOI： 10.1016/j.physc.2005.02.108

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DOI： 10.1016/j.physc.2005.03.055

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DOI： 10.1016/j.physc.2005.01.070

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DOI： 10.1109/TASC.2005.849173

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DOI： 10.1109/TASC.2005.849469

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DOI： 10.1109/TASC.2005.849661

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Language：English   Publishing type：Research paper (international conference proceedings)  

A numerical model to calculate current density distribution in a parallel conductor assembled by multiple high temperature superconducting tapes was proposed. The numerical calculations on the current distribution in the parallel conductor of three high-temperature superconducting tapes were performed by using the developed model. The numerical results showed that the current density distribution in the parallel conductor were affected by the tape arrangement in the conductor. © 2004 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2004.01.137

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

An approximated method to estimate the electro-magnetic AC loss in high temperature superconducting coil was proposed. We applied this method to estimate the AC loss characteristics of the high temperature superconducting coil made by stacking pancake coils which are wound by Bi2223/Ag tape. The analytical AC losses of the HTS coil were compared with the measured AC losses before and after impregnation by epoxy resin obtained in the previous work by National Institute of Advanced Industrial Science and Technology. The analytically estimated electro-magnetic AC losses in the HTS coil were much smaller than the measured AC loss of the coil. The AC loss characteristics could not be explained by the method proposed in this paper. The main reason of this discrepancy was considered that the measured AC losses in the HTS coil contained large amount of the mechanical AC loss due to the AC vibration of the winding even if the HTS coil was impregnated.

DOI： 10.1109/TASC.2004.830027

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

An numerical model to calculate the inter-layer current distributions and the AC losses in co-axial multi-layer cable conductors was developed. The inter-layer current distributions and the AC losses in 4-layer cables were numerically calculated for various combinations of twist pithes and twist directions of the cables. With the numerical model, the twist pitch combinations to equalize the inter-layer current distribution and to reduce the AC loss were also investigated. The analytical results showed the remarkable dependence of the losses and the current distributions on the twist pitches and the directions of the cables. The optimized cable structures were discussed by the numerical results.

DOI： 10.1109/TASC.2004.830028

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

The authors studied a dc hybrid three-phase fault current limiting interrupter composed of a superconducting reactor and an S/N transition element, connected in series each other. This device has the capability of interruption of a limited fault current. In this paper, the experiments on this device with a test model are described and compared with analyses. A proper combination of a reactor coil and an S/N transition element is also discussed for 6.6 kV-2000 A power systems.

DOI： 10.1109/TASC.2004.830276

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

The fitting equations of critical current and n value were derived from measured data of Ag-sheathed Bi-2223 tapes concerning the magnetic fields and flux angles at 77 K. The critical current of a high temperature superconducting coil can be calculated with these equations together with a magnetic field analysis of a coil. Critical current, magnetic field, flux How loss, stored energy, etc. were calculated for HTS coils with various coil aspect ratios along the curve, where Fabry Factor F of 1.97 was kept constant. It was made clear that the central magnetic field per coil volume is maximum at the coil aspect ratio providing the minimum coil volume.

DOI： 10.1109/TASC.2004.830883

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

When a Dyneema fiber reinforced plastic (DFRP) is used as a superconducting coil's bobbin, the decrease of the winding tension during cooling down is prevented. Therefore, it is expected that a mechanical loss generated in AC superconducting coils is reduced. The DFRP has the high Young's modulus to the fiber direction, however the modulus to the perpendicular direction is lower than the modulus of the glass fiber reinforced plastic (GFRP), which is generally used as a structural material in superconducting coils. Therefore, a Dyneema and glass fiber reinforced plastic (DGFRP), which was compounded the Dyneema and the glass fibers, was fabricated for the purpose of the intensification of the DFRP, and total losses of the superconducting coils having some kinds of DGFRP bobbins were measured under AC operation. In loss measurements, the coil whose loss was the smallest was the coil having the strongest winding tension at cryogenic temperature, and the coil whose loss was the largest was the coil having the weakest tension. Moreover, comparing with the results of the loss measurement in the coils fabricated with the DFRP bobbin, the losses of the coils having the DGFRP bobbins were slightly large due to the mixture of the Dyneema and the glass fibers. This paper reports the effect of the reduction of the mechanical losses in the coils having the DGFRP bobbins and the comparison with the coils whose bobbins are the DFRP and the GFRP.

DOI： 10.1109/TASC.2004.830464

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

This paper describes the conceptual design of 100 MVA high temperature superconducting (HTS) autotransformers as well as coreless ones having the rating of three-phase, 50 Hz, and 132 kV/66 kV. Each one is compared with conventional oil-immersed autotransformers. The HTS autotransformers with core will provide a percent impedance of 11.7% close to the conventional one and require an air gap field of 0.33 T, while the coreless HTS autotransformers require 25.8% and 0.7 T respectively. HTS antotransformers with a core have a 22% smaller window size and 0.3% higher efficiency than conventional ones. The coreless HTS autotransformers need HTS tapes about two times the length required for the HTS autotransformer with a core.

DOI： 10.1109/TASC.2003.813101

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

AC loss characteristics of. high temperature superconducting tape in external AC magnetic field carrying AC transport current were theoretically and experimentally studied. We measured the AC transport current and magnetization losses in a conventional Bi2223/Ag multifilamentary tape in the external AC magnetic field carrying the AC transport current. The measured AC magnetization loss, AC transport current loss and total AC loss were compared with the numerical results. The measured AC loss characteristics of the tape can not be explained by the critical state model. Numerical calculations on the AC losses in the tape were performed by using our analytical model introducing the electric field and current density relation locally determined in the filamentary region of the tape. The numerical losses by our model reasonably agree with the measured data.

DOI： 10.1109/TASC.2003.813071

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

AC loss characteristics of the twisted multifilamentary high temperature superconducting tape,with different twist pitches in external AC longitudinal magnetic field as well as those of the nontwisted tape were experimentally investigated. We measured the AC transport current loss and the AC magnetization loss in the twisted and nontwisted Bi2223 multifilamentary tapes in the external AC longitudinal magnetic field. The measured transport loss and magnetization loss in the twisted tapes were affected, by the amplitude and the direction of the AC longitudinal field. It was also shown that the AC losses in the tapes under the external AC longitudinal field depended on the twist pitch.

DOI： 10.1109/TASC.2003.813072

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Language：English   Publishing type：Research paper (international conference proceedings)  

AC loss characteristics of the twisted and not-twisted Bi2223/Ag/AgMg multifilamentary tapes in the longitudinal and transverse AC external magnetic fields were experimentally investigated. The measured AC transport current loss and the AC magnetization loss in the twisted tape were affected by the amplitude and the direction of the longitudinal AC magnetic field. The magnetization loss in the transverse AC magnetic field in the twisted tape was smaller than that in the not-twisted tape. The total AC loss properties were discussed.

DOI： 10.1109/TASC.2002.1018713

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Language：English   Publishing type：Research paper (international conference proceedings)  

A new type of open gradient magnetic separator using multiple magnetic field sources was proposed. As an example, a open gradient magnetic separation using quadrupole magnetic field was focused in this paper. Simulation study was performed to investigate the basic separation characteristics of our separation method by analyzing the dynamic behavior of magnetic particles. The simulation results show that the continuous separation of paramagnetic particles of several tens micrometer diameter is enough feasible. We demonstrated the validity of proposed method by an experiment using quadrupole electromagnet.

DOI： 10.1109/TASC.2002.1018559

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER INST PHYSICS  

Numerical studies on characteristics of electromagnetic behaviors of co-axial multi-layer superconducting cable conductors with superconducting magnetic shields were performed. The inter-layer current distribution and AC loss dependencies on the twist pitches and the diameter of the superconducting layers of the cables were investigated. The numerical results showed that the magnetic field produced by the shielding currents flowing in the superconducting magnetic shield strongly affected the current distribution and the AC losses of the cable. Based on the model, the optimized cable structures for uniform current distribution and to reduce the AC losses were studied. Examples of the optimized structures obtained by our model were presented and compared with those obtained by the conventional electrical circuit model. The differences between the results obtained by our model and those obtained by the circuit model were discussed.

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Cryogenic Association of Japan  

DOI： 10.2221/jcsj.37.765

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Institute of Electrical and Electronics Engineers Inc.  

We have studied a measuring method of a critical current (Ic) for NbTi superconducting wires having a Cu matrix and Cu/CuNi matrices. To prevent the wires from prematurely quenching before reaching their Ic, we tried grease impregnations to sample coils. When the NbTi/Cu wire was used for the coil, silicon grease impregnation was enough to fix the wire and to measure the Ic of the wire. In case that we used the NbTi/Cu/CuNi wire when measuring the Ic, restraint of wire motion by the silicon grease impregnation was insufficient and the sample coil easily quenched before reaching the Ic. Accordingly, the Apiezon® (type: M) grease was used for an impregnation material, and we found the Apiezon grease was effective in fixing the NbTi/Cu/CuNi wire.

DOI： 10.1109/TASC.2002.1018589

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

We developed a de S/N transition type three-phase fault current limiter, the primary element of which, is made of the superconducting bulk material, Bi-2223. This element is connected to secondary windings of transformers, through a modified half control bridge. This unique, single element device simultaneously limits and interrupts fault current, in three-phase power systems, This paper describes the results of tests, using a small scale model and simulation results.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

We propose a single de reactor type fault current limiter, This fault current limiter is for a three-phase power system, employing only one superconducting coil that is connected with the secondary windings of transformers through a diode-bridge. In this composition, a de reactor type fault, current limiter using a high temperature superconducting coil takes advantage of coil inductance and does not require a superconducting coil normal transition. This paper presents the most suitable case of a single de reactor type fault current limiter for a 6.6 kV power system and compares it with a three de reactors type fault current limiter.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

We propose a single de reactor type fault current limiting interrupter (FCLI) for a three-phase power system, The device uses a single high temperature superconducting (HTS) coil that operates in conjunction with a modified half control bridge composed of thyristors and diodes connected to a transformer's secondary windings. One variety is an automatic interrupter, which automatically blocks fault current through the application of de bias current to the bridge. Another is a gate interrupter, which does the same thing by blocking the thyristor's gate pulses. We will examine the results of various simulations on a new device that both limits and interrupts fault current in a three-phase power system.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

A numerical model to calculate current distribution and AC loss in a high temperature superconducting tape carrying Ae transport current exposed to external magnetic field was developed. The model considers the electric field and current density relation and the shape of wire, We measured the AC transport current losses in a conventional Bi2223/Ag multifilamentary tape in the DC external magnetic field and the results were compared with the numerically calculated AC losses. The measured AC transport current losses increase as the DC external field increases. It is also shown that the measured losses per cycle decrease with increasing frequency, These characteristics can not he explained by the critical state model. The numerical losses by our model reasonably agree with the measured data. The transport loss characteristics under the DC field and their dependence on the frequency are explained by the numerically calculated current distribution in the wire.

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Language：English   Publishing type：Research paper (international conference proceedings)  

This paper presents an application of single dc reactor type fault current limiter (FCL) with a gate turn-off (GTO) thyristor bridge as an emergency power source. A dc reactor type FCL stores electrical energy in a superconducting coil at a steady state. When a power transmission stops due to line faults, the FCL supplies stored energy to the load. This conception can be realized by operating the GTO thyristor bridge as an inverter. On the other hand, the function of fault current limitation is possible by keeping all GTO thyristors turned-on. The proposed FCL is numerically tested in a three-phase distribution system.

DOI： 10.1109/77.920272

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCI LTD  

Various types of fault current limiters (FCLs) that use superconducting element are proposed and are under development, but there are many technical issues: their cost, fault current limiting ability, etc., so at present are far from commercialization. The purpose of this work is to study single de device type FCL for three-phase power system. The device uses single high temperature superconducting coil that is operated in the diode bridge connected to transformer's secondary windings. The bias power supply is required in the bridge for a rectification type limiter to avoid the distortion of load currents. This bias power supply requires the ability to supply at least the load currents in the steady state in the power system. Also the ability that many times of rated line currents can flow in the bias power supply at the time of a fault is required. In this paper, we study technical items that are demanded on this bias power supply and provide the optimal constitution of simple de device type FCL for three-phase power system. (C) 2001 Published by Elsevier Science Ltd.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

Numerical studies on characteristics of the inter-layer current distribution and the AC losses in co-axial multi-layer cable conductors with superconducting magnetic shields were performed by using the model developed by our previous study. The inter-layer current distribution and AC loss dependencies on the twist pitches, the diameter and the numbers of the superconducting layers of the cables were investigated. The analytical results showed that the magnetic field produced oy the shielding currents strongly affected the current distribution and the AC losses of the cable. Based on the model, the optimized cable structures to achieve the uniform current distribution and to reduce the AC losses were studied. Examples of the optimized structures obtained by our model were presented and compared with those obtained by the conventional electric circuit model.

DOI： 10.1109/77.828436

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

Quench characteristics of NbTi superconducting wires with Cu and CuNi matrices have been experimentally studied to standardize a measuring method of critical current (Ic) for the wires. In the previous work, the superconducting wires frequently quenched caused by abrupt wire motions before reaching their Ic. Hence, we tried to measure Ic under various fixing conditions of the wires (1, 5, and 10 N of winding tensions to the wires, dry windings and impregnation by two types of grease (silicone and Apiezon(R)), non-metallic and metallic sample mandrels). The experimental results showed that 10 N of the winding tension was best in the three tensions, and Apiezon grease impregnation was effective. However, the wire currents in the above conditions did not reach their Ic at low magnetic fields. When the wires were fixed by soldier on the metallic mandrels, the wire currents always reached their Ic with no premature quenches due to the wire motions. We obtained a prospect to standardize the measuring method of Ic for the three-component superconducting wires.

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DOI： 10.1109/77.920297

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DOI： 10.1109/77.783355

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DOI： 10.1109/77.783367

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

In the paper we report the experimental results for ac losses in HTS coil carrying ac transport current. The tests of ac losses mere performed by the electrical method at 77 K and 4.2 K. The HTS tape consists of 55 multi-filamentary silver-sheathed BSCCO 2223 tape without twisting. The sample coil was made by the React and Wind method using insulated tape conductor. The coil has the inner diameter of 46 mm, outer diameter of 87 mm, height of 14.5 mm, and the number of turns is 300 turns. The measured ac losses were compared with the conventional theory based on the critical state model. In the region of low transport current, the theoretical results agreed well the experimental results. We also investigated the distribution of the sc losses in the coil to study the effects of magnetic field distribution on ac losses.

DOI： 10.1109/77.783425

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

The purpose of this work is to study a de reactor type fault current limiter(FCL) using high temperature superconducting(HTS) coil which is connected with the secondary winding of a transformer through a diode bridge. The high temperature superconductors provide the advantages of a higher temperature operation and they are preferably applied to the de reactor type fault current limiters, where a superconducting coil can be operated at both low voltage and de current. We propose a de reactor type three-phase FCL incorporating only one HTS coil and present its test result. Three-phase FCL for 6.6 kV-2000 A requires one HTS coil of 2.9 MJ.

DOI： 10.1109/77.783452

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

AC loss characteristics of multifilamentary NbTi AC wires under AC external magnetic field were experimentally and theoretically investigated. We measured AC transport current losses of AC wires with various twist pitches under the AC external magnetic field which included longitudinal and transverse components. It was shown by the measurement that the AC transport current losses decreased with decreasing the twist pitches when the external and self longitudinal fields were in the same direction. The measured dependence of the AC losses on the twist pitches and the applied longitudinal field component were well explained by our previously derived model. We also theoretically estimated the AC losses caused by the longitudinal and azimuthal field components. It was found that the AC losses can be substantially reduced by properly selecting the twist pitch and the external longitudinal field component.

DOI： 10.1109/77.783424

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

HTS wires of various structures and synthesizing processes are being developed for power apparatus applications in Super-CM project. AC transport current losses of four of those wires were measured by an electric method using a lock-in amplifier, Measured wires are a Bi2212 rod fabricated by the laser pedestal growth method, a Y123 tape fabricated by the pulsed laser deposition and multilayered Bi2223/Ag sheathed wires of square and tape shapes. In this paper, based on measured data, dependence of AC transport current losses of HTS wires on their structures and synthesizing processes is discussed.

DOI： 10.1109/77.783510

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

We have excited two types of superconducting coils having same specifications. One is made of DFRP (Dyneema(R) fiber reinforced plastic) bobbins which expands during cooling process from room temperature to liquid helium temperature. The other is made of GFRP (glass fiber reinforced plastic) bobbins which contract during the cooling process. Firstly, we have numerically estimated changes of winding tensions of the two types of the coils by the cooling down. And then, training behaviors of the DFRP and GFRP coils wound with the same winding tension at room temperature were measured. Influence of quench currents in the training quenches and the number of quenches to reach to the maximum currents on the winding tensions at liquid helium temperature is systematically discussed.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We investigated the AC loss characteristics of a low temperature NbTi AC wire by measuring the AC transport current losses in the external AC magnetic field whose components are the longitudinal and transverse ones. The measurement results showed that the AC losses were significantly dependent on the directions and magnitudes of the external longitudinal field component. The AC losses caused by the longitudinal and azimuthal field components were estimated by our previously derived model. The theoretical results well explained the dependence of the AC losses on the longitudinal field components. It was also shown that the AC losses can be substantially reduced by the proper choice of the twisting way. (C) 1998 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4534(98)00450-X

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

An analytical model to estimate the AC losses and the inter-layer current distributions of co-axial multi-layer cable conductors was developed. The interlayer current distributions and the AC losses were numerically calculated and their dependence on the twist pitches and directions of the cables was evaluated. It was shown that when the twist pitches were longer than 200mm, the AC losses and the current distributions of the cables were not clearly dependent on the twist directions. It was also shown that the AC losses decreased and the uneven current distributions improved by decreasing the twist pitches less than 150mm.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

A number of fault current limiters(FCL) have been proposed, but so far they are not installed in the power grid because of cost, performance and others. The purpose of this work is to study DC reactor type current limiter using high temperature superconducting coil A three-phase de reactor type fault current limiter composed of only one superconducting coil is proposed.

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DOI： 10.1109/77.614485

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Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

DOI： 10.1109/77.614473

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DOI： 10.1109/20.511469

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

We have measured the self magnetic field AC losses of a typical AC multifilamentary superconducting wire in an AC transverse external magnetic field at 50 Hz. The experimental results show that the self field AC losses are much higher than the transverse field AC losses and strongly depend on the phase difference between the transport current and the background field. We derived a theory to estimate the dependence of the self field AC losses on the phase of the external field based on the critical state model. In the paper, the measured loss characteristics are compared with the theoretical results. The theoretical model well explains the measured loss characteristics.

DOI： 10.1109/77.402652

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

The paper presents results obtained in the study of the current - voltage characteristics, critical and quenching currents as well as of the AC transport current and magnetization losses in four series of fine filament NbTi composites prepared by Showa Electric Wire & Cable Co., Japan. Complementary measurements have been made also on a commercially available type T AC superconducting wire produced by GEC Alsthom Company, France.
The samples differ in filament diameter, matrix thickness between filaments and in the number and structure of filament bundles in the composite. In two series of samples barrier type artificial pinning centers have been introduced to increase J(c) and decrease the normalized magnetization losses. The conclusions concerning the optimization of multifilamentary composites and cables for AC use at low working magnetic field are briefly reviewed.

DOI： 10.1109/20.305580

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Studies of the current-voltage characteristics of typical superconducting wires under static and dynamic conditions up to 50 Hz frequency have revealed much higher instantaneous local loss power densities in an external DC magnetic field than those which occur by magnetization in a transverse AC magnetic field. In this paper, the measured loss characteristics are compared with theoretical ones derived from the critical state model, and the self field AC losses are also estimated. Possible errors that can result by applying the critical state model are discussed.

DOI： 10.1109/20.305763

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：BUTTERWORTH-HEINEMANN LTD  

Current-voltage characteristics of two superconducting wires for a.c. use in d.c. magnetic fields up to 1 T were studied. They were measured with d.c. transport current as well as with sinusoidal a.c. transport current at frequencies up to 50 Hz. Voltage signals due to the self-field effect were obtained and analysed. The measured self-field voltage is also compared with that obtained from calculations based on the critical state model. The self-field losses were estimated by numerical integration of the measured self-field voltage multiplied by instantaneous transport current.

DOI： 10.1016/0011-2275(94)90112-0

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

Because the inductive component of the voltage across a superconducting coil is 10(2) - 10(4) times larger than its loss component, in the most of electric methods for AC loss measurements compensating voltage proportional to partial derivative I/partial derivative t is needed, I being the current in the coil.
This paper presents a description of an air-core transformer whose mutual inductance M can be smoothly changed in a very large interval and the secondary voltage of which has a negligible phase shift with regard to partial derivative I/partial derivative t in the primary as well as strongly suppressed noise voltage.
It also describes a very sensitive method of measuring AC losses in superconducting coils based on the use of such mutual as well as of a selective nanovoltmeter. This method eliminates the need off an integrator with small drift and phase shift error, of a high resolution voltage divider with sliding contact and of a shunt resistor with small phase shift which are required by other analog electric methods.
Experimental compensating coil and their parameters as well as AC Toss measurements in different superconducting coils are also described.

DOI： 10.1109/77.233693

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

The study of the influence of filament diameter d(f), filament spacing d(n) and of B on critical current density J(c) and AC losses p(s) in one type of fine filament composite (FFC) has revealed 2 components of J(c), bulk and surface critical current density J(cb) and J(cs), respectively. A simple relation between them, J(c) and geometrical parameters of FFC including those with different types of artificial pinning centers (APC) is formulated. It is used to explain measured J(c)(B, d(f)) dependence in FFC with island type APC as well as to discuss the prospects of both types of APC in FFC for AC use. To verify the obtained theoretical results another series of FFC samples with sheet type APC has been prepared and measured. Experimental values of J(c)(B, d(f)) obtained in this series samples are smaller than expected. One possible reason for this difference is suggested.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：I E E E  

DOI： 10.1109/77.233644

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：I E E E  

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Language：Japanese   Publishing type：Article, review, commentary, editorial, etc. (scientific journal)   Publisher：Cryogenic Association of Japan  

DOI： 10.2221/jcsj.57.87

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Language：Japanese   Publishing type：Article, review, commentary, editorial, etc. (scientific journal)   Publisher：Cryogenic Association of Japan  

DOI： 10.2221/jcsj.55.149

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© 2019 Published under licence by IOP Publishing Ltd. Aiming to develop the compact nuclear magnetic resonance devices, the authors have been developing the uniform magnetic fields in the space between the face-to-face magnetic poles which contain HTS bulk magnets. The authors deformed the original convex shape of the magnetic field distribution to concave by attaching a ferromagnetic iron plate of 20 mm and 100 mm in diameter on one of the pole surfaces. The magnetic field distribution precisely measured along x-axis changed from concave to convex with increasing distance from the magnetic pole surface, which suggested the presence of magnetically flat region. From the results of mapping on the field uniformity data, we have succeeded in obtaining the field uniformity under 500ppm in the space of 4 x 4 x 0.8 mm3 when the magnetic poles emitting concave and convex field distributions were settled face-to-face with various gaps 30-70 mm. This performance exceeded the target value of 1,500ppm which is necessary to detect the NMR signals.

DOI： 10.1088/1742-6596/1293/1/012044

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© Published under licence by IOP Publishing Ltd. The authors have developed a useful technique for extracting the Ni-sulfate compound from the "processed waste/fluid" of the electroless Ni-plating processes. After forming the fine NiHPO3 precipitate from the waste, the coarse NiSO4 crystals were synthesized through the reaction with the concentrated sulfuric acid. In the experiment, the open-gradient magnetic separation was employed to collect the NiSO4 crystals from the muddy mixture composed of these compounds. The experiments were practically conducted with use of the Gd123-based HTS bulk magnets generating up to 3.99 T, which were activated by the field cooling magnetizing method operated at 35 K. Intense magnetic field emitted from the bulk magnet attracted the paramagnetic fluid and coarse NiSO4 crystals on the bottom of the water channel, forming a giant drop of NiSO4-saturated clear fluid and the slurry of nickel sulfate hexahydrate. This preferential collection suggests a feasible recycling process of Ni resource as a raw material in the plating processes.

DOI： 10.1088/1742-6596/1054/1/012047

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© Published under licence by IOP Publishing Ltd. A peculiar magnetic separation technique has been examined in order to remove the Cs-bearing Fe precipitates formed of the waste ash from the withdrawn incinerator furnaces in Fukushima. The separation system was constructed in combination with high temperature superconducting bulk magnets which generates the intensive magnetic field over 2 T, which was activated by the pulsed field magnetization process. The separation experiment has been operated with use of the newly-built alternating channel type magnetic separating device, which followed the high-gradient magnetic separation technique. The magnetic stainless steel filters installed in the water channels are magnetized by the applied magnetic fields, and are capable of attracting the precipitates bearing the Fe compound and thin Cs contamination. The experimental results clearly exhibited the positive feasibility of HTS bulk magnets.

DOI： 10.1088/1742-6596/897/1/012024

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© 2015 IEEE. A characteristic magnetic separation technique has been developed to recycle the Ni resource from the waste fluid of electroless Ni-plating processes. Since the plating waste still contains high concentration of Ni ions after several cycles of plating processes, the magnetic separation experiments were conducted to collect the precipitates containing Ni compounds. After extracting the fine NiHPO 3 precipitate from the waste liquid, we synthesized the NiSO 4 crystals from the NiHPO 3 precipitates through the reaction with the concentrated sulfuric acid. The open-gradient magnetic separation was employed to separate the NiSO 4 crystals from the mixed slurry composed of the NiSO 4 and the NiHPO 3 due to the difference between their magnetic properties. The experiments were practically conducted with use of the Gd123-based HTS bulk magnets with very steep gradient of magnetic field of over 100 T/m, which were activated to the magnetic fields of 1.53 and 1.95 T by the pulsed field magnetization process operated at 35 K. The ratio of NiSO 4 content in the collected slurry attracted to the magnetic poles reached up to 98%, which suggests that the paramagnetic NiSO 4 crystals are preferentially collected by the magnetic force from the mixed slurry in the strong magnetic field generated by the HTS bulk magnet.

DOI： 10.1109/TASC.2016.2517923

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© 2015 The Authors. A unique experimental attempt aiming to obtain a uniform magnetic field space as required for NMR has been carried out with use of HTS bulk magnets. The magnetic poles were activated as 1.8 T (North) and 1.4 T (South) at 30 K by applyinga pulsed magnetic field up to 7 T, and positioned face-to-face with gaps less than 70 mm. The uniformity of the magnetic field required for detecting the NMR signals isless than 1,500 ppm at more than 0.3 T in the cross sectional plane of 2 x 2 mm 2 . After thepreliminary trials whichrevealed auniformity of 5,421 ppm at 0.44 T in a70 mm gap, we attached a ferromagnetic iron plate to a magnetic pole surface to change the magnetic field distribution to be concave. The best uniformity of 358 ppm at 1.11 T was obtained at 9 mm distance from the iron plate surface in a gap of 30 mm. It is stated that the concave magnetic field distribution was compensated by the counter conical-shape field, resulting in the uniform field plane.

DOI： 10.1016/j.phpro.2015.06.187

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