Motohiro Miki

An axial gap-type HTS bulk synchronous motor excited by pulsed-field magnetization with vortex-type armature copper windings

We studied a high-temperature superconducting synchronous motor assembled with melt-textured Gd-Ba-Cu-O bulk field magnets. The motor is an axial gap-type, brushless synchronous motor with eight rotating bulk field magnet poles. Liquid nitrogen is circulated to cool down the rotor components. Pulsed field magnetization was performed to excite the bulk field magnets by using a pair of the vortex-type armature copper windings under the zero-field cooling. The trapped peak field density on the surface of the bulk was varied from 0.5 T to 0.8 T. The trapped peak magnetic field 0.5 T on the surface of the bulk magnets provided the motor performance of 3.1 kW with 720 rpm. The field density distribution on the pole bulk magnet surface is anisotropic and different from the ideal conical shape. The optimized pulsed current waveform applied to the armature and the employing of a composite of bulk crystal magnets leading to a spatially homogeneous flux trapping are promising methods for reinforcement of the field flux from the rotor and the motor torque.

Development of a synchronous motor with Gd-Ba-Cu-O bulk superconductors as pole-field magnets for propulsion system

Rotating machines with high-temperature superconductors (HTS) usually consist of pole-field magnets having coils wound with Bi-2223 HTS wire. We have successfully used Gd–Ba–Cu–O bulk HTS in pole-field magnets in an axial-gap type rotating machine. These HTS pole-field bulk magnets were assembled in the rotor plate. They are cooled down with a liquid cryogen supplied via a rotary joint and circulated inside the rotor plate. The present design provides a small air gap and a bulk HTS gives a high magnetic field around the armature coils. Successful mechanical design has enabled us to magnetize the pole-field bulk to more than 1 T by using a pulsed current applied to the copper armature coils. These techniques imply the possibility of smaller and lighter rotating motors or generators with a HTS bulk magnet for a sub-megawatt class propulsion system. We report several essential techniques for both mechanical and cryogenic designs, and deduce the characteristic features of the present axial-gap type machine using a HTS bulk magnet.

Trapped Flux and Levitation Properties of Multiseeded YBCO Bulks for HTS Magnetic Device Applications—Part II: Practical and Achievable Performance

After clarifying the essential characteristics of the multiseeded YBCO bulk in the previous work, that is, the existence of strongly connected or coupled grain boundaries between two grains by means of intergrain super current with a big flowing loop size, this paper proceeds to report its achievable performance in actual excitation conditions by working as trapped-field magnets or levitation devices. Besides evaluation of the trapped-flux characteristics by two most popular excitation methods, namely, static field-cooling magnetization (FCM) and pulsed-field magnetization in liquid nitrogen (LN2), the trapped-flux capability at lower temperatures up to 30 K, an interesting temperature for present superconducting machine applications, is also reported. From LN2 temperature to 30 K, the trapped flux with over six times improvement from 0.64 to 3.91 T was observed by FCM, at the same time along with a much decreased relaxation rate from 13.7% to 0.9% in 1-h measurements. In the aspect of levitation applications, the magnetic force density (both levitation force and guidance force, two most important performance parameters for maglev) above a permanent magnet guideway with Halbach style is measured and found to be able to promote the loading and stability performance of the first high-temperature superconducting maglev test vehicle simultaneously. As the multiseeding technique of the top-seeded melt-growth process has provided a promising way to fabricate bulk superconductors with large size, time saving and cost reduction, the presented trapped-flux and magnetic force results may help to comprehend, refer, and finally make better use of the multiseeded bulks in the large-scale superconducting magnetic devices.

Trapped Flux and Levitation Properties of Multiseeded YBCO Bulks for HTS Magnetic Device Applications—Part I: Grain and Current Features

The strongly connected or coupled grain boundaries (GBs) between adjacent grains and their macroembodiment as flowing intergrain supercurrents crossing the GBs inside multi- seeded bulk high-temperature superconductors were elucidated by trapped-flux evaluation. Trapped-fleld measurements, after cutting and polishing two multiseeded YBCO bulk samples, were conducted to present the existence of coupled GBs and their distribution along the c-axis growth. The intensive trapped-flux density observed near the GB areas inside the whole sample is direct evidence of a strongly connected or coupled GB. The relatively strong trapped flux near the GB areas and the significant improvement of the total trapped flux compared with the isolated single-grain bulks were ascribed to the intergrain supercurrent flowing across the GBs in large macroscopic loops with coordination of the intragrain supercurrent circulating in each grain of the multiseeded bulk. Based on the experimental results, a simplified simulation model that incorporates two forms of the intra- and inter supercurrents flowing inside the multiseeded bulk is introduced to reproduce the trapped-flux density features, and qualitative agreement is obtained by comparison with the experimental ones.

HTS Bulk Pole-Field Magnets Motor With a Multiple Rotor Cooled by Liquid Nitrogen

High-Tc superconductor (HTS) rotating machine with melt-textured Gd-123 bulk pole-field magnets was assembled and tested. The axial gap type motor with neither brush/slip ring nor iron core was composed of a kind of multiple rotor, i.e., a twinned rotor. On each rotor disk, there are eight Gd-123 HTS bulk magnets. The bulk magnets were cooled down with circulating liquid nitrogen supplied inside the rotors. Separately, triple-layered armature copper coils cooled with liquid nitrogen were fixed for pulsed current magnetization for the bulks. Rotation of the twinned rotor provide a better single-phase waveform in 200 rpm compared to in single rotor operation. Magnetic flux density and its distribution on each bulk are stable with averaged decay of 4% for seven hours continuous synchronous operation. The results give a poof on the realization of strong flux collection motor.

Study of a new split-type magnetizing coil and pulsed field magnetization of Gd?Ba?Cu?O high-temperature superconducting bulk for rotating machinery application

A new type of magnetization coil was designed to increase the maximum trapped magnetic flux density and the total flux associated with an appropriate trapped magnetic flux density distribution in a high-temperature superconducting bulk magnet. The coil is composed of an inner vortex-type coil wound with an outer solenoid coil. A pulsed current is applied to the inner or the outer coils. Successive applications of pulsed current from both the inner and the outer coils to only the inner coil provide a distribution of trapped magnetic flux density closer to being conical in addition to an increase of the maximum flux density and total integrated flux. The present magnetization technique, the controlled magnetic flux density distribution coil method, is useful for magnetized high-temperature superconducting bulk applications such as in rotating machines, generators and magnetic separation.

Development of a Cryogenic Helium-Neon Gas Mixture Cooling System for Use in a Gd-Bulk HTS Synchronous Motor

Temperature Superconductors (HTS) applied to rotating machines require an efficient cooling system. It is necessary to increase the maximum trapped flux density in the bulk HTS magnets and decrease the overall cooling time. In this paper, we added a gaseous helium phase to a condensed-neon closed-cycle thermosyphon. The latent heat of neon-film cooling is combined with heliums high thermal conductivity. Different mixture proportions were evaluated in terms of resistance to variable heat loads. More helium decreased the temperature variation of the evaporator. The mixture was then used to cool down a 30 kW-grade gadolinium-bulk HTS synchronous motor. The eight bulk HTS conductors of the rotor were cooled to 40 K in less than six hours. The application of this thermosyphon is envisioned for larger rotating machines.

Recovery of trapped field distribution around a growth sector in a Gd?Ba?Cu?O HTS bulk with pulsed-field magnetization

Pulsed-field magnetization is a useful magnetization technique for high-temperature superconductivity bulk crystals for practical applications. However, the trapped magnetic flux is reduced due to the temperature increase from the mobile flux, and the distribution of the flux density is considerably distorted in the a–b crystallographic axes plane. We present two proposals to improve the trapped magnetic flux and its distribution upon pulsed-field magnetization. Firstly, split-type vortex pulsed copper coils with diameters smaller than that of the bulk disk. Secondly, two HTS bulks are stacked layered with misalignment of their crystal a–b axes to reduce the flux motion in the four-fold growth sectors, since the critical current density Jc in the growth sectors is lower. Combining these techniques, the decayed trapped field distribution was recovered and the total flux was increased up to 30% in comparison with a conventional pulsed-field magnetization.

Influence of AC Magnetic Field on a Rotating Machine With Gd-Bulk HTS Field-Pole Magnets

A synchronous rotation study with eight field rotor pole of Gd123 bulks was performed with the axial-type motor. The field pole of Gd123 packs with and without doped amorphous magnetic particles Fe-B-Si-Nb-Cr-Cu (MP) were employed. Before and after the synchronous operation, the change of the trapped magnetic flux on the bulk was investigated at the operating temperature of 40 K. The observed decay of the trapped integrated flux on the surface of Gd123 packs after 5 hours synchronous rotation were 7.2% for the Gd123 pack without MP doping and 4.1% for the Gd123 bulk doped with MP. The present result indicates that the lowering operation temperature may be beneficial to avoid the AC loss. This also leads to another complementally conclusion that the employment of the condensed neon gas cooling system is a suitable choice from the viewpoint of practical applications of the HTS bulks in contrast to liquid nitrogen cooling. In addition, the Gd123 doped with MP alloy particles provide a practical magnetic flux trapping function at 40 K.

Optimization of a condensed-neon cooling system for a HTS synchronous motor with Gd-bulk HTS field-pole magnets

The axial-gap synchronous machine developed in our laboratory is based on Gd-bulk HTS field-pole magnets, able to trap a part of the magnetic flux they are submitted to when cooled down below Tc. At the liquid nitrogen temperature, by the Pulsed-Field Magnetization (PFM), 1.04 T was trapped in 60 mm-diameter and 20 mm-thickness magnets, leading to an output power of the motor of 10 kW at 720 rpm. To enhance this performance, we have to increase the total amount of trapped flux in the bulk, the shortest way being to decrease the temperature of the bulk HTS. Thus, we focused on the improvement of the condensed-neon cooling system, a closed-cycle thermosyphon, so that it provided enough cooling power to lead the rotor plate enclosing the magnets to a low temperature. The present study implied coming out with a new fin-oriented design of the condensation chamber; hence, the numeric calculations and FEM software (ANSYS) heat transfer simulations were conducted for various shapes and positions of the fins. The trapezoidal design offering the best efficiency was then manufactured for testing in a heat-load test configuration, leading to cooling times divided by three and a maximum heat load endured of 55 W.