Masahiro Kitano

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.

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.

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.

Pulsed-field magnetization of bulk HTS magnets in twinned rotor assembly for axial-type rotating machines

We study the pulsed-field magnetization of HTS bulks and its application to a rotating synchronous machine of axial-gap type. To increase the output performance of the rotating machine, the multiple rotor structure is designed with the alternation of the rotor with Gd-bulks and the fixed armature. Thus, we obtain a redundancy for the rotor magnet operation without additional current leads or slip rings. To assess the present redundancy, we show the result of the pulsed-field magnetization for Gd-bulks in the above-mentioned multiple rotor geometry. Two Gd-bulks 60 mm in diameter and 19 mm thick were inserted into three vortex-type Cu coils. They were immersed in liquid nitrogen. The pulsed current was applied to three serial vortex-type coils. The maximum applied magnetic filed was 5.7 T with a rise time of 6.6 ms. The trapped fields for the two Gd-bulk were 0.851 and 0.835 T, respectively. We showed that it is possible to perform pulsed-field magnetization for two Gd-bulks sandwiched between three armature vortex-type Cu coils, i.e., a double-layered twin rotor assembly.

Study of single pulsed-field magnetization of Gd-Ba-Cu-O bulk high-temperature superconductor with a split type of armature coil for rotating machinery

We employed a Gd-bulk HTS as rotating field magnet poles aiming for a smaller and lighter axial-gap-type motor. The bulk was inserted in the split-type armature pulsed copper coils and cooled down to 77 K under zero field. Employing the bulk magnet to HTS rotating machinery, the number of pulsed field magnetizations should be reduced for practical use. Thereby, a single pulsed current was applied to the pulsed copper coils to magnetize the bulk. The trapped field distribution and transient flux behaviour strongly depend on the radial dimension of the armature coil with a vortex-type winding. On decreasing the diameter of the pulsed copper armature coil, the distribution of the trapped flux density on the surface of the bulk becomes close to a conical shape. In contrast to the use of a solenoid, the application of vortex-type armature coils to magnetization of Gd-bulk HTS shows a quick intervention of the external magnetic flux into the centre of the bulk. The magnetization to the bulk HTS of the vortex-type copper coils with an optimum radius is useful and may be an effective technique for applied bulk HTS for rotating machines such as motors and/or generators.

Mechanical design of a synchronous rotating machines with Gd-Ba-Cu-O HTS bulk pole-field magnets operated by a pulsed-field magnetization with armature copper coils

We studied a high-temperature superconducting (HTS) synchronous motor assembled with melt-textured Gd-Ba-Cu-O bulk pole-field magnets. The structure of a HTS motor is an axial gap type with neither brushes/slip rings nor iron core. The specific feature is that the rotor pole-field magnets of bulk are magnetized with pulsed current flow through vortex-type armature copper windings. The rotor pole bulks and armature coils are cooled down with liquid nitrogen. Cooling and magnetization of bulk pole field magnets are performed inside of the rotor. The trapped peak magnetic field of more than 0.5 T of the bulk magnets provided the motor performance of 3.1 kW with 720 rpm. In order to attain high output, single rotor plate with 8 bulks was substituted with a twinned rotor plates with 16 bulks together with triple layer armature units. We report on the test results and performance of the present twinned rotor-type HTS synchronous motor.

Pulsed Field Waveforms for Magnetization of HTS Gd-Ba-Cu-O Bulk Magnets

Progress in pulse magnetization technique for high-temperature superconductor bulks of melt-textured RE-Ba-Cu-O with large diameter is important for the realization of power applications. We studied the pulsed power source and pulsed field waveforms to enhance to improve the magnetization properties for Gd-Ba-Cu-O bulk. The risetime and duration of pulse waveform effectively varied distribution of magnetic flux.

Single pulsed-field magnetization on Gd-Ba-Cu-O Bulk HTS assembled for axial-gap type rotating machines

We employed Gd-bulk HTS magnets as rotating poles for a smaller and lighter axial-gap type rotating machine. The bulk was placed between two vortex-type armature coils and cooled down to 77 K under zero-field. Pulsed current was applied to the vortex-type magnetizing coils. The trapped field distribution and transient flux behaviour strongly depend on the radial dimension of the armature vortex-type coil. In the present study, we show that there is an optimal radial dimension of magnetizing coils to the given bulk disk size to give a homogeneously conical distribution of the trapped flux.

Pulsed-Field Magnetization of Bulk HTS magnets in Twinned Rotor Assembly for Axial-type Rotating Machines

We study a pulsed-field magnetization of HTS bulks and application to rotating synchronous machine of axial-gap type. To increase the output performance of the rotating machine, the multiple rotor structure is designed with the alternation of the rotor with Gd-bulks and the fixed armature. Thus, we obtain a redundancy for rotor magnets operation without additional current leads nor slip rings. To assess the present redundancy, we show the result of the pulsed-field magnetization for Gd-bulks in the above mentioned multiple rotor geometry. Two Gd-bulks with 60 mm in diameter and 19 mm thick were inserted into three vortex-type Cu coils, alternatively. They are immersed into liquid nitrogen. The pulsed current was applied to three vortex-type coils as serial. The maximum applied magnetic filed was 5.7 T with the rise time of 6.6 ms. The trapped field for two Gd-bulk were 0.851 T and 0.835 T, respectively. We evidenced it is possible to perform the pulsed field magnetization for two Gd-bulks sandwiched among three armature vortex-type Cu coils, i.e., double layered rotor assembly.