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.Index Terms-High-temperature super-conducting motor, hightemperature superconductors, melt-processed bulk superconductors, melt-textured Gd123.

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.

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.

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 J c 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.