High-performance Magnetic Bearings using Field Shaping Function of Bulk Superconductors

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In this study, static and dynamics characteristics of superconducting magnetic bearings using field shaping function of bulk superconductors have been examined with a small-scale experimental equipment. Then its application as low-loss bearings for large high-speed rotating machines and further improvement of the characteristics have been discussed.The small-scale experimental equipment consists of Bi-2223 superconducting coil for magnetic field generation, YBCO bulk superconductors for field shaping, and an iron component as a rotor. The superconducting coil and the bulk superconductors were directly cooled down by a GM refrigerator. The maximum flux density at the center of the coil was about 0.8 T. Fundamental characteristics were measured using this equipment: Cooling characteristics, excitation of the superconducting coil, field shaping characteristics of the bulk superconductors, influence of division structure of the field shaping material, electromagnetic forces, etc. These characteristics were compared with the results of numerical analysis, which indicates fairy good agreement between them.Characteristics of electromagnetic force and rotational losses of the superconducting magnetic bearing and their dependence on several parameters such as the magnetic flux density were measured. The maximum rotational speed was as low as 9000 rpm due to the resonance of rotational vibration and the rotational losses were larger than expected. This is because superconducting properties of the bulk superconductors for field shaping were deteriorated in a higher field, which was almost confirmed by numerical analysis. Therefore, the increase of critical current density of the bulk superconductors and homogeneous and uniform superconducting properties in each superconductor sample and among superconductors are important to improve the performance of the superconducting magnetic bearings and to develop them as more practical low-losses bearings.