Stiffness Enhancement of a Superconducting Magnetic Bearing Using Shaped YBCO Bulks

Abstract

High-speed superconducting motors and generators<br>stand to benefit from superconductor magnetic levitation bearings if their stiffness characteristics can be improved. Here we investigate a novel thrust bearing geometry, comprising a conical frustum shaped permanent magnet and matching superconducting toroid and puck assembly, aimed at producing high stiffness coupled with high levitation force. To this end, we have constructed a bearing test rig enabling measurements of the levitation force and stiffness of the assembly of YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-d</sub> melt-textured bulks and Nd<sub>2</sub>Fe<sub>14</sub>B permanent magnet at temperatures down to 47 K. The experimental results are supported by finite element modeling that is validated against the experiment, and used to quantify the advantages of this configuration over a conventional cylindrical magnet and HTS puck arrangement.<br>For axial displacements, the assembly produces higher and more consistent stiffness together with stronger restoring forces. For lateral displacements, the assembly produces up to double the lateral force and up to four times the stiffness. Our study also shows that the force contribution to the assembly from the small inner puck is negligible and it can therefore be eliminated from the bearing design.

AC Losses in Superconducting Magnetic Bearings

“…Considering the thermal conductivity of YBCO, thermal resistance and time constant between the feedback sensor on the outer side of the toroid and the cryocooler coldhead, the temperature at the inner surface of the toroid may be different to that read by the results. This J c was calculated before by our group [111]. According to the data shown in figure 4.18, the n-index of the HTS assembly in this bearing matches with the simulation data for n-index of 40.…”

“…Under ideal conditions, forces in x and y directions should be zero. However, a finite force was measured in x and y directions which could be caused by PM-HTS misalignment or inhomogeneities in HTS bulk and/or PM [111]. Although the same PM and a different HTS bulk with the same dimensions as described in section 3.4 were used, the maximum levitation force measured in the vacuum chamber is smaller than that measured in section 4.3.…”

“…Another experimental setup was designed and built to measure levitation force decay from cryo-cooled bulks in a vacuum chamber [111]. Two SMBs were studied in this experimental rig.…”

“…Since the main disadvantage of SMBs is their low stiffness compared to mechanical bearings, a novel thrust SMB was designed and built at the Institute aiming at enhancing bearing stiffness. Levitation force and stiffness of this SMB have been studied before [100,111]. In order to estimate the flux creep exponent (n-index) of the trapezoidal toroid HTS assembly in this bearing (described in section 3.5), levitation force decay experiments were performed and results are presented in this section.…”