Characterization and Analysis of Magnetic Fields of Multiple Piece Levitation Magnets used in Flywheel Energy Storage Systems

Abstract

Superconducting magnetic bearings are being used in energy storage systems because they have little to no frictional losses, and therefore have high energy storage efficiencies. Any frictional losses that do occur are mostly due to inhomogeneities in the magnetic field of the permanent magnets in the superconducting magnetic bearing. The magnetic field of the permanent magnets used as rotors in the magnetic bearings must therefore be examined for any inhomogeneities, which could cause frictional losses and instabilities in the system The magnetic fields of the two permanent magnet rotors that are to be used in the Flywheel Energy Storage system at Argonne National Laboratory were examined in detail for any inhomogeneities. In order for the magnet to have low frictional losses and low instabilities, the inhomogeneities need to be less than 10% of the total magnetic field of the rotor magnet. The first rotor magnet examined, Rotor A, had magnetic field inhomogeneities of 14-17% of its total magnetic field. In addition, the inhomogeneities showed extreme dependence on the locations of the seams between the magnet pieces. This clearly exceeded the expected amount of inhomogeneities, and it was decided to manufacture a second rotor magnet, B. Rotor B Magnet had magnetic field inhomogeneities of 4-6% of its total magnetic field. Although the inhomogeneities were dependent upon the positions of the seams between the magnets, even the largest local inhomogeneity was only 6% of the total magnetic field. Thus Rotor B Magnet's field was within the allowable inhomogeneity margins for the permanent magnet rotors to be used in the Flywheel Energy Storage system.