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.
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