Losses in magnetic bearings

The application of HTSC bulk materials in magnetic bearings requires an understanding and minimization of the friction in such systems. Since the rotors in these bearings are not completely axially symmetric, their rotation generates an ac magnetic field acting on the superconductors and the metallic components of the arrangement. This results in hysteresis and eddy current losses. In this paper we present systematic experimental investigations of these loss mechanisms obtained from various arrangements with well defined deviations from the axial symmetry of the rotor. Theoretical models of the loss contributions have been developed and compared with the experimental results. The basis of these models is to describe deviations from the axial symmetry of the rotor as a distribution of magnetic dipoles and to calculate the force on such dipoles caused by hysteresis or eddy currents. These models allow the estimation of the losses of a given arrangement and can be used for the design of superconducting magnetic bearings.