Residual Drag Torque on Magnetically Suspended Rotating Spheres

An almost quantitative explanation of the residual drag torque on freely spinning steel spheres in magnetic suspension is given by comparing previously published experimental data with theoretical results of Keith (1963). Accordingly, the residual drag torque is mainly caused by the earth's rotation. The associated decay ratio −ω̇/ω increases quite linearly with the rotor angular velocity ω. A distortion of the rotational symmetry of the supporting magnetic field enhances the decay ratio. This effect decreases with increasing rotor speed. The influences of rotor imperfections and of lateral rotor vibrations are also considered.