Impact of the Yoke Material on the Performance of Wounded Electrodynamic Bearings

This paper concerns null-flux, centering electrodynamic bearings with an internal rotor made of permanent magnets creating a multipole radial magnetic field and with an external airgap wounded conductor attached to a yoke. The inclusion of a ferromagnetic yoke in front of the permanent magnets has the two following effects. On the one hand, it increases the magnetic flux density in the airgap and the magnitude of the centering Lorentz forces between the rotor and the stator of the device. This is positive since the stiffness associated with the centering force of the bearing increases too. On the other hand, it also induces a negative stiffness due to the reluctant force between the yoke and the magnets usually referred to as the unbalanced magnetic force. The goal of this paper is to investigate about the gain in stiffness and performance associated with the presence of a ferromagnetic yoke in multipole electrodynamic bearings. To this purpose, a method is exposed to evaluate the stiffness and stability of the bearing objectively using a root-loci plot. The results are based on a 2-D analytical model of electrodynamic bearings which is briefly presented in the paper. It is shown that in some cases, the magnetic permeability of the yoke has a very low impact on the bearing performance.