Active Axial Electromagnetic Damper

In magnetic bearing applications with rather limited axial loads and/or relaxed requirements for the axial positioning accuracy of the rotor it can be attractive to achieve the axial suspension using passive reluctance centering. In this case, however, it might be difficult to obtain the damping necessary to limit transient response to unexpected dynamic loads. A concept of an active electromagnetic axial damper developed for a 1MW 15,000RPM motor on magnetic bearings with passive axial reluctance centering is presented. The damper features a PM-biased actuator with a flux feedback. Using the flux feedback allows reducing the negative axial stiffness of the actuator to a level sufficient for using it in combination with a passive axial bearing exhibiting rather low positive axial stiffness 2400 lbf/in (420kN/m). The other advantages of the flux feedback include more linear relationship between the output force and the control command than in conventional current control as well as significant reduction of the gain and phase roll offs caused by the eddy currents. The design delivers 8760 N⋅s/m damping coefficient, which represents 20% of the critical damping, and 670N load capacity sufficient to address the worst-case-scenario dynamic response.