Touchdown bearing contact forces in magnetic bearing systems

Under contact-free levitation, rotors supported by active magnetic bearings have many advantages such as allowing near frictionless rotation and high rotational speeds. They also provide the designer the capability to achieve increased machine power density. However, magnetic bearings possess limited load capacity and operate under active control. Under certain operational conditions, the load capacity may be exceeded or a transient fault may occur. The rotor may then make contact with touchdown bearings and the ensuing rotor dynamics may result in transient or sustained contact dynamics. The magnetic bearings may have the capability to restore contact-free levitation, though this will require appropriate control strategies to be devised. An understanding of the contact dynamics is required, together with the relationship between these and applied magnetic bearing control forces. This paper describes the use of a contact force measurement system to establish the force relationship. The contact force components measured by the system are calibrated against forces applied by an active magnetic bearing. The data generated can be used to validate non-linear dynamic system models and aid the design of control action to minimize or eliminate contact forces.