Rotor/Auxiliary Bearing Dynamic Contact Modes in Magnetic Bearing Systems

Magnetic bearings systems usually incorporate auxiliary bearings so that rotor/stator contact is prevented. The rotor must then make contact with the auxiliary bearings if any clearance boundaries are exceeded. It is then possible for a rotor with a given unbalance condition to exhibit either contact free levitation or to become trapped in various contact modes. For passively mounted auxiliary bearings, the contact modes are determined by the system design parameters. However, it is now feasible to mount the auxiliary bearings in an active system using piezoelectric actuators. If closed loop control is applied, the mounting (e.g. stiffness/damping) characteristics may be varied on-line. The potential also exists to apply open-loop actuation to the auxiliary bearing to make the clearance domain variable. For example, appropriate synchronous translation of a circular auxiliary bearing may cause it to appear as having a different radius or to be elliptical to the rotor. The question that needs to be addressed is whether this is useful in the destabilisation of contact modes so that contact free levitation may be restored. This paper will investigate whether this is a feasible option.