Active Magnetic and Touchdown Bearings to Encourage Rotor Re-Levitation from a Persistent Contact Condition

Magnetic bearing systems are commonly used for high speed rotor applications, having particular advantages in low pressure/vacuum environments. The benefits of using magnetic bearing systems are well documented in terms of low fiction and controllable stiffness and damping. In order to protect magnetic bearings in cases of power failure, intermittent faults and unexpected external disturbances, secondary back-up or touchdown bearings are usually included so that rotor/stator contact is prevented. If, for any reason, a rotor should make contact with a touchdown bearing, the ensuing rotor dynamic response will depend on operational parameters and any residual rotor unbalance distribution. The influence of unbalance force distributions on the contact rotor dynamics are demonstrated in this paper. Interactions between a rotor and touchdown bearings can cause relatively high contact forces that could be damaging in terms of direct mechanical stresses and through induced heat inputs when slip occurs. Inorder to alleviate the contact problem an actively controlled touchdown bearing system has been proposed to reduce contact forces and, where possible, recover rotor positional control and return it to a condition of contact-free levitation. Active touchdown bearing control may be applied together with magnetic bearing control and suggested strategies to encourage rotor contact-free levitation are assessed in this paper.