The use of magnetic bearings allows rotor dynamic systems to be developed for high speed applications, including low pressure/vacuum environments. They provide an alternative to conventional journal, rolling element and gas bearings. The benefits of using magnetic bearings are well documented in terms of low friction operation, together with controllable dynamic characteristics such as stiffness and damping. Magnetic bearings are usually equipped with touchdown bearings to protect the system in cases of power failure, transient loadings, system faults or unexpected influences that may induce system control malfunction. A rotor assembly invariably exhibits residual unbalance due to manufacturing imperfections. The underlying unbalance forces have an influence of the rotor dynamics that arise from contact between a rotor and a touchdown bearing. When considered with the system dynamics, larger unbalance tends to increase the possibility that a rotor will be able to remain in persistent contact with a touchdown bearing. A system has therefore been developed in which the touchdown bearings may be actuated so as to induce the rotor to return to contact-free levitation. This paper provides an assessment of the touchdown bearing motions that will realistically achieve this goal.

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