To prevent rotor/stator contact in a rotor/magnetic bearing system, auxiliary bearings may be located along the shaft and at the magnetic bearings. Rotor responses after a contact event may include periodic trapped modes where repeated contact may lead to highly localized thermal stresses. This paper considers an active auxiliary bearing system with a control strategy designed to limit the trapped contact modes in a rotor/magnetic bearing system that are induced by rotor unbalance. The controller is evaluated from a system model and its responses to short duration contact events. An active auxiliary bearing model is introduced to the system where the dynamic response of the bearing is dependent on the controller. From a harmonic decomposition of rotor/bearing contact, dynamic controllers are sought which limit the numbers of possible periodic solutions for a given rotor unbalance and operating speed. A case study is performed considering a simple two degree of freedom system with passive and active auxiliary bearings. Recovery of a rotor trapped in an asynchronous contact mode is shown with variation of the auxiliary bearing controller parameters.

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