All high performances of active magnetic bearings (AMBs) greatly depend on the reliability of the AMBs in normal and emergency conditions. Since the loading capability of the AMBs is limited and the rotating rotor will drop to the stator when the current of the coils is suddenly cut off or some parts of the AMBs, such as sensor, control system, power amplifiers, fail. In order to improve the reliability of the AMBs, a parallel back-up support system, as part of the AMBs design, should be used. Therefore, the back-up bearing is one of key elements to use successfully the AMBs in the rotating machinery and to improve the reliability of the AMBs. In order to develop a good performance back-up bearing for the AMBs, a novel controllable back-up bearing consisted of a general back-up bearing and a controllable flexible support which is based on eddy-current principle is presented in this paper. After shown the basic structure and principle of the novel controllable back-up bearing, the dynamic behaviors of the controllable back-up bearing with a fixed clearance ball bearing to improve the dropping dynamics after the AMBs failure are experimentally investigated, and the effectiveness of the novel controllable back-up bearing in controlling the transient vibration during rotor dropping is validated. It is shown that the dynamic behaviors of the novel controllable back-up bearing can be controlled and the novel controllable back-up bearing can greatly suppress the transient vibration during rotor dropping.
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