Semi-Active and Active Magnetic Stabilization of Supercritical Rotor Dynamics by Contra–rotating Damping

High speed rotors are usually operated above the critical speed to achieve a good selfcentring and to reduce the reactions of bearings. Damping associated to the rotating parts induces some dynamic instability, when the system rotates faster than an angular velocity defined as 'instability threshold'. To increase the range between the critical speed and the instability threshold the designer usually applies to the stator a suitable amount of non-rotating damping, being always stabilizing. In some application there is no stator available (spacecrafts, satellites) and in rotors suspended on active magnetic bearings the control current may be fairly large, if the dynamic stability has to be assured at a high spin speed. A new kind of active magnetic stabilization is therefore proposed to overcome those limits. The damping action of the stator is here rotating, but its vector and the rotor spin speed are just opposite. Therefore that action appears as 'contra-rotating'. This approach provides an asymptotic dynamic stability of both the forward and backward whirling motions, even in the high supercritical regime. Design issues, implementation and results obtained in case of a semiactive system based on permanent magnets as well as of an active magnetic device are herein discussed.