Vibration condition monitoring of rotors on AMB fed by induction motors

The presence of induction driver motors in rotors on active magnetic bearings (AMBs) can affect significantly the vibration monitoring operation because of the effect of the so-called unbalanced magnetic pull (UMP) on the rotor whirling cage. A sort of beat appears in monitored radial displacements, caused by the interaction between the unbalance response of the rotor and the UMP, depending on the slip occurring in the induction motor. This effect is analysed in the range of spin speeds where the rotor behaves as a rigid body. The equations of motion of the system include the negative stiffness provided by the motor in the presence of non-uniform air gap and are written under the assumptions of linearized forces for the AMB and rotor axis-symmetry. Some experiments have been performed to validate the proposed model of a spindle on magnetic suspension, with five active axes