Unbalance Control for High-Speed Active Magnetic Bearing Systems Without Speed Sensors

In high-speed active magnetic bearing (AMB) systems, synchronous vibration caused by rotor mass unbalance poses a significant challenge. To address this issue, the article proposes an unbalance control strategy that works without speed sensors. This strategy integrates a coordinate transformation (CT)-based unbalance compensator for displacement vibration suppression with a phase-locked loop (PLL)-based estimator for precise rotor frequency detection. Through the transfer function model, the frequency tracking capability of the PLL-based estimator is illustrated and a guideline for parameter tuning is provided. For the CT-based compensator, the system stability criterion is clarified based on the complex-vector analysis method, revealing the significant role of an additional phase compensation angle in enhancing system stability. The optimal control parameters are also tuned via the root locus method. Moreover, the control model of an unbalanced AMB rotor with the proposed strategy is constructed and validated through simulations. Finally, the experiments including both constant speed and run-up load tests are conducted on a magnetically suspended high-speed machine. All the results exhibit the highly accurate estimation in rotor frequencies with the amplitudes of synchronous displacement vibrations being nearly suppressed to zero, confirming the effectiveness of the proposed strategy.