A Negative Stiffness Injection Method to Reduce Vibration for PMSM in 5-DoF Magnetic Levitation Systems

Magnetic bearings (MBs) are widely applied in high-speed permanent magnet synchronous motors (PMSMs), which can avoid the friction loss and mechanical vibration by levitating the shaft. However, there is no active stiffness on the rotor of PMSMs, which can also generate the unbalanced magnetic pull (UMP) and vibration due to the changing air gap between stator and rotor. Further, it can deteriorate the stability of magnetic levitation system. Therefore, a negative stiffness injection method for PMSM in five degrees of freedom (5-DoF) magnetic levitation systems is proposed in this article, which is realized by injecting negative d-axis current to weaken the UMP and without other windings or equipments. Simulation and experimental results both verify that the multifrequency vibration accelerations are greatly reduced and the operation safety of magnetic levitation system is improved. Moreover, the proposed method is benefit for PMSM to pass the resonant frequency safely as well.