Resonant amplitude reduction of a rotor supported by a superconducting magnetic bearing with an axial electromagnet

Superconducting magnetic bearings (SMBs) have various merits because of noncontact stable levitation. However, SMBs tend to make the rotor amplitude larger near a critical speed because of their low damping. Furthermore, complicated vibrations can be caused due to the nonlinearity of the electromagnetic force. Therefore, it is necessary to reduce the amplitude by considering the effect of the nonlinearity on dynamics during passing through the critical speed in applications. In this paper, we investigate resonant amplitude reduction of a rotor supported by an SMB using axial translation of the rotor. For this purpose, we first introduced an analytical model consisting of an SMB, a permanent magnet, and an electromagnet (EM) that can translate a rotor in the axial direction. We then examined the effect of exerting electromagnetic force at an appropriate rotational speed on amplitude reduction of a rotor. Numerical results show that considerable reduction of the resonant amplitude can be achieved by switching the EM at a proper rotational speed both in steady-state response and in transient response. We also carried out experiments. Steady-state responses were measured with changing the distance between the superconducting bulk and the rotor, magnetic induction of the EM, and so on. Experimental results show qualitative agreement with numerical ones, which verifies the effectiveness of the proposed method of amplitude reduction. We further discussed effect of nonlinearity of electromagnetic forces on amplitude reduction.