# Suppression of Harmonic Current in Magnetic Bearing-Rotor System with Redundant Structure
## 1 Introduction
Nevertheless, there are several scientific questions that have been studied and solved in magnetic bearings under the differential control method. For a rotor suspended with magnetic bearings under the differential control strategy, some advanced control methods have been applied to solve specific problems; a typical example is the mass unbalances of a rotor, and some researchers have proposed a series of approaches to suppress unbalanced rotor vibration. [PERSON] et al. [19] designed a real-time variable step polygon iterative search algorithm that includes an unbalance compensator to suppress rotor vibration. [PERSON] et al. [20] proposed an online unbalance compensation algorithm based on the least mean squares method and the influence coefficient method, and the experiment was carried out on a maglev rotor platform; the results demonstrated the effectiveness of the proposed control algorithm. In order to suppress the harmonic current of coils and rotor vibration produced by the mass unbalances and sensor runout, [PERSON] et al. [21; 22; 23; 24] analyzed the characteristics of the disturbance signal and proposed several repetitive controllers with different structures. Simulation and experimental results were given to show the superiority of proposed repetitive controllers. Moreover, to suppress the harmonic components in the full speed range, [PERSON] [25] and [PERSON] [26] proposed multiple phase-shift notch filters in series and in parallel modes. Moreover, in order to suppress the harmonic vibration of a magnetically suspended control moment gyro, [PERSON] et al. [27] proposed a control algorithm that uses the orthogonal characteristics of the output signals of the \(x\)-direction and \(y\)-direction displacement sensors. The electromagnetic force of the magnetic bearing in a magnetically suspended control moment gyro was directly used as the input signal of the control to produce zero magnetic force control, and the experimental results demonstrated the validity of the algorithm. [PERSON] [28] designed a linear active disturbance rejection (LADRC) for magnetic bearings, and the experiments demonstrated that the LADRC had better anti-interference performance compared to the PID controller. [PERSON] et al. [29] analyzed the dynamic characteristic of a flexible rotor suspended by magnetic bearings, and, according to the model of the system, the robust H\({}_{\infty}\) controller was proposed. The experimental results indicated that the controller had superior performance of vibration suppression, enabling the flexible rotor to pass the first bending critical speed. Furthermore, [PERSON] et al. [30] proposed a nonlinear adaptive algorithm whose asymptotic stability is guaranteed by [PERSON]'s theory to suppress the harmonics vibration of the magnetic bearing system. Using an unbalance suppression algorithm applied to piezoelectric active bearings for rotating machinery, [PERSON] et al. [31] presented a novel generalized notch filter for harmonic suppression control for magnetically levitated rotors, and the experimental results demonstrated that the control algorithm reduced the rotor in the level of generated vibrations by at least one order of magnitude. [PERSON] et al. [32] proposed a modified iterative learning control including an extended state observer to suppress the disturbance of the hybrid magnetic bearing system, and the experimental results showed that the proposed strategy had better reference tracking and disturbance suppression ability than PID and neural network inverse control. According to the aforementioned analysis, there has been no advanced control method studied for magnetic bearings with redundant structures. However, similar harmonic vibration exists in magnetic bearing systems with redundant structures. In this paper, magnetic bearings with redundant structures and no coil failure were considered as the research object.