# Modeling and Design of a Novel 5-DOF AC-DC Hybrid Magnetic Bearing
## 1 Introduction  
Currently, with the increase of rotor speed, the requirements for bearing performance are getting higher and higher. New types of bearings such as air-floated bearings, liquid-floated bearings, and magnetic bearings (MBs) have been employed. The friction between the rotor and bearing is reduced in high-speed electric machines supported by air-floated or liquid-floated bearings. However, special air-floated and liquid-floated equipment is required, which makes the motor system bulky, complicated in structure, and increased in cost. Moreover, there are inevitable problems of air and liquid leakage that prevent their use in pollution-free, ultra-clean special electric drives. MBs supported by electromagnetic force are new types of non-contact bearings. They have the advantages of no wear and lubrication, low vibration and noise [1; 2; 3; 4; 5; 6; 7; 8; 9], and can achieve higher rotor speed and power operation. Therefore, special machines use MBs, such as vacuum pumps, blood pumps, and heart pumps [10; 11; 12; 13]. Commonly, thrust MBs are adopted to control the axial directional translation. The radial two-degree-of-freedom (2-DOF) MBs are employed to control the two radial directional translations [14]. A 3-DOF MB is used to control the axial directional translation and two radial tilting motions [15; 16]. Recently, several studies have been proposed to develop the structures of MBs. In [8], a slice of 5-DOF four-pole MB was studied. The structure is compact. However, it is driven by DC power amplifiers, resulting in a bulky and costly system. In [17], a 3-DOF MB with a biased permanent magnet was designed and applied to a high-speed motor. Compared with the MBs using only solid magneticmaterials, a laminated radial stator was applied in the 3-DOF MB to reduce eddy currents. Considering the eddy-current effects and leakage effects, an accurate mathematical model was established. Meanwhile, in [18], a 3-DOF MB was designed and optimized with a hierarchical multiobjective optimization structure to improve both the radial and axial suspension. However, a large thrust disk causes many problems such as speed limitation, increased rotor unbalance, and rotor dynamic performance degradation. In [19], a novel four-pole 4-DOF MB with two active parts and one passive part was proposed. The structure of the 4-DOF MB was compact, so the size and power consumption were reduced. However, due to the application of four permanent magnets, the 4-DOF MB has high costs and is difficult to manufacture and assemble [20]. Moreover, because the axial single DOF is passively stabilized by reluctance forces, the axial load-carrying capacity is limited, and the output power of the motor is low. It is only suitable for applications where the axial impact and motor output power are small. Commonly, a full magnetic suspension drive system needs axial MBs, radial MBs, or radial-axial MBs, and a high-speed electric machine [21; 22; 23]. A combined structure of two or three MBs results in a rather long axial length of the rotor shaft and makes the structure complicated. It has low axial utilization, critical speed, and power density. Compared with bearingless motors such as bearingless permanent magnetic motors [24] and bearingless flux-switching permanent motors [25], combined structures have obvious advantages in terms of simple control and small coupling [26; 27]. In addition, traditional MBs generally have an eight-pole or four-pole structure [28; 29; 30]. The radial 2-DOF MB is driven by four unipolar or two bipolar DC power amplifiers. The whole MB system is bulky and costly [31; 32]. In order to reduce the size and save cost, a three-pole MB was proposed. At present, three-pole MBs mainly use two driving modes, namely a three-phase power inverter drive [33; 34; 35] and a DC power amplifier drive [36]. In comparison with DC MBs, AC MBs can effectively reduce the number of inverters, system volume, and cost [37; 38].