Principles of AC Magnetic Bearings and Current Situation of Research and Development

Magnetic bearing (referred to as magnetic bearing) is a new type of bearing that uses electromagnetic force to suspend the rotor in space and realizes no mechanical contact between the rotor and the stator. Therefore, it has no friction, no wear, no lubrication and sealing, high speed, high The advantages of precision and long life have changed the traditional form of contact support [1]. According to the type of power amplifier control current, the magnetic bearing can be divided into DC type and AC type. DC magnetic bearing power amplifiers are expensive and bulky, and a radial magnetic bearing usually requires four-way unipolar power amplifier circuits [2-4]; while AC magnetic bearings use AC three-phase power inverters to control coil windings Provide excitation current, a three-phase power inverter can completely control two radial degrees of freedom, and three-phase inverter application technology is mature, cheap, small size, using vector control strategy, easy to control system software programming and programming. transplant, thereby reducing the cost of the magnetic bearing control system as a whole [5-8]. Therefore, the AC magnetic bearing has a good application prospect in the high-speed motion occasions that require magnetic suspension support, such as the magnetic bearing electric spindle system, the bearingless motor, and the high-speed flywheel energy storage system.

1 The composition of the AC magnetic bearing system and its basic

Principle The motion of a rigid body in space includes translation and rotation, with a total of 6 degrees of freedom [9]. The state of the active magnetic bearing described in this paper is to rotate around the main axis, that is, the z direction, so the remaining 5 degrees of freedom need to be controlled, which requires two radial bearings (each responsible for the x, y directions) and a shaft. To the bearing (responsible for the z direction), which constitutes a complete electromagnetic bearing system.

Here, an AC two-degree-of-freedom hybrid magnetic bearing is used as an example to illustrate its working principle [7-8,10]. Figure 1 is a schematic diagram of the working principle of the AC two-degree-of-freedom hybrid magnetic bearing. The working principle of the AC magnetic bearing is based on the principle of a bearingless motor, so that the number of pole pairs P1 of the torque winding is 0, and the number of pole pairs P2 of the suspension force winding is 1, which satisfies the condition P2=P1±1 generated by the radial suspension force. The three-phase power inverter provides driving current to the suspension force winding, so the bearingless motor with this structure actually becomes a magnetic bearing that only generates radial suspension force [8, 10-11]. According to the motor theory, after the three-phase winding is connected to the three-phase alternating current, a rotating magnetic field can be generated to form a unipolar synthetic magnetic flux. When the magnetic bearing is subjected to radial disturbance, the rotor deviates from the radial reference equilibrium position, the position sensor detects the radial offset position x and y of the rotor, and feeds it back to the radial controller, which is in phase with the reference equilibrium position x* and y* After the comparison, the controller calculates the offset x and y of the rotor, and then converts it into a control signal, and provides three-phase control currents ia, ib and 3 to the three radial control coils through the current tracking power inverter (CRPWM). ic, so that the synthetic monopole magnetic flux generated by the control current in the radial three-phase winding can be directed in the opposite direction to the position offset, and the corresponding radial magnetic levitation force is generated to make the rotor return to the radial equilibrium position.

2 Classification and characteristics of AC magnetic bearings

2.1 Active and Hybrid

AC magnetic bearings are classified according to the principle of levitation force, and can be divided into active magnetic bearings [2-3, 5, 7] and hybrid magnetic bearings [4, 6, 8]. The magnetic flux that produces the static levitation force is called the bias magnetic flux; the magnetic flux that produces the dynamic levitation force, that is, the magnetic flux that overcomes the disturbance or load is called the control magnetic flux.

Active Magnetic Bearing (AMB) uses electromagnets to provide bias magnetic flux and control magnetic flux at the same time, which can provide larger static bearing capacity. However, due to the large number of ampere turns of the coil, the volume is large, and the power amplifier circuit is still used to provide the bias magnetic flux when it is suspended in a steady state, so the energy consumption is high and the loss is large. At present, the most studied is the three-phase AC active magnetic bearing [7, 10, 12]. As shown in Figure 2, the magnetic bearing consists of a stator core with three magnetic poles, a three-phase excitation coil and a rotor core. When the synthetic levitation force generated after the three-phase excitation coil is connected to the three-phase alternating current is equal to the gravity of the rotor core itself and the external interference force, the alternating current magnetic bearing rotor can realize the suspension. At present, Jiangsu University in China mainly studies the magnetic bearing of this structure, and the University of Navarra in Spain and the Korean Institute of Science and Technology are studying this structure in foreign countries. Different from the three-pole structure magnetic bearing, there is also a six-pole structure AC active magnetic bearing [13], as shown in Fig. 3, the AC active magnetic bearing is composed of three U-shaped iron cores with three-phase excitation coil stators and three-phase excitation coils. The rotor is composed of the same working principle as the above-mentioned magnetic bearing.

Hybrid Magnetic Bearing (HMB) relies on permanent magnets to provide bias magnetic flux, that is, the magnetic force generated by the permanent magnet supports the self-weight of the rotor, and the electromagnet only needs to provide the control magnetic flux to ensure that the rotor can return to the rotor when it is disturbed. The original suspension position; therefore, its power amplifier is small in size, compact in structure, small in energy consumption during steady-state operation, and low in loss [4, 6, 8-9]. At present, the structure of domestic research is two-piece (3 × 2) six-pole AC two-degree-of-freedom hybrid magnetic bearing [8], as shown in Figure 4. The stator of the magnetic bearing is superimposed by two identical three-phase AC active magnetic bearings, so that there are two sets of control coils (excitation coils), and the two sets of control coils are connected in series respectively. Set the magnetic flux, when there is no external interference and static suspension, the bias magnetic flux can make the rotor levitate in the center position; when disturbed, the two sets of coils are energized at the same time to generate a magnetic force opposite to the direction of the interference force to pull the rotor back to the original position Central location. The main domestic research institution is Jiangsu University. There is also a six-pole AC two-degree-of-freedom hybrid magnetic bearing [14], as shown in Figure 5, by embedding permanent magnets on three of the poles to generate bias magnetic flux; the other three poles are wound with three-phase control coils , when the rotor is disturbed, it is used to control the rotor to remain suspended in the equilibrium position. The foreign research institution is mainly the Korea Institute of Science and Technology.

2.2 Heteropolar and homopolar

The magnetic bearing with the magnetic field line perpendicular to the axis of the rotor is called a heteropolar magnetic bearing. In this structure, the magnetic bearing is similar to the motor and is easy to manufacture. Therefore, most magnetic bearings currently use heteropolar type, but the disadvantage of this structure is that the hysteresis loss is relatively large. Therefore, in order to make the hysteresis loss as small as possible, the rotor must be laminated, that is, the magnetic action part of the rotor must be It is made of pressed circular punching sheets. Since the simulation of the mechanical system, the design of the control system and the measurement of the rotor motion are usually based on the Cartesian coordinate axis, in order to simplify the control of the bearing, the layout of the radial magnetic bearing generally adopts a three-pole structure [10] , 14], as shown in Figure 2. In the application of large magnetic bearings, it is sometimes possible to increase the number of magnetic poles to achieve its purpose.

A magnetic bearing whose magnetic field lines are parallel to the rotor axis is called a homopolar magnetic bearing. As shown in Figure 6. This layout is usually called homopolar magnets, and its hysteresis loss is relatively small, and the rotor may not need laminations [8]. This structure is mainly used in those occasions where laminations cannot be used for the rotor due to various reasons. This structure uses a quadrupole electromagnet, and the differential structure is also used in the two degrees of freedom.

2.3 Number of poles

At present, there are only two types of magnetic poles in AC magnetic bearings: 3 poles and 6 poles. Figures 2 and 4 are magnetic bearings with a three-pole structure, and Figures 3 and 5 are magnetic bearings with a 6-pole structure. AC magnetic bearings mostly use three-phase power inverters to provide excitation current to the control coil windings. One three-phase power inverter can completely control the radial two degrees of freedom, and the three-phase inverter application technology is mature, cheap and bulky. It is compact and adopts the vector control strategy, which is easy to program and transplant the control system software, thereby reducing the cost of the magnetic bearing control system as a whole. The three-pole structure is not only easy to realize the centralized control of the rotor radial displacement, but also has the advantages of low cost, low iron loss, good heat dissipation performance, and easy installation of windings and sensors. It is widely used in magnetic bearing systems, bearingless motors, and high-speed flywheel energy storage systems. It has a good application prospect in high-speed motion occasions such as magnetic suspension support.

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