The working principle and classification of magnetic bearings

 

Magnetic bearing systems can be divided into three categories according to their working principles: Active Magnetic Bearing, Passive Magnetic Bearing, and Hybrid Magnetic Bearing.

 

Active magnetic bearing

 

Active magnetic bearings use controllable electromagnetic force to levitate the rotating shaft, which is mainly composed of rotors, solenoids, sensors, controllers, and power amplifiers. The solenoids are mounted on a stator that is suspended in a magnetic field generated by the electromagnets placed in radial symmetry, each of which is equipped with one or more sensors to continuously monitor changes in the position of the shaft. The signal output from the sensor, with the help of the electronic control system, corrects the current through the electromagnet, so as to control the attraction of the electromagnet, so that the rotating shaft runs in a stable and balanced state, and achieves certain accuracy requirements.

 

Active magnetic bearings can be divided into current control and voltage control according to different control methods, and can be divided into radial magnetic bearings and axial magnetic bearings according to different support methods. At present, among the active magnetic bearings, the most widely used is the DC controlled magnetic bearing.

 

The mechanical part of the active magnetic bearing is generally composed of a radial bearing and an axial bearing, and the radial bearing is composed of a stator (electromagnet) and a rotor; Axial bearings consist of a stator (electromagnet) and a thrust plate.

 

Because the active magnetic bearing has the advantages of rotor position, bearing stiffness and damping can be determined by the control system, it has been the most widely used in the field of magnetic levitation, and the research of active magnetic bearing has always been the focus of magnetic levitation technology research. After years of hard work, its design theory and methods have become more and more mature.

 

Passive magnetic bearing

 

As a form of magnetic bearing, passive magnetic bearing has its own unique advantages, it is small in size, no power consumption, and simple in structure. The biggest difference between passive magnetic bearings and active magnetic bearings is that the former does not have an active electronic control system, but uses the characteristics of the magnetic field itself to levitate the rotating shaft. At present, the most widely used passive magnetic bearings are permanent magnet bearings composed of permanent magnets. Permanent magnet bearings can be divided into two types: repulsion type and suction type.

 

Passive permanent magnet bearings can be used as both radial bearings and thrust bearings (axial bearings), both of which can be suction or repulsion. Depending on the magnetization direction and relative position of the magnetic ring, permanent magnet bearings have a variety of magnetic circuit structures. But there are two basic structures

 

The other type of passive magnetic bearing is based on the suction force, which acts between the magnetized soft magnetic components. When the rotor component moves radially, the suction effect comes from the change in magnetoresistance, so it is also called a "magnetoresistive bearing". This kind of bearing can be designed so that the permanent magnet part does not rotate, and only the soft iron part rotates, so that the system has better stability.

 

The combination of the stabilizing effects of reluctance bearings and active solenoids results in a magnetic bearing system with minimal energy consumption.

 

Hybrid magnetic bearings

 

Hybrid magnetic bearings are formed on the basis of active magnetic bearings, passive magnetic bearings and some other auxiliary support and stabilizing structures - a kind of combined magnetic bearing system. It takes into account the comprehensive characteristics of active magnetic bearings and passive magnetic bearings.

 

The hybrid magnetic bearing is to use the magnetic field generated by the permanent magnet to replace the static bias magnetic field of the electromagnet, which can not only significantly reduce the power consumption of the power amplifier, but also reduce the number of ampere turns of the electromagnet by half, reduce the volume of the magnetic bearing, and improve the load-bearing capacity.

 

Since a bias magnetic field is generated by a permanent magnet and a controlled magnetic field is generated by the electromagnet, permanent magnet offset hybrid magnetic bearings have the following advantages:

 

1) The permanent magnet is used to provide the bias static magnetic field, and the electromagnet only provides the control magnetic field for balancing the load or external interference, which can avoid the power loss caused by the bias current of the system and reduce the heating of the coil.

 

2) The number of turns required for the electromagnet of the hybrid magnetic bearing is much smaller than that of the active magnetic bearing, which is conducive to reducing the volume of the magnetic bearing and saving materials. This kind of bearing has the advantages of small size, light weight and high efficiency, and is suitable for miniaturization and small size applications.