The Working Principle and Classification of SKF Magnetic Bea

Writer: Eric Bearing Limited

SKF magnetic bearing systems can be divided into three categories according to their working principles: active magnetic bearings, passive magnetic bearings, and hybrid magnetic bearings.

1. Active Magnetic Bearing

Active magnetic bearings use controllable electromagnets to suspend the shaft. It is mainly composed of rotors, electromagnets, sensors, controllers, and power amplifiers. The electromagnet is installed on the stator, and the rotor is suspended in the magnetic field generated by the electromagnets placed symmetrically in the radial direction. Each electromagnet is equipped with one or more sensors to continuously monitor the position change of the rotating shaft. The signal output from the sensor, with the help of an electronic control system, corrects the current passing through the electromagnet, thereby controlling the attractive force of the electromagnet, so that the rotating shaft runs in a stable and balanced state and meets certain accuracy requirements. After the sensor detects the displacement of the rotor from the reference point, the microprocessor as the controller converts the detected displacement into a control signal, and then the power amplifier converts the control signal into a control current, which is generated in the actuator electromagnet Magnetism thus enables the rotor to maintain its stable floating position. The stiffness, damping and stability of the suspension system are determined by the control system.

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 active magnetic bearings, the most widely used is the DC-controlled magnetic bearing.

The mechanical part of active magnetic bearings-generally consists of radial bearings and axial bearings. The radial bearing is composed of a stator (electromagnet) and a rotor; the axial bearing is composed of a stator (electromagnet) and a thrust plate. In order to overcome the eddy current loss, the stator and rotor (shaft journal part) collars are all laminated by punching sheets. The electromagnet of the radial bearing is similar to the stator structure of the motor, and the number of magnetic poles can be 8 poles, 16 poles or more.  SKF 6306-2Z bearings online , pls click here : 

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

2. Passive magnetic bearings

Passive magnetic bearing, as a form of magnetic bearing, has its own unique advantages. It has small size, no power consumption, and simple 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 suspend the shaft. From the current point of view, in passive magnetic bearings, permanent magnetic bearings composed of permanent magnets are the most widely used. Permanent magnetic bearings can be divided into two types: repulsion type and suction type.

Passive permanent magnetic bearings can be used as radial bearings and thrust bearings (axial bearings) at the same time. Both types of bearings can be of suction or repulsion. Depending on the magnetization direction and relative position of the magnetic ring, permanent magnetic bearings have a variety of magnetic circuit structures.

Permanent magnetic bearings can be composed of radial or axial magnetized rings. Rigidity and bearing capacity can be increased by superposing multiple pairs of magnetic rings.

Another type of passive magnetic bearing is based on the suction force, which acts between the magnetized soft magnetic parts. When the rotor parts move in the radial direction, the attraction effect comes from the change of magnetic resistance, so it is also called "reluctance bearing". This kind of bearing can be designed so that the permanent magnet part does not rotate, only the soft iron part rotates, so that the system has better stability.

Combining the stabilizing effects of reluctance bearings and active electromagnets can form a magnetic bearing system with minimal energy consumption.

For permanent magnetic bearings, when a certain load is applied to the shaft, the working air gap between the rotor and the stator ring will change. The repulsive force at the minimum working air gap is greater than the repulsive force at the maximum air gap, so that the shaft is radially The position changes and tends to balance. As mentioned earlier, it is impossible to obtain a stable balance only with permanent magnetic bearings, and it is unstable at least on one coordinate. Therefore, for a permanent magnetic bearing system, at least one direction must attract external forces (such as electromagnetic force, mechanical force, aerodynamic force, etc.) to achieve system stability.

3. Hybrid magnetic bearings

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

Hybrid magnetic bearings use the magnetic field generated by the permanent magnet to replace the static bias magnetic field of the electromagnet, which not only can significantly reduce the power consumption of the power amplifier, but also can reduce the ampere turns of the electromagnet by half, reducing the volume of the magnetic bearing , Improve the carrying capacity and so on.

Since the permanent magnet generates the bias magnetic field and the electromagnet generates the control magnetic field, the permanent magnet bias hybrid magnetic bearing has the following advantages:

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

(2) The number of ampere turns required by the electromagnet of the hybrid magnetic bearing is much smaller than that of the active magnetic bearing, which is beneficial to reduce the volume of the magnetic bearing and save material. This kind of bearing has the advantages of small size, light weight and high efficiency, and is suitable for miniaturization and small size applications.