[Guide]The electric motor must be known to everyone, and the rotor “huhhuhu” rotates, driving the development of the industry. In the field of industry and defense, rotating machinery is the most important source of power. Whether it is aircraft engines in heavy industry, ship propeller propulsion, automobile high-speed rail power systems, or compressors for central air conditioning in daily life, rotating machinery can be seen in these devices. In rotating machinery, the bearing is a very critical component, which supports the rotor in the rotating machinery. If there is no bearing, the rotor cannot rotate freely in a fixed position.
This article is the second prize work of the Electricity, Power Supply and Power Electronics Popular Science Essay Contest in Life. The original title “Magnetic Bearings-Application of Power Electronics in the Field of High Speed Rotating Machinery”, author: Jiang Dong, Sun Hongbo, Yang Jichang, Liu Zicheng, Shuai Yixuan , From Huazhong University of Science and Technology, State Key Laboratory of Strong Electromagnetic Engineering and New Technology.
01 Background Introduction of Magnetic Bearings
Everyone knows the electric motor, and the rotor “huhhuhu” rotates, driving the development of industry. In the field of industry and defense, rotating machinery is the most important source of power. Whether it is aircraft engines in heavy industry, ship propeller propulsion, automobile high-speed rail power systems, or compressors for central air conditioning in daily life, rotating machinery can be seen in these devices. In rotating machinery, the bearing is a very critical component, which supports the rotor in the rotating machinery. If there is no bearing, the rotor cannot rotate freely in a fixed position.
The importance of bearings in industry and national defense is irreplaceable. In the Second World War, the German army’s rapid lightning tactics relied on a large number of fighters and tanks, all of which were inseparable from the ball bearings in various rotating machinery. In order to contain the German forces, the British and American allies used large-scale air strikes to strike at the German ball bearing factory in Schweinfurt. This caused a serious shortage of German equipment bearings, which directly affected the German combat capabilities and brought about the battle. Had an important impact.
Figure 1. Allied forces bombed the German ball bearing factory (from the Internet)
At present, the most widely used type of bearing in the industry is a ball bearing. Use low-friction balls to achieve contact support. In addition, the ball bearings need to be added with lubricating oil to further reduce friction. However, with the increase of speed and friction, this method of contact support will encounter a lot of trouble at high speeds. In addition, in many applications, in order to ensure the cleanliness of the environment, the use of lubricating oil is prohibited.
I believe you may have some ideas. Since the friction in contact bearings is difficult to reduce, try to use non-contact bearings. Based on this idea, two other mechanical bearings were invented. One is the Foil Bearing. During rotation, the foil and the shaft form a film, which can be out of contact. The other is an air bearing, in which compressed gas is passed. When the rotor rotates to a certain speed, it can also be separated from the stator. Compared with traditional contact ball bearings, these two methods have the advantage of low friction under high-speed operating conditions. However, it is generally only suitable for high-speed, and the process is relatively complicated. Currently, it is mainly used in special application scenarios.
Figure 2. Several typical mechanical bearings (picture from the Internet)
Different from the above-mentioned mechanical bearings, the magnetic suspension bearing developed in recent years adopts controllable electromagnetic force to realize the non-contact support of the rotor, which can adapt to the application of a wide speed range from static to high speed, and has the advantages of no lubricating oil, no contact friction, etc. . Therefore, magnetic bearings have become a new generation of “leading protagonists” in the support of rotating machinery in various high-end equipment.
The picture on the left below shows a typical magnetic levitation bearing, which is essentially an electromagnetic mechanism, which achieves levitation by generating controllable electromagnetic force. The principle structure diagram of the magnetic bearing is shown in the right figure below. The main structure includes the controller, sensor, power amplifier (driver), winding and so on. High-end device, but its principle is not complicated. The basic principle of magnetic bearing can be explained by knowledge of high school physics: in the direction of each axis, the electromagnetic force generated by the current on both sides of the winding can attract the rotor. In a certain balance state, the electromagnetic forces on both sides are just in balance with the other forces on this axis, and the rotor can remain suspended at this position without contacting the stator.
Figure 3. Magnetic bearing
It can be seen from the above figure that the magnetic bearing is an unstable system: when the rotor is disturbed and shifts to one side, the air gap on this side decreases and the magnetic attraction force increases. If there is no fast and accurate current control at this time, the rotor will eventually be attracted to the limit position by the electromagnetic force on this side, and the suspension will inevitably fail.
Therefore, the current of the magnetic bearing system needs to be dynamically adjusted according to the position of the rotor at this time. For example, when the rotor is lower, the upper current needs to increase, and the lower current needs to decrease. The difference between the upper and lower electromagnetic forces will lift the rotor upward. . The prerequisite for the realization of dynamic adjustment is that the position detection must be fast enough and accurate to detect micron-level displacements. The current command (small signal) is a “decision” signal made by the controller after learning the current rotor position. With the “decision” signal, the implementation of the “decision” is needed next. Performing this step can be understood as turning the current command, the “bit world” quantity, into the real current in the winding, which is exactly the task of the power amplifier. After the generated real current flows in the windings, electromagnetic force will be applied to the rotor, and the position of the rotor will change. The sensor collects the rotor position information again, and sends it to the controller to generate a current command, the power amplifier generates the real current, and the electromagnetic force adjusts the rotor position. Such a reciprocating cycle forms a “closed loop control”, which makes the rotor dynamically levitate in the expected position.
In fact, Figure 3 is a schematic diagram of the principle of a magnetic bearing with only the upper and lower positions controlled. In real rotating machinery applications, the rotor generally needs to be supported in five positions (that is, five degrees of freedom), including four radial directions and one stop. Push the bearing. The figure below is a schematic diagram of a typical five-degree-of-freedom magnetic bearing system. In order to support the rotor, two-axis radial magnetic bearings with a plane on each side are required. Each plane contains a pair of bearings shown in Figure 3(b) to control the suspension in two orthogonal directions. In order to control the axial (z direction) displacement of the rotor, a thrust bearing loaded on the thrust plate is also required. In addition to magnetic suspension bearings, mechanical protection bearings are often equipped on the periphery of the rotor. The protective bearing is mainly used to support the rotor when the rotor is unstable and falls to prevent damage to the electromagnetic mechanism. It is also used to support the rotor after the system is shut down. In order to achieve good support characteristics, the control of the magnetic bearing system is essential.
Figure 4. Five-degree-of-freedom magnetic bearing
02 Magnetic Bearings and Power Electronics Technology
Through the above introduction, I believe that everyone has a certain understanding of magnetic bearing. It can be seen that magnetic bearing is a multidisciplinary field, including mechanics, electromagnetics, sensor and signal processing, control theory, and power electronics technology, etc. knowledge. Among them, the power amplifier based on power Electronic technology is an important link in the magnetic bearing control, and it is the actuator that realizes the conversion of the “bit world” in the controller into real current.
In order to make the magnetic bearing performance better, the power electronic controller has certain index requirements. The first is that the response speed must be fast enough to quickly respond to position fluctuations; the second is to output current with low ripple and noise, which is to ensure the stability of the electromagnetic force; the converter also needs low loss and high efficiency to ensure that the heat dissipation meets the requirements Further, in order to reduce costs, the converter hopes to use as few devices as possible while ensuring performance; in addition, when the device fails, it can continue to maintain a stable suspension to avoid safety accidents such as unit damage.
Let’s understand the working principle of magnetic bearings from a deeper level. It can be seen intuitively that the electromagnetic force is related to the control current and the air gap. In fact, there is a more complicated mathematical relationship between them. In order to improve the linearity of the electromagnetic force in the electromagnet with respect to the control current and the air gap, and facilitate the simplification of the control model, it is first necessary to inject a DC bias current into the winding through a power electronic converter to excite the magnetic field in the electromagnet to a stable operation point. On this basis, the control current is superimposed according to the demand of electromagnetic force increase and decrease. For example, if the bias current is 5 amperes, when the control current is 1 ampere, the currents of the two windings facing the same degree of freedom are (5+1=6) amperes and (5-1=4) amperes respectively, then The force of the 6 ampere winding is greater than the force of the opposite 4 ampere winding, so that the force effect of the 1 ampere control current is loaded on the rotor. The sum of the two winding currents is 10 amperes, which is twice the bias current. In addition, the electromagnetic force of the magnetic bearing is the magnetic resistance. So what is magnetic resistance? To give a simple example, the magnet can attract ferromagnetic materials regardless of the N- or S-polarity, and the direction of the electromagnetic force has nothing to do with the polarity. Therefore, passing currents in different directions is equivalent to forming the N/S extreme facing the ferromagnetic material, with different polarities, but the electromagnetic force plays an attractive role. The electromagnetic force is only related to the amplitude of the current, and our daily contact is more The electromagnetic force of an AC motor is related to the direction of the current. The above-mentioned characteristics are the main difference in principle between the magnetic bearing power electronic controller and the AC motor controller. Figure 5 is a schematic diagram of the magnetic field polarity and the direction of electromagnetic force. Figure 5(a) is a magnetic bearing. The electromagnetic attraction generated by controlling the winding current makes the rotor levitate in the center position. Into the alternating current, the generated magnetic field rotates in space, and its torque drives the rotor to rotate.
Figure 5. Schematic diagram of magnetic field polarity and electromagnetic force
Cooperating with power electronic control, the magnetic suspension bearing winding current can be characterized by the three methods shown in Figure 6. Each winding of the magnetic bearing can be equivalent to an inductance. Under the condition of DC bus power supply, the switch tube combination loads the DC bus voltage on the winding in the forward direction, and the winding current will increase rapidly (Figure 6(a)); if the switch tube is turned off, the winding current will reverse through the anti-parallel diode Connect the DC bus to load the negative bus voltage on the windings, and the winding current will drop rapidly (Figure 6 (b)); if one switch tube is turned on and the other is turned off, the winding current will flow through the switch tube and the anti-parallel diode , The voltage loaded on the winding is close to zero, and the winding current is close to stable (Figure 6(c)). The magnetic bearing system can achieve high dynamic performance under the main mode (a) and (b), and can maintain a small current ripple ripple under the main mode (c).
Figure 6. The basic combination of magnetic bearing power electronic control current
There are the following two common magnetic bearing power electronic circuits evolved from the three working modes. The left picture below shows the “half-bridge” bridge arm structure. By turning on and off the switch tubes Sa and Sb, the basic combination of the various modes in Figure 6 can be realized, and the current can be quickly controlled. From the voltage point of view, it can be considered that a controllable voltage is applied to both ends of the winding, namely 0, VDC, and -VDC. In addition, as mentioned above, the currents of the two facing windings that control the same axis satisfy the sum of two times the bias current, which is a fixed value. Therefore, the two facing windings can be set as shown in Figure 7 (b ) Is connected in the manner shown, each of the two windings has one end connected to the intermediate bridge arm, and the intermediate bridge arm controls the potential at this point. This structure is called the “common bridge arm” structure. Compared with the “half bridge” structure, when the “common bridge arm” structure is adopted, the switching devices required for each degree of freedom can be reduced, while the driving circuit of the switching devices and other electrical devices are saved, and the overall device volume can also be reduced.
Figure 7. Topological structure of magnetic bearing power electronic power amplifier
As mentioned earlier, the electromagnetic force of the magnetic bearing has nothing to do with the direction of the current. Both forward and reverse currents can achieve the same electromagnetic force. This feature provides a new idea for fault-tolerant control under faults. In power electronic circuits, open-circuit failures of switching devices, loss of gate drive signals, and damage to the gate drive circuit are frequently encountered problems. When the above failure occurs, if the rotor happens to be in a high-speed running state, the rotor will fall at a high speed, causing serious safety accidents such as impact wear and even destruction of the unit structure. Therefore, it is necessary to automatically determine the occurrence of the fault in time and accurately, and take corresponding measures to ensure that the rotor does not fall.
Fortunately, there are redundant devices in power electronic power modules based on the full-bridge structure on the market today, which provides the possibility of fault tolerance. Taking Figure 8 as an example, the structure shown in Figure 8 (a) is a common three-phase full bridge power module, which can be regarded as the two “common bridge arms” structure shown in Figure 8 (b) and Figure 8 (c) Overlay. Under normal conditions, the power amplifier works in the mode of Figure 8(b), and the sum of the two winding currents is twice the bias current; when an open-circuit fault occurs, the sum of the two winding currents is less than twice the bias current, so In order to judge the occurrence of the fault; after the controller judges that the fault has occurred, it sends a switching command to switch the mode of Fig. 8(b) to the mode of Fig. 8(c). Although the current is reversed, it does not affect the direction of the electromagnetic force. Based on the above mechanism, “fault-tolerant control” under fault conditions such as open circuit of switching devices is realized.
Figure 8. Magnetic bearing power electronic fault-tolerant control
03 A wide range of applications of magnetic bearings
Driven by power electronics technology, the technology of magnetic bearings has become more mature and has begun to be widely used in all walks of life.An earlier application of magnetic bearing technology is the French military’s application of magnetic bearing technology to satellite attitude control in 1972. Since then, the application of magnetic bearings in the aerospace field has gradually increased and expanded to other fields. The main applications at present are:
(1) High-speed motor: The magnetic suspension bearing can avoid friction and collision when the rotor is rotating, and the rotor is easy to obtain a higher speed.A typical case is: In February 2021, Calnetix Technologies of the United States developed high-speed online magnetic levitation fans and dual-controller products, and applied them to NASA’s next-generation carbon dioxide removal system.
Figure 9. Magnetic levitation high-speed motor (from the network)
(2) Centrifugal compressor: The compressor is the core rotating machinery of the air-conditioning unit, and the magnetic suspension bearing can replace the lubricant and lubrication device of the traditional bearing, and it does not pollute the compressor refrigerant. In addition, the magnetic bearing can improve the operating efficiency of the compressor, which can save energy by about 48%. At present, all major air-conditioning companies have applied compressor products with magnetic suspension bearings. Representative companies mainly include Danfoss, Gunes, Mecos of Switzerland, Seiko of Japan, OKBM of Russia, etc. In 2019, Wuhan Metro was equipped with a magnetic levitation central air-conditioning product launched by Gree.
Figure 10. Magnetic levitation central air-conditioning unit (from the network)
(3) Flywheel energy storage: The working principle of flywheel energy storage is to use electrical energy to drive the flywheel to rotate and convert electrical energy into mechanical energy for storage. When the electrical energy needs to be released, the flywheel decelerates and the mechanical energy is converted into electrical energy. Energy storage and output are achieved through the acceleration and deceleration of the flywheel. When the flywheel rotates rapidly, friction loss is inevitable, and then the magnetic levitation can come in handy again. The frictionless support characteristics of the magnetic suspension can significantly reduce the mechanical loss during the rotation of the flywheel and improve the energy conversion efficiency. In July 2019, at the Guangyangcheng Station of Beijing Subway, the megawatt-level flywheel energy storage device that applied magnetic bearing technology was officially commercialized, filling the domestic application of flywheel energy storage devices in the field of regenerative braking energy recovery in urban rail transit. Whitespace.
Figure 11. Magnetic levitation flywheel energy storage system (from the network)
(4) Artificial heart pump: The loss or failure of the heart pumping function can cause serious diseases and even endanger life safety. For the treatment of patients with heart failure, in addition to the use of heart transplantation, another important treatment method is an artificial heart pump. The ECMO that everyone often hears during the 2020 epidemic also has a similar principle. The magnetic levitation artificial heart pump combines the magnetic levitation technology with the artificial heart pump to provide power for human blood circulation, and the non-contact support advantage of magnetic levitation can avoid the damage of traditional bearings to blood cells, while effectively reducing the patient’s myocardial oxygen consumption and promoting the patient’s cardiac function recover. In July 2021, Union Hospital Affiliated to Tongji Medical College of Huazhong University of Science and Technology performed an external magnetic levitation artificial heart assisted operation for a 31-year-old heart failure patient, and successfully performed a heart transplantation 12 days later. Clinical Use Case of Asphyxia Assist Device.
Figure 12. Magnetic levitation artificial heart (from the Internet)
In addition to the above-mentioned typical applications, magnetic bearing technology is also used in satellite attitude control, motorized spindle machine tool processing and other fields. Magnetic bearings are an important part of the future intelligent machinery, with the advantages of no friction, no collision, no lubrication, and active control. The author believes that magnetic bearings will play a more important role in industry, national defense, medical and other fields.
 Gerhard Schweitzer, Eric H. Maslen (author), Xu Yang, Zhang Kai, Zhao Lei (translated). Magnetic Bearings—Theory, Design and Application of Rotating Machinery[M]. Beijing: Mechanical Industry Press, 2013.147~179
 Calnetix Technologies Supplies Key components for NASA’s Next-Generation CO2 Removal System | Calnetix Technologies
Source: Infineon Industrial semiconductor, original: Jiang Dong, Sun Hongbo, etc.