Working principle of Shanghai maglev train

Working principle of maglev train (diagram)

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Since 1825, when the world's first standard gauge railway appeared, wheel-rail trains have been the means of transportation for people to travel. However, with the increase of train speed, the violent collision between wheels and rails makes the train produce strong vibration and noise, which makes passengers feel uncomfortable. Because the higher the train speed, the greater the resistance. So when the speed of the train exceeds 300 kilometers per hour, it is difficult to speed up any more.

If the train can float from the rail, the friction between the train wheels and the rail will be eliminated, and the speed of the train will be greatly improved. But how to make the train float from the tracks? Scientists have thought of two solutions: one is air flotation, which can make the train float by its reaction even if it blows a lot of air to the rail ground; The other is the magnetic levitation method, which uses the magnetic repulsion between two magnetic poles with the same name or the magnetic attraction between two magnetic poles with different names to make the train float from the track. Using air flotation on land will not only arouse a lot of dust, but also produce a lot of noise, which will cause great pollution to the environment, so it is not suitable for use. This makes maglev train become the main means of research and test.

At present, there are two main forms of maglev train in the world, one is repulsion; The other is inhalation. The repulsion formula is to use the repulsion generated by two magnets with the same polarity to make the train float. Superconducting electromagnets with strong magnetic fields are installed on both sides of the carriage of this maglev train. When the vehicle is running, the magnetic field of this electromagnet cuts the aluminum rings installed on both sides of the track to generate induced current, and at the same time produces opposite magnetic fields with the same polarity, so that the vehicle is pushed off the track surface and floats in the air. But at rest, because there is no cutting potential and current, the vehicle cannot be suspended, and it can only support the body with wheels like an airplane. When the vehicle is driven by a linear motor and the speed reaches above 80 km/h, the vehicle stops. The suction type is based on the principle that two magnets attract each other. The electromagnet is placed under the track and fixed on the bogie of the car body. When two magnets generate a strong magnetic field and attract each other, the train can be suspended. This kind of suction maglev train can maintain a stable suspension state whether it is stationary or moving. The medium and low speed maglev train developed by our country this time belongs to this type.

"If you leave" is the basic working state of maglev train. The maglev train uses electromagnetic force to counteract the gravity of the earth and make the train suspend on the track. During the operation, the car body and the track are in a state of "at arm's length", and the magnetic levitation gap is about 1 cm, so it has the reputation of "zero altitude aircraft". Compared with ordinary wheel-rail trains, it has the characteristics of low noise, low energy consumption, no pollution, safety and comfort, high speed and high efficiency, and is considered as a new type of transportation with broad prospects. Especially this kind of medium and low speed maglev train has small turning radius and strong climbing ability, which is especially suitable for urban rail transit.

Germany and Japan are the first countries in the world to carry out research on maglev trains. Transrapid, a maglev train developed in Germany, reached a speed of 436 kilometers per hour on the Emsland test line in 1989. Maglev is a maglev train developed in Japan, which set a world record of 550km/h on the test line in Yamanashi Prefecture in February 1997. After a long and repeated argument, both Germany and Japan believe that it is possible for maglev trains to be put into operation in their own countries before the middle of the next century.

Working principle of maglev train

Maglev train is the product of modern high-tech development. Its principle is to use electromagnetic force to counteract the gravity of the earth, and make the train suspend on the track by linear motor (suspension gap is about 1 cm). Its research and manufacture involve automatic control, power electronics technology, linear propulsion technology, mechanical design and manufacture, fault monitoring and diagnosis and many other disciplines. Technology is very complex, which is an important symbol of a country's scientific and technological strength and industrial level. Compared with ordinary wheel-rail trains, it has the characteristics of low noise, no pollution, safety and comfort, high speed and high efficiency, and has the reputation of "zero-altitude aircraft". It is a new type of transportation with broad prospects, especially suitable for urban rail transit. Maglev trains are generally divided into repulsion type and suction type according to the different suspension modes, and there are high speed and medium-low speed according to the running speed. The maglev train developed by the National Defense Science and Technology University this time belongs to the medium and low speed maglev train with constant conductivity and suction.

Types of maglev trains

Maglev trains can be divided into two categories: normally guided and superconducting. Normally-guided maglev, also known as normally-guided maglev, is represented by transrapid, a German high-speed normally-guided maglev train. It uses the electromagnetic attraction principle of ordinary DC electromagnet to suspend the train, and the suspension air gap is relatively small, generally around10 mm. The speed of the normally oriented high-speed maglev train can reach 400 ~ 500 kilometers per hour, which is suitable for long-distance rapid traffic between cities. Superconducting maglev train, also called superconducting magnetic repulsion, is represented by Japanese maglev. It uses the strong magnetic field generated by superconducting magnet to interact with the coil arranged on the ground when the train is running, and generates electric repulsion to suspend the train. The suspension air gap is large, generally around 100 mm, and the speed can reach more than 500 kilometers per hour. These two kinds of maglev trains have their own advantages and disadvantages, and their economic and technical indicators are also different. Germany tends to the former, concentrating on the development of constant conductivity high-speed maglev technology. Japan, on the other hand, is optimistic about the latter and is fully committed to high-speed superconducting magnetic levitation technology.

Maglev train in normal operation in Germany

When the normally guided maglev train works, firstly, the electromagnetic attraction of the lower suspension of the vehicle and the guiding electromagnet is adjusted, and the train is suspended through the magnetic reaction with the windings on both sides of the ground track. Under the action of the guiding electromagnet and the track magnet at the lower part of the vehicle, the wheel and the track keep a certain lateral distance, thus realizing the non-contact support and non-contact guidance of the wheel and the track in the horizontal and vertical directions. The suspension gap between the vehicle and the running track is 10 mm, which is guaranteed by a high-precision electronic adjustment system. In addition, because the suspension and guidance are actually independent of the running speed of the train, the train can still enter the suspension state even when it stops.

The principle of synchronous linear motor is used to drive the normally-guided maglev train. The electromagnet coil supported on the lower part of the vehicle is equivalent to the excitation coil of the synchronous linear motor, and the three-phase moving magnetic field driving winding on the inner side of the ground track is equivalent to the armature and the long stator winding of the synchronous linear motor. According to the working principle of the motor, when the armature coil as the stator is energized, the rotor of the motor is driven to rotate due to electromagnetic induction. Similarly, when the substations arranged along the line provide three-phase frequency modulation and amplitude modulation power to the driving windings inside the track, the bearing system and the train are pushed to do linear motion similar to the motor "rotor" due to electromagnetic induction. Therefore, in the suspended state, the train can completely achieve non-contact traction and braking.

Superconducting maglev train in Japan

The most important feature of superconducting maglev train is that its superconducting elements have complete conductivity and diamagnetism at quite low temperature. Superconducting magnets are composed of superconducting coils made of superconducting materials. It not only has zero current resistance, but also can conduct powerful current that ordinary wires can't match. This feature makes it possible to make electromagnets with small volume and high power.

The vehicle of superconducting maglev train is equipped with on-board superconducting magnets to form an induction power integration device, and the driving winding and floating guide winding of the train are installed on both sides of the ground guide rail. The induction power integrated device on the vehicle consists of three parts: power integrated winding, induction power integrated superconducting magnet and floating guide superconducting magnet. When three-phase alternating current with the same speed and frequency is provided to the driving windings on both sides of the track, a moving electromagnetic field will be generated, thus generating electromagnetic waves on the train guide rail. At this time, the vehicle-mounted superconducting magnet on the train will receive a thrust synchronous with the moving magnetic field, and it is this thrust that pushes the train forward. It works like surfing. Surfers stand on the waves and are pushed forward by the waves. Like surfers, superconducting maglev trains have to deal with the problem of how to accurately control the motion at the peak of moving electromagnetic waves. Therefore, a high-precision instrument is installed on the ground guide rail to detect the position of the vehicle, and the supply mode of three-phase alternating current is adjusted according to the information sent by the detector, so as to accurately control the electromagnetic wave shape and make the train run well.

The superconducting maglev train is also driven by three-phase alternating current provided by substations distributed along the line to the driving windings on both sides of the ground guide rail, and is induced by the power integrated winding below the train to realize non-contact traction and braking. The guide windings suspended on both sides of the ground guide rail have nothing to do with external power supply. When the train approaches the winding, the strong electromagnetic induction of the superconducting magnet of the train will automatically induce current in the ground winding, so electromagnetic force will be generated between the induced current and the superconducting magnet, so that the train will be suspended, and the gap between the track and the train will be detected by the precision sensor, so that it will always maintain the suspension gap of100 mm. At the same time, the guiding winding electrically connected with the suspension winding will also generate electromagnetic guiding force, ensuring that the train can run stably in the center of the track at any speed.

Technical problems existing at present

Although maglev train technology has many advantages mentioned above, it still has some disadvantages:

(1) Because the magnetic levitation system completes the functions of levitation, guidance and driving by electromagnetic force, the safety measures of magnetic levitation after power failure, especially the braking of trains after power failure, are still a problem to be solved. Its high-speed stability and reliability need long-term testing.

(2) The suspension height of the normally conductive magnetic levitation technology is lower, so the requirements for line flatness, subgrade settlement and turnout structure are higher than those of superconducting technology.

(3) Superconducting magnetic levitation technology consumes more energy than conventional magnetic levitation technology due to eddy current effect, and the cooling system is bulky, and the strong magnetic field has an impact on human body and environment.