In this blog post, we’ll explore the principles behind maglev trains and superconductors, as well as how superconductors are utilized in maglev trains.
Is it possible to travel from Seoul to Busan in just one hour by train? Even the KTX, currently South Korea’s fastest high-speed train, takes over two hours to travel from Seoul to Busan. If there were a train capable of traveling at speeds two to three times faster than that, it would be possible to make the trip in under an hour. A prime example of such an ultra-high-speed train is the maglev train. Conventional trains move forward by utilizing the friction between the wheels and the rails. However, as speeds increase significantly, it becomes difficult to maintain stable contact between the wheels and the rails, and limitations such as friction, vibration, and noise become apparent. In contrast, maglev trains travel while floating several centimeters above the track, so there is virtually no friction between the wheels and the rails, enabling them to reach speeds exceeding 500 km/h. They also produce less vibration and noise, providing a more comfortable travel experience. So how do maglev trains float in the air as they move? At the heart of this technology lies superconductors.
Materials in the world can be classified as conductors or insulators based on how well they allow electricity to flow. The flow of electricity can be understood as the movement of electrons, and the property that hinders this movement is called electrical resistance. In other words, the lower the resistance, the better electricity flows; the higher the resistance, the worse electricity flows. Since the magnitude of resistance is an inherent property of a material, materials with low resistance that allow electricity to flow well are called conductors, while those with high resistance that do not allow electricity to flow well are called insulators. Typical conductors include metals such as copper and iron, while insulators include glass, rubber, and plastic.
A superconductor is a material whose electrical resistance drops to zero at very low temperatures. In other words, it is a conductor in which resistance completely disappears under specific conditions. The phenomenon of superconductivity was first discovered in 1911 when Dutch physicist Heike Kamerlingh Onnes observed that the electrical resistance of mercury suddenly disappeared at a temperature of approximately –269°C. Since then, superconductivity has been discovered in various metals and compounds, such as niobium (Nb) and vanadium (V). Since superconductors exhibit superconductivity only below a certain temperature, each material has a specific temperature at which superconductivity begins. This is called the critical temperature.
Superconductors possess two important properties: the Meissner effect and the locking of magnetic flux. First, the Meissner effect is the phenomenon in which a superconductor, when cooled below its critical temperature, expels the internal magnetic field outward. A magnetic field can be understood as the space in which the force of a magnet acts. As the magnetic field inside the superconductor is expelled outward, a repulsive force is generated between the superconductor and the magnet, and this force allows the superconductor to levitate above the magnet. This phenomenon was discovered by Walter Meissner in 1933 and is therefore called the Meissner effect.
However, the Meissner effect alone is not sufficient for a superconductor to remain stably suspended above a magnet. This is because the Meissner effect essentially exerts only a repulsive force against the magnetic field. Flux pinning plays the role of stabilizing the superconductor in this situation. To understand this phenomenon, we first need to examine magnetic field lines. Magnetic field lines form around a magnet, indicating the direction of the magnetic force; the total amount of magnetic field lines passing through a given area is called magnetic flux. Flux pinning is the phenomenon in which a portion of this magnetic flux is confined to specific locations within the superconductor. Since some of the magnetic field that is not repelled by the Meissner effect remains confined within the superconductor, an attractive force is generated between the magnet and the superconductor. Ultimately, the repulsive force of the Meissner effect and the attractive force of flux pinning balance each other out, allowing the superconductor to float stably in the air above the magnet.
So, how are superconductors used in maglev trains? Maglev trains essentially use magnetic forces and electromagnetic induction to levitate the train and propel it forward. Since installing magnets along the entire track would be very costly, strong magnets are typically used on the train cars, while conductive coils are installed along the track. The principle utilized here is electromagnetic induction. Electromagnetic induction is the phenomenon in which a changing magnetic field generates an electric current. When a train equipped with magnets passes over the track, a current is induced in the coils on the track; this current, in turn, creates a magnetic field that lifts the train and generates the force to propel it forward. By using superconducting magnets in this process, a very strong magnetic field can be maintained for a long time with minimal power consumption. While conventional electromagnets require a continuous power supply and suffer from significant power loss, superconductors have zero resistance, allowing for a substantial reduction in power loss; as a result, they are utilized as a core technology in maglev trains.
Despite their advantages of speed and comfort, maglev trains have not yet become widely adopted worldwide. Currently, Japan is continuing to develop superconducting maglev technology, and China operates high-speed maglev train lines. In South Korea, the Incheon International Airport Maglev Train operated on a trial basis for some time, but service has since been suspended. Although maglev trains still face challenges such as high construction and maintenance costs, they continue to be researched and developed as a future ultra-high-speed mode of transportation as superconducting and transportation technologies continue to advance. If these technical challenges are resolved, we will be able to usher in an era of even faster and more convenient rail transportation.
