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Magnetically levitated trains are common nowadays. However, the MagLev train the Central Japan Railway Company developed is quite unique and superior to the other trains. Running at more than 600 km per hour, it has achieved the status of ‘fastest train.’ This train uses superconducting magnets, which is why it is called SC MagLev. Once charged with an exciting current, the superconducting magnets of this train produce a circulating DC current and strong magnetic field forever, with zero loss. Let’s understand more about this successfully tested train technology, which is projected to overtake other magnetic levitation technologies by the year 2027. The same technology is poised to connect New York city to Washington DC in just one hour by 2030.
To successfully operate a magnetically levitated train, we have to achieve the following three objectives: 1) propel, 2) levitate, and 3) guidance. However, before we get into the details of how the SCMagLev train achieves these objectives, let’s study about the heart of this train — the superconducting magnets.
Levitating trains require enormously powerful electromagnets. The stronger the magnets, the more lift force and propelling force they have, resulting in higher train speed. A normal electromagnet is not able to increase the current value beyond a certain limit, due to the heating issue. In the superconducting electromagnets the temperature of the conductor is lowered below a critical limit. After this the material suddenly produces a huge amount of current flow with zero resistance. That result is exactly what we want. The interesting thing is that you only need to charge the superconducting coil once, using an exciting current, in order for the short-circuited coils to produce a circulating DC current forever, with no energy loss. The current circulated by the superconducting coils is huge: 700 kilo Amperes, almost 10000 times the current value of the conventional household copper gauge wires! The superconducting electromagnets are obviously the most powerful and efficient electromagnets.
The challenge is to keep the coils in a superconducting stage. For this purpose, an onboard liquid helium refrigeration system is used. The superconductor in the SCMagLev train is a niobium-titanium alloy, which has a critical temperature of 9.2 Kelvin. To keep the alloy temperature below this limit, liquid helium at a temperature of 4.5 Kelvin is circulated around it. After passing over the conductor, the liquid helium evaporates. To bring it back to the initial stage, a helium compressor and refrigeration unit is used. The refrigeration unit works on the principle of Gifford-McMahon refrigeration cycle.
Still, the cryogenic department’s engineering task is not finished yet. The superconductor can absorb heat from outside in the form of radiation. To prevent this absorption from occurring, a radiation shield is added around it. However, during the train’s operation, eddy current formation and heating issues can happen in this shield. To neutralize this heating, the radiation shield also needs cooling, which is achieved by supplying liquid nitrogen to the unit. To prevent convective heat transfer, a vacuum is maintained inside the radiation shield. Four such superconductors with opposing current polarity are arranged in a unit. Although in an SCMagLev the electromagnets work without any power supply, the cryogenics department demands a good amount of power. Such many units are attached along the length of the train on both sides.
As mentioned, the first task is propulsion. Propelling the train forward is an easy task. For this purpose we use a series of normal electromagnets, they are called propelling coils. The propelling coils are powered in an alternative manner as shown and are placed inside a guideway. Next, we need to find out the force the propelling coils are producing on the train’s superconducting magnets. Please note that to understand the direction of force one magnet produces on the other, you just have to consider the nearest poles. In this way, let’s analyze the force acting on the superconducting coils due to the propelling coils. If you take the result of all these forces, the net force will be in the forward direction, so the train moves forward. As soon as the train reaches the next mean position, switch the electromagnets to the alternate polarity so that the net force is again in the forward direction. Just by controlling the frequency of this switching, you can control the train speed.
Now let’s get to the most interesting part of this technology: the levitation of the SC MagLev trains. You may be surprised to learn that the SCMagLev train’s levitation is achieved with the help of these simple, figure-eight-shaped coils, which are not even powered. Many such eight-figure-shaped coils are arranged in the guideway. To understand the levitation technology, we should first learn something about the nature of a pair of superconducting magnets. The resultant magnetic field produced by this pair of SC magnets is very similar to a long permanent magnet. So for simplification of the analysis, let’s replace this pair with a long bar magnet.
To successfully operate a magnetically levitated train, we have to achieve the following three objectives: 1) propel, 2) levitate, and 3) guidance. However, before we get into the details of how the SCMagLev train achieves these objectives, let’s study about the heart of this train — the superconducting magnets.
Levitating trains require enormously powerful electromagnets. The stronger the magnets, the more lift force and propelling force they have, resulting in higher train speed. A normal electromagnet is not able to increase the current value beyond a certain limit, due to the heating issue. In the superconducting electromagnets the temperature of the conductor is lowered below a critical limit. After this the material suddenly produces a huge amount of current flow with zero resistance. That result is exactly what we want. The interesting thing is that you only need to charge the superconducting coil once, using an exciting current, in order for the short-circuited coils to produce a circulating DC current forever, with no energy loss. The current circulated by the superconducting coils is huge: 700 kilo Amperes, almost 10000 times the current value of the conventional household copper gauge wires! The superconducting electromagnets are obviously the most powerful and efficient electromagnets.
The challenge is to keep the coils in a superconducting stage. For this purpose, an onboard liquid helium refrigeration system is used. The superconductor in the SCMagLev train is a niobium-titanium alloy, which has a critical temperature of 9.2 Kelvin. To keep the alloy temperature below this limit, liquid helium at a temperature of 4.5 Kelvin is circulated around it. After passing over the conductor, the liquid helium evaporates. To bring it back to the initial stage, a helium compressor and refrigeration unit is used. The refrigeration unit works on the principle of Gifford-McMahon refrigeration cycle.
Still, the cryogenic department’s engineering task is not finished yet. The superconductor can absorb heat from outside in the form of radiation. To prevent this absorption from occurring, a radiation shield is added around it. However, during the train’s operation, eddy current formation and heating issues can happen in this shield. To neutralize this heating, the radiation shield also needs cooling, which is achieved by supplying liquid nitrogen to the unit. To prevent convective heat transfer, a vacuum is maintained inside the radiation shield. Four such superconductors with opposing current polarity are arranged in a unit. Although in an SCMagLev the electromagnets work without any power supply, the cryogenics department demands a good amount of power. Such many units are attached along the length of the train on both sides.
As mentioned, the first task is propulsion. Propelling the train forward is an easy task. For this purpose we use a series of normal electromagnets, they are called propelling coils. The propelling coils are powered in an alternative manner as shown and are placed inside a guideway. Next, we need to find out the force the propelling coils are producing on the train’s superconducting magnets. Please note that to understand the direction of force one magnet produces on the other, you just have to consider the nearest poles. In this way, let’s analyze the force acting on the superconducting coils due to the propelling coils. If you take the result of all these forces, the net force will be in the forward direction, so the train moves forward. As soon as the train reaches the next mean position, switch the electromagnets to the alternate polarity so that the net force is again in the forward direction. Just by controlling the frequency of this switching, you can control the train speed.
Now let’s get to the most interesting part of this technology: the levitation of the SC MagLev trains. You may be surprised to learn that the SCMagLev train’s levitation is achieved with the help of these simple, figure-eight-shaped coils, which are not even powered. Many such eight-figure-shaped coils are arranged in the guideway. To understand the levitation technology, we should first learn something about the nature of a pair of superconducting magnets. The resultant magnetic field produced by this pair of SC magnets is very similar to a long permanent magnet. So for simplification of the analysis, let’s replace this pair with a long bar magnet.
If a bar magnet moves parallel to these figure-eight-shaped coils, can you predict what will happen? The varying magnet flux will induce EMF on both the loops according to Faraday's law. Are these EMFs in the same direction? Please note that this is a twisted coil, only when we unwind it, we will understand the right direction. It is clear the induced EMFs are opposite in direction, which means net EMF induced on this coil due to the bar magnet movement is zero, and no current will flow through the loop. In short a bar magnet, moving through the center of the loop won’t have any effect on the loop.
Now consider the same case, but this time the magnet is slightly offset to the loop, as shown. Here, the bottom loop faces magnetic flux of higher strength, which means the EMF induced on the bottom loop will be higher than on the top. This higher strength also means that a net current will flow through the loop. This current flow produces a south pole on the top loop and a north pole on the bottom loop. If you analyse the force interaction between the magnetic poles, it’s clear that a resultant upward force is imposed on the superconducting magnet. If this force is more than the gravitational pull, the magnet will move up. Yes, movement of a superconducting magnet parallel and offset to a figure-eight-shaped coil produces levitation.
As the magnet moves up, the difference between EMF values and the current flow in the loop reduces, which means the force on the loop also reduces. Finally, when the upward force becomes equal to the gravitational pull, the magnet balances or the train has achieved levitation. Japanese engineers achieved a levitation of 3.9 inches using this technology.
Clearly, the higher the train’s speed, the greater the levitation force, which means that when the train is at rest, it cannot levitate. This is why the SCMagLev train uses normal tires for starting and low-speed operation. When the train achieves a critical speed, the tires retract, as the electromagnetic force is strong enough to levitate the train.
Next comes the question of train guidance. Guidance means the train should always be centered; it should move without hitting the sidewalls. In other words, it should achieve lateral stability. Japanese engineers achieved this stability quite easily by interconnecting the figure-eight-shaped coils we saw earlier as shown. If the train is in the center, the induced EMFs on the right and left coils will be equal and no current will flow through the interconnecting coils.
However, suppose the train has moved slightly towards the right. This shift will cause an EMF difference between the right and left coils, resulting in the interconnecting coils having a current flow. The current flow through the interconnecting coils will drastically affect the current flow in both the bottom loops, and thus the pole strength, of each loop.
Let’s analyze the forces acting on the train now. You can see that the vertical components of the forces remain the same, but a net horizontal component manifests towards the left, which forces the train to move back to the center. As the train nears the center, the currents in the interconnecting loops decrease, and finally the horizontal component of the force disappears. What an easy and brilliant mechanism to stabilize the train, right?
From the discussion so far, you might understand that the cryogenic system of the train and the other electrical appliances of the train require a huge amount of electrical power. How do you transfer electrical power to such a high speed train? The Central Japan Railway used a technique called ‘inductive power collection’ for this purpose. Here, using the principle of electromagnetic induction, electric power is transferred from the ground coils to the power collection coil in the train without any material contact.
The strong magnetic field the superconducting magnets produce can have health hazards on passengers. To avoid this unwanted effect, magnetic shields are used on the rolling stock and passenger embarkation facility, thus keeping the strength of the magnetic field below ICNIRP guidelines.
SCMagLev train test rides began in 1997 on the Yamanashi Maglev test line. The test rides were quite successful and continued for 10 consecutive years without missing a single day. A world record speed of 603 km/hr was achieved during this time. These highly positive results encouraged the Japanese authorities, and they granted permission to conduct commercial SCMagLev operations between Tokyo and Nagoya by the year 2027, with more SCMagLev trains to follow. The SCMagLev train technology revolves around the physics of superconductivity which is a crazy and amazing phenomenon. To understand what superconductivity is in a logical way please check out this interesting video from Arvin Ash. Also, please don’t forget to be part of our team! Thank you!
Now consider the same case, but this time the magnet is slightly offset to the loop, as shown. Here, the bottom loop faces magnetic flux of higher strength, which means the EMF induced on the bottom loop will be higher than on the top. This higher strength also means that a net current will flow through the loop. This current flow produces a south pole on the top loop and a north pole on the bottom loop. If you analyse the force interaction between the magnetic poles, it’s clear that a resultant upward force is imposed on the superconducting magnet. If this force is more than the gravitational pull, the magnet will move up. Yes, movement of a superconducting magnet parallel and offset to a figure-eight-shaped coil produces levitation.
As the magnet moves up, the difference between EMF values and the current flow in the loop reduces, which means the force on the loop also reduces. Finally, when the upward force becomes equal to the gravitational pull, the magnet balances or the train has achieved levitation. Japanese engineers achieved a levitation of 3.9 inches using this technology.
Clearly, the higher the train’s speed, the greater the levitation force, which means that when the train is at rest, it cannot levitate. This is why the SCMagLev train uses normal tires for starting and low-speed operation. When the train achieves a critical speed, the tires retract, as the electromagnetic force is strong enough to levitate the train.
Next comes the question of train guidance. Guidance means the train should always be centered; it should move without hitting the sidewalls. In other words, it should achieve lateral stability. Japanese engineers achieved this stability quite easily by interconnecting the figure-eight-shaped coils we saw earlier as shown. If the train is in the center, the induced EMFs on the right and left coils will be equal and no current will flow through the interconnecting coils.
However, suppose the train has moved slightly towards the right. This shift will cause an EMF difference between the right and left coils, resulting in the interconnecting coils having a current flow. The current flow through the interconnecting coils will drastically affect the current flow in both the bottom loops, and thus the pole strength, of each loop.
Let’s analyze the forces acting on the train now. You can see that the vertical components of the forces remain the same, but a net horizontal component manifests towards the left, which forces the train to move back to the center. As the train nears the center, the currents in the interconnecting loops decrease, and finally the horizontal component of the force disappears. What an easy and brilliant mechanism to stabilize the train, right?
From the discussion so far, you might understand that the cryogenic system of the train and the other electrical appliances of the train require a huge amount of electrical power. How do you transfer electrical power to such a high speed train? The Central Japan Railway used a technique called ‘inductive power collection’ for this purpose. Here, using the principle of electromagnetic induction, electric power is transferred from the ground coils to the power collection coil in the train without any material contact.
The strong magnetic field the superconducting magnets produce can have health hazards on passengers. To avoid this unwanted effect, magnetic shields are used on the rolling stock and passenger embarkation facility, thus keeping the strength of the magnetic field below ICNIRP guidelines.
SCMagLev train test rides began in 1997 on the Yamanashi Maglev test line. The test rides were quite successful and continued for 10 consecutive years without missing a single day. A world record speed of 603 km/hr was achieved during this time. These highly positive results encouraged the Japanese authorities, and they granted permission to conduct commercial SCMagLev operations between Tokyo and Nagoya by the year 2027, with more SCMagLev trains to follow. The SCMagLev train technology revolves around the physics of superconductivity which is a crazy and amazing phenomenon. To understand what superconductivity is in a logical way please check out this interesting video from Arvin Ash. Also, please don’t forget to be part of our team! Thank you!