We may perhaps learn to deprive large masses of their gravity and give them absolute levity, for the sake of easy transport.
- Benjamin Franklin
What is Maglev?
Magnetic Levitation Transport, or maglev, is a revolutionary form of railway transport. It uses magnetic forces to make the vehicles hover above the track rather than running along them, thus reducing the friction between the two and allowing much higher speeds to be attained than are safely achievable on any other form of ground based public transport.
A Brief History of the Technology
Though the idea of high speed trains levitated by magnetic forces seems revolutionary, even to us today, the concept is over a century old. In the early 1900s, Bachelet in France and Goddard in the United States discussed the possibility of using magnetically levitated vehicles for high speed transport. However, they were unable to propose a practical way to achieve this goal.
The practical means that these men required were not properly researched until after World War II1. It wasn't until 1966 that American scientists James Powell and Gordon Danby proposed the first practical system for magnetically levitated transport. For the next two decades the technology began to be properly explored and tested in the USA, Germany and Japan.
Since the idea was first coined in the early 1900s, the methods and the implementations of the technology have been greatly explored and refined. The use of superconducting magnets and the advances made in the field of high temperature superconductors have made the technology a practical possibility and today there are full scale prototypes in Japan and the USA and even a commercial high speed maglev train in service in China, operated by the company Transrapid International.
As yet the widespread implementation of maglev as an efficient form of public transport has been hindered, primarily by cost2 but also because of the many different ways this technology has been adapted. With so much variation available in maglev systems, no two companies use the technology the same way, so collaboration between them is often slow and conflicted. However, the future sees many more commercial uses of maglev technology, not just for trains but for other projects as well, such as launching rockets into space at greatly reduced cost.
How Does It Work?
Maglev trains differ from normal trains in that they are levitated and propelled by magnetic forces rather than resting on the track and being propelled by a conventional steam, diesel or electric engine. They 'fly' above the track, or guideway, dramatically reducing friction and allowing very large speeds to be achieved.
Maglev technology works in principle in one of two ways; either by Electromagnetic Suspension (EMS), which uses attractive forces, or by Electrodynamic Suspension (EDS), which uses repulsive forces. A third adaptation of this technology, which normally uses the EDS principal, is called Inductrack. This is different from normal maglev systems because it uses permanent magnets instead of electromagnets or superconductors. In almost all applications of this technology either EDS or EMS principals are used, though they all differ in how these principals are adapted. This section is intended to explain the basics of each system. Details of specific examples will be given along with the examples themselves in a later section.
Electromagnetic Suspension (EMS)
This uses the attractive forces of magnets, positioned below the guideway as in a monorail, to levitate the train and to guide it in order that it avoids contact with the guideway. The guideway is made of a ferrous metal and the magnetic field generated by the magnets attracts to this. This system is complex and EMS was only made practical by significant advances in electronic control systems that maintain a necessary distance between the train and the guideway, both horizontally and vertically. This distance is measured constantly and the magnetic field generated by the coils is carefully controlled, via feedback systems, to compensate for any variations in the weight of the train or in the orientation of the guideway.
This system levitates because of the constant power supplied to the electromagnets, which remain switched on even when the train is at rest. This means EMS systems require no auxiliary wheels when they are not moving or moving at low speeds. There is a risk here that a sudden power failure could cause the levitation to fail and the train to crash, however EMS trains are equipped with emergency battery power supplies to cope with such an emergency.
Electrodynamic Suspension (EDS)
This uses the repulsive forces of magnets to lift the train from the guideway and to keep it away from the sides. Magnetic fields generated on board the train induce currents in the coils in the track as the train passes. When these currents are induced they induce a magnetic field that opposes the original one on the train and repels it, pushing the train away. This system is simpler than EMS in terms of vehicle guidance and stability because of the inverse relationship connecting the magnetic field and the distance between the train and the guideway. As the train gets closer to the side of the guideway the repelling magnetic forces between this and the train increase exponentially, pushing the train away and keeping it centralised on the guideway3. This means that EDS does not require the complex guidance systems that EMS trains do.
The EDS system uses superconducting magnets, which, in the event of a power failure, will continue to levitate the train until it drops below a critical speed and comes to rest on the auxiliary wheels. This is because of the lack of internal resistance within superconducting coils, meaning that they can still conduct electricity when the power supply has been turned off. This provides something of a failsafe against power failure but since the critical speed for levitation is around 100km/h the wheels would need to be sufficient to safely bring the train to a halt.
Magnetic Levitation Systems
Either Electromagnets or Superconductors can be used in EMS and EDS systems to produce magnetic fields powerful enough to levitate the train.
Wires that carry currents produce a magnetic field orientated according to the 'Right Hand Rule'. If the thumb of the right hand is aligned with the direction that the current flows in the wire (positive to negative) then the fingers, when curled around the wire, will follow the paths of the magnetic field lines. Magnetic fields caused by coils of wire follow a form of the right-hand rule. If the fingers of the right hand are curled in the direction of current flow through the coil (they follow the wires), then the thumb points in the direction of the field inside the coil.
This is the basis upon which electromagnets are formed. They generally consist of a current carrying wire coiled around an iron core. The current running through the wire produces a magnetic field, which is increased by increasing the number of turns in the coil. Increasing the number of turns in the coil or varying the current in the wire are two ways to increase the strength of the magnetic field, as is the addition of an iron core, which concentrates the magnetic field, making it stronger than it would be from the coil alone.
The core is often made of 'soft iron' because this material does not retain magnetism as permanent magnets do. Electromagnets, therefore, can be turned off altogether, unlike permanent magnets, which were not considered an option for the original maglev designs because they weren't thought strong enough. Electromagnets though, given a sufficient power source, would be capable of levitating a maglev train using the EMS principle of attraction to the guideway from beneath because this, unlike the EDS system, requires only one magnetic field to levitate the train rather than two opposing fields.
Electromagnets themselves, though, are not widely used in maglev trains because in order to create a strong enough magnetic field you would need a very large, bulky electromagnet and a massive power supply, which would be particularly difficult to attain consistently and safely on a fast-moving vehicle. Even then much of the electrical energy would be lost as heat because of the internal resistance of the wire coils. These are some of the reasons why superconductors have become the method of choice for maglev trains.
These are materials that have no resistance to the flow of electricity, though normally they only behave in this way below certain temperatures. Most materials begin to exhibit superconducting properties when the temperature approaches absolute zero (0K on the Kelvin scale, -273.15°C or -459.67°F) but significant advances in high temperature superconductors have found new materials that will behave as superconductors at temperatures as high as 138K. The following extract shows how a magnetic field interacts with a superconducting coil of wire as opposed to a regular coil of wire.
The next great milestone in understanding how matter behaves at extreme cold temperatures occurred in 1933. German researchers Walter Meissner and Robert Ochsenfeld discovered that a superconducting material will repel a magnetic field. A magnet moving by a conductor induces currents in the conductor. This is the principle upon which the electric generator operates. But, in a superconductor the induced currents exactly mirror the field that would have otherwise penetrated the superconducting material - causing the magnet to be repulsed. This phenomenon is known as strong diamagnetism and is today often referred to as the 'Meissner effect' (an eponym). The Meissner effect is so strong that a magnet can actually be levitated over a superconductive material.
- Extract taken from Superconductors.org.
Because there is no resistance in the superconductor, the magnetic field produced in it when the current flows is equal to and opposing the magnetic field of the original magnet and repels it. However, when superconductors are used in maglev trains the roles of the non-superconducting and superconducting coils of wire are reversed; the superconductor is used in the electromagnet, making a superconducting magnet, and the normal coils line the guideway. In this configuration the magnetic field reacts with the normal coil of wire as is described above, but since the magnetic field is produced by a superconducting magnet, rather than a normal magnet or electromagnet, the field is much stronger and the coils in the guideway repel the magnetic field in the same was that a superconductor would repel a normal magnet. This system can achieve levitation of up to 10cm, much more than the 1cm EMS systems are capable of.
Superconducting maglev systems save energy by chilling the coils at frigid temperatures, but this requires complicated and expensive cryogenic cooling systems. Also the magnetic fields generated would be very large, necessitating magnetic shielding within the train to protect passengers with pacemakers or other critical electrical equipment.
Many maglev systems use electromagnets and superconducting magnets to levitate the trains, but some EDS systems use permanent magnets arranged in Halbach Arrays.
This version of the technology is passive; no power is required to levitate the train when it is moving. The motion of the magnets relative to the guideway induces currents in the track's coils, which produces an electromagnetic field that repels the array and causes the train to levitate. When the train is moving the only power supply needed is the one to the propulsion system, either magnetic coils interspersed among the levitating circuits in the track, similar to those used in regular EDS maglev trains, or a different form of propulsion like a propeller system driven by an on-board gas turbine.
At low speeds and when the train is stationary the Halbach Arrays are not capable of levitating the train, therefore auxiliary wheels are required for 'take off' and 'landing' as they are with regular EDS systems. Once the train is set in motion on the wheels it isn't long before it takes to the air. As long as the train is moving above a low critical speed of a few kilometres per hour - a bit faster than walking speed and much slower than the critical levitation speed of regular EDS systems - the Halbach arrays will be levitated a few centimetres above the track's surface.
Though it has been said that permanent magnets wouldn't be strong enough to hold up the train, theoretical and practical work being done by Lawrence Livermore scientists has predicted levitation forces of up to 50 metric tons per square meter of magnet array using modern permanent magnet materials such as neodymium-iron-boron. This is because these materials, when combined in this way, have a greater number of magnetic field lines per area, increasing the strength of the overall magnetic field produced.
So, using this combination of materials for the magnets, and arranging them in Halbach Arrays, has produced a system using permanent magnets that could prove to be more efficient than both EMS and EDS. The scientists have even produced a scale model of their design, though there is no full-scale prototype operational yet.
Pros and Cons of different Technologies
|Electromagnetic Suspension (EMS)|
|Electrodynamic Suspension (EDS)|
Current applications of Maglev technology
Transrapid International is currently the only company able to boast a fully-operational commercial application of maglev technology. It is a joint venture of the German companies Siemens AG and ThyssenKrupp, is currently established in China, Germany and the USA and on 31 December, 2002 the Chinese branch staged the maiden voyage of the Transrapid Maglev Train on its first commercially-operated route worldwide, which runs from Shanghai's Long Yang Road to the Pudong International Airport.
This system uses the EMS principal of attractive forces aimed upwards at the guideway from beneath, and travels at top speeds of 430 km/h: however, the real advantage is the good acceleration. As there is no mechanical friction loss, the distance required by the Transrapid to accelerate to 300 km/h from a standing start is just five kilometres.
This project was designed primarily to demonstrate the capabilities of the Transrapid system4 but in terms of safety, reliability, availability, and functionality it has shown that this technology is ready for widespread implementation. In November, 2004 there were even talks about extending the track from Shanghai to Hangzhou, 180km away.
This project hasn't been without its flaws. Observations of the trains in motion have shown that at a speed of about 400km/h they produce noise levels of up to 95 decibels. At a higher speed, say of 435km/h, the noise reaches even 97 decibels, which is comparable to the noise of a low flying plane and which is liable to upset the homeowners in the vicinity of the train. These high levels of noise, sound waves caused by friction with the air around the fast moving train, have made many fear for their safety and health and even go so far as to move house.
The Birmingham Maglev
It should be noted here that the somewhat less ambitious, and fairly unreliable Maglev link between Birmingham International Airport and the adjoining railway station operated between 1985 and 1995, but this low-speed service suffered so many problems that it has now been replaced with a regular shuttle train service.
Proposed applications of Maglev technology
Many applications of maglev technology have been suggested in lots of different countries. Aside from the Transrapid projects in China, Germany and the USA, and the Yamanashi Maglev Test Line in Japan, Switzerland has drawn up plans for an intercity maglev link that will run underground in a climate controlled environment.
The concept of the Swissmetro, or the Eurometro as it is sometimes called on an international stage, is an underground version of the maglev train system. The tunnel through which it runs would be under partial vacuum, reducing the air resistance and allowing even greater speeds than normal over-ground maglevs, however this vacuum would necessitate the trains to be pressurised like aeroplane cabins.
The Swissmetro system, if it goes to plan, could be in operation by 2020 and some have even speculated that it would be a good alternative to air travel across Europe if it is implemented further.
One very different use of maglev technology has been suggested by NASA. It is looking into using the Inductrack to aid it in launching rockets into space.
Inductrack Launcher could provide the initial boost for future spacecraft. The craft could be mounted on a levitation 'launch cradle' that would glide up a sloping one-kilometre-long track. Unimpeded by wheel friction, the cradle could accelerate the spacecraft to 950 kilometres per hour. The craft's engines would then ignite and propel it into orbit.
This system was conceived after NASA heard about Livermore's inductrack model and offered the laboratory a contract to design and build another model to simulate rocket-launching. NASA studies have shown that if such a system proves possible then it could cut the amount of rocket fuel required for a launch by 30 to 40 percent.
Maglev railway systems have been successfully demonstrated in both testing phases and commercial use, and they have proved to be much faster and just as reliable as conventional railways. However, since maglev systems are unable to use conventional railway infrastructures they have to be designed as entire transport systems. This means that a new maglev railway link will have a phenomenal set up cost encompassing the building of terminals, locomotives and guideways. It is this initial cost that has thus far held back the widespread implementation of maglev technology. However, with so many companies competing to adapt this technology, in the future it is highly likely that Magnetic Levitation will see much greater use all over the world and will become part of everyday transport.