Magnetic levitation
A magnetic levitation train or maglev is a train-like vehicle that is suspended in the air above the track, and propelled forward using the repulsive and attractive forces of magnetism. Because of the lack of physical contact between the track and the vehicle, the only friction is that between the carriages and the air. Consequently maglev trains can travel at very high speeds with reasonable energy consumption and noise levels (systems have been proposed that operate at up to 650 km/h, which is far faster than is practical with conventional rail transport). Whilst the very high maximum speeds make maglev trains potential competitors to airliners on many routes, the enormous cost of constructing the tracks has limited maglev vehicles largely to demonstration projects.
Technology
There are two primary types of maglev technology: One that relies on superconducting magnets and a newer, potentially more economical system that uses permanent magnets.
Japan and Germany are active in maglev research producing several different approaches and designs. In one design, the train can be levitated by the repulsive force of like poles or the attractive force of opposite poles of magnets. The train can be propelled by a linear motor on the track or on the locomotive or both. Massive electrical induction coils are placed along the track in order to produce the magnetic field necessary to propel the train, leading some to speculate that the cost of constructing such tracks would be enormous.
Magnetic bearings are unstable because of the Earnshaw theorem. Conventional maglev systems are stabilized with electromagnets that have electronic stabilization. The electromagnets and electronics tend to be large, power-hungry, and expensive.
The weight of the large electromagnet is a major design issue. A very strong magnetic field is required to levitate a massive train, so conventional maglev research uses superconductor research for an efficient electromagnet.
The effect of a powerful magnetic field on the human body is largely unknown. For the safety of the passengers, shielding might be needed, which would add additional weight to the train. The concept is simple, but the engineering and design aspects are complex.
A newer, perhaps less-expensive system is called "Inductrac." The technique has a load-carrying ability related to the speed of the vehicle, because it depends on currents induced in a passive electromagnetic array by permanent magnets. In the prototype, the permanent magnets are in the track, and the array is in a cart. The magnets and cart are unpwoiered, except for the speed of the cart. Inductrack was originally developed as a magnetic motor and bearing for a flywheel to store power. With only slight design changes, the bearings were unrolled into a linear track. Inductrack was developed by physicist William Post at Lawrence Livermore Laboratories.
Inductrack uses Halbach arrays for stabilization. Halbach arrays are arrangments of permanent magnets that stabilize moving magnets without electronic stabilization. Halback arrays were orginally developed for beam guidance of particle accelerators.
Currently, some space agencies, such as NASA, are researching the use of maglev systems to launch spacecraft. In order to do so, the space agency would have to get a maglev-launched spacecraft up to escape velocity, a task which requires elaborate timing of magnetic pulses or a very fast, very powerful electric current (electric current, when passed through metals, can turn those metals into magnets, and the same is true of rare earth elements, which can also be metals.)
Maglev systems
In Berlin, the M-Bahn was built in the 1980s: a driverless maglev system with a 1.6 km track connecting 3 metro stations. Testing with passenger traffic started in August 1989, and regular operation started in July 1991. Because of traffic changes after the fall of the wall, deconstruction of the line started only 2 months later and was completed in February 1992. The line was replaced with a regular metro line.
A maglev service ran from the airport terminal of Birmingham International Airport (UK) to the nearby railway station from 1984 till 1995. The length of the track was 600 metres, and "flew" at an altitude of 1.5 cm. It operated for nearly eleven years, but it was unreliable and was replaced by a bus.
Transrapid (a German maglev company, which has a test track in Emsland, Germany), constructed the first operational maglev railway in the world, from Shanghai, China to the new Shanghai airport in Pudong. It was inaugurated in 2002. It has a peak speed of 430 km/h and a track length of 30 km.
Japan has a test track in Yamanashi prefecture where test trains have reached 581 km/h, far faster than wheeled trains.
Unimodal is a proposed personal rapid transit system using Inductrack suspension to achieve average commute speeds of 160kph (100mph) in the city.
On December 31, 2000, the first manned high-temperature superconducting maglev was
successfully tested in Southwest Jiaotong University, Chengdu, China. This system is based on the principle that bulk high-temperature superconductors can be levitated or suspended stably above or below a permanent magnet. The load is over 530 kg and the levitation gap is over 20 mm. The system uses liquid nitrogen, which is very cheap, to cool the superconductor.
See also
External links
Referenced By
List of electronics | List of electronics topics
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