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MAGLEV
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MAGLEV

The flying train

courtesy Central Japan Railway CompanyIf a Maglev wants to use this force to levitate, it needs a strong magnetic field in its wagons. We could use normal magnets, but their magnetic power is limited. The most efficient way to produce the most powerful magnetic field we know of today, with a reasonable energy cost, is the use of superconducting coils, as in MRIs. This is the solution chosen by the Japanese Railway Technical Research Institute (JR-RTRI). This institute built a 40 km long track to test their Maglev in Yamanashi region, in Japan. It was on this track that the prototype MLX01 won the speed record for a train: 581 km/h

courtesy Central Japan Railway Companycourtesy Central Japan Railway Company

For efficiency reasons, the superconducting coils are placed on the sides of the wagons (four on each side). As in MRIs, these coils are made with conventional superconductors that require very low temperatures, a few kelvins above absolute zero: they are hence always surrounded with liquid helium. Permanent currents of about 700 000 amperes go through these superconducting coils, hence creating a strong magnetic field of almost 5 teslas, i.e. 100 000 times stronger than the earth magnetic field. The coils are closed and the magnetic field they generate is hence constant and does not change over time.

courtesy Central Japan Railway CompanyThe magnetic field created by these coils enables both the levitation and the propulsion of the Maglev, thanks to metal coils in the beams located along the tracks. In these tracks, there are two types of coils: propulsion coils and levitation coils.

The propulsion coils are active, which means they are supplied by a source of energy: this makes sense; the train must accelerate and defeat air resistance. Since these coils are made of metal, they consume energy. courtesy Central Japan Railway CompanyNevertheless, they can be totally controlled: when the direction and the intensity of the currents going through them are controlled, the sign and the intensity of the created magnetic field are also controlled. To make the Maglev accelerate, you only need to send an electric current in the propulsion coils located in the beams upstream from the Maglev in order to attract it; and to send an electric current in the coils down stream in order to push it. Attracted in the front and pushed in the back, the Maglev accelerates. The engine of the Maglev is hence located in the tracks! To slow down, we only need to invert the current, pushing the front of the Maglev and attracting its back. Furthermore, the wagons are equipped with air brakes in order to slow down without consuming any energy.

The levitation coil, as their name indicates, enable the Maglev to levitate. They are made of metal, just as the propulsion coils, but contrary to them, they are not linked to any source of energy. They are short-circuited and totally passive. When the Maglev moves thanks to the propulsion coils, the magnetic field it carries (created by superconducting coils) scans the levitation coils. These coils are short-circuited and inducted currents flow through them, similar to the Eddy currents in the experiment of the magnet falling in a copper tube. These inducted currents, since they flow in a coil, create a magnetic field. The shape of the levitation coils was designed so that the magnetic field created by the inducted currents apply a levitation force on the superconducting coils in the wagons.

courtesy Central Japan Railway CompanyThe levitation of the Maglev hence does not require any other energy than the energy required for it to move: the train naturally floats in the air. However, this only works if the Maglev moves fast enough (about 100 km/h), since the levitation coils have to be scanned by the magnetic field of the superconducting coils: the faster the Maglev moves, the faster the magnetic field passes in front of the levitation coils, the stronger the inducted currents and the better the levitation. If the Maglev moves slowly, or stops, the inducted currents become too weak and the Maglev stops levitating: the wagons are hence equipped with “landing wheels” that appear when the speed is too limited, as in planes. In many respects, the Maglev looks more like a plane than a train!

The 40–km-long tracks that are used for testing are for now the only existing tracks; no commercial line provides a trip on board a 500 km/h superconducting train. Today (in 2011), the only way to experience this is to take part in the tests that are done with passengers by the RTRI. However, a railway line between Tokyo and Osaka should be built, but it will take a few more years…

CNRSSociété Française de PhysiqueTriangle de la physique
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CNRSSociété Française de PhysiqueTriangle de la physique