October 2001

Magnetically Levitated Trains

How a maglev train works.

By Technology Review

The future of transportation may find travelers flying on vehicles that have no wings. Magnetically levitated trains, which use the attracting and repelling forces of magnets, jet through the air just millimeters off a specialized track-some at speeds of 550 kilometers per hour. Maglevs are quieter and consume less energy than trains with wheels that touch the track.

The city of Shanghai, China, is building a high-speed German maglev dubbed Transrapid, which will whisk people the 33 kilometers between downtown Shanghai and Pudong International Airport. And by 2004, the U.S. Department of Transportation will fund a $950 million project to build a maglev train either between Baltimore and Washington or between Pittsburgh International Airport and downtown Pittsburgh.

The Transrapid train is propelled, guided and levitated by magnetic forces. Frames attached to the bottom of the train curve down around a T-shaped guideway; electromagnetic levitation magnets attached to the frames are attracted upward to magnetic rails on the guideway's underside, lifting the train up about 15 centimeters. An alternating current passing through the guideway creates an electromagnetic field that travels down the rails. The magnets on the frames are attracted to this traveling field, which pulls the train forward in much the same way that a refrigerator magnet moved underneath paper pulls a second magnet across the top. To slow the train down, the traveling field is made to move in the reverse direction. Sensors monitor the distance between the magnets and the guideway, and a computer regulates the strength of the current sent to the magnets to keep the gap at a constant 10 millimeters. Guidance magnets and sensors located along the sides of the frames work to keep the vehicle centered above the guideway.

Another maglev system, being developed in Japan, uses superconducting magnets to levitate and propel a train and takes advantage of both attractive and repulsive forces. The magnets are situated along the sides of the train and along the inside of a U-shaped guideway. The vehicle rolls on rubber tires until it reaches 100 kilometers per hour. An electric current then creates two opposing magnetic fields that lift the train 10 centimeters above the guideway. The array of magnets along the sides pulls and pushes the train along. Both trains provide a smooth, quiet, frictionless ride unmatched even by air flight-even though the train is literally flying above the ground.

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