IIT JEE 2006 Paper : PHYSICS Section III - Comprehension Passage 4
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Magnetic leviation (maglev) trains are being developed to eliminate the loss of power due to friction (as in conventional trains) between the train and the track. Maglev trains using the electrodynamic suspension (EDS) technology run on the principle of Lenz's law without any physical contact with the tracks.
In EDS technology the train is fitted with superconducting electromagnets which produce their own magnetic field and the tracks are lined with conducting coils. When the train moves there is a change of flux in the conducting coil which produces induced currents in these coils which in turn give rise to another magnetic field. The train is levitated by the repulsive force between these two magnetic fields.
At slow speeds, the current induced in these coils and the resultant magnetic flux is not large enough to support the weight of the train. For this reason the train must have wheels to support the train until it reaches a speed that can sustain levitation. Propulsion coils on the track are used to exert a force on the electromagnets in the train to make it move forward. The train also encounters an electromagnetic drag force according to lenz's law as it moves forward.