magnetic levitation(redirected from Diamagnetic levitation)
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magnetic levitation or maglev (măgˈlĕv), support and often propulsion of objects or vehicles by the use of magnets. The magnets used in magnetic levitation suspend an object free of contact with any surface, making it particularly appropriate for high-speed (275–300 mph/435–475 km/h) transportation, where it greatly reduces friction and allows for fast, quiet operation. In a typical system, the vehicle, which resembles a railroad or monorail car, travels along a guideway; use of a low-pressure tube to enclose the guideway, reduce drag, and increase speeds by two to three times standard maglev speeds has been proposed.
In one version of the guideway system, magnets of like polarity repel each other to lift the train above the guideway; in another, magnets of opposite polarity attract the part of the car suspended below the guideway up toward the guideway, raising the rest of the car above it. By continuously changing the polarity in alternate magnets, a series of magnetic attractions and repulsions is created that moves the vehicle along the track. The electrical energy required for such a system is great and an obstacle to wide use, but the use of magnets made of superconducting materials (see superconductivity) reduces energy needs.
A maglev transportation system was first proposed by Robert Goddard in 1909, but research into such systems has been conducted only since the 1960s, in the United States, Great Britain, Japan, Germany, and other nations. Research into the use of a low-pressure tube to enclose a maglev train was begun in the 21st cent. Maglev technology was applied in England in the construction of a fully automated, low-speed shuttle in Birmingham, but the line was closed because of maintenance problems. In 1996 funding was approved in Germany for a maglev train linking Berlin and Hamburg, but it was canceled in 2000. In 2004 a maglev line linking Shanghai's financial district with its airport began commercial operation; the train can reach speeds of 268 mph (432 km/h) along its 18.6 mi (30 km) route.
See I. Baldea, New Ways: Tiltrotor Aircraft and Magnetic Levitating Vehicles (1991).
magnetic levitation[mag′ned·ik ‚lev·ə′tā·shən]
A method of supporting and transporting objects or vehicles which is based on the physical property that the force between two magnetized bodies is inversely proportional to their distance. By using this magnetic force to counterbalance the gravitational pull, a stable and contactless suspension between a magnet (magnetic body) and a fixed guideway (magnetized body) may be obtained. In magnetic levitation (maglev), also known as magnetic suspension, this basic principle is used to suspend (or levitate) vehicles weighing 40 tons or more by generating a controlled magnetic force. By removing friction, these vehicles can travel at speeds higher than wheeled trains, with considerably improved propulsion efficiency (thrust energy/input energy) and reduced noise. In maglev vehicles, chassis-mounted magnets are either suspended underneath a ferromagnetic guideway (track) or levitated above an aluminum track. See Magnet
In the attraction-type system, a magnet-guideway geometry is used to attract a direct-current electromagnet toward the track. This system, also known as the electromagnetic suspension (EMS) system, is suitable for low- and high-speed passenger-carrying vehicles and a wide range of magnetic bearings. The electromagnetic suspension system is inherently nonlinear and unstable, requiring an active feedback to maintain an upward lift force equal to the weight of the suspended magnet and its payload (vehicle).
In the repulsion-type system, also known as the electrodynamic levitation system (EDS or EDL), a superconducting coil operating in persistent-current mode is moved longitudinally along a conducting surface (an aluminum plate fixed on the ground and acting as the guideway) to induce circulating eddy currents in the aluminum plate. These eddy currents create a magnetic field which, by Lenz's law, oppose the magnetic field generated by the travelling coil. This interaction produces a repulsion force on the moving coil. At lower speeds, this vertical force is not sufficient to lift the coil (and its payload), so supporting auxiliary wheels are needed until the net repulsion force is positive. The speed at which the net upward lift force is positive (critical speed) is dependent on the magnetic field in the airgap and payload, and is typically around 80 km/h (50 mi/h). To produce high flux from the traveling coils, hard superconductors (type II) with relatively high values of the critical field (the magnetic field strength of the coil at 0 K) are used to yield airgap flux densities of over 4 tesla. With this choice, the strong eddy-current induced magnetic field is rejected by the superconducting field, giving a self-stabilizing levitation force at high speeds (though additional control circuitry is required for adequate damping and ride quality). See Eddy current
Due to their contactless operation, linear motors are used to propel maglev vehicles: linear induction motors for low-speed vehicles and linear synchronous motors for high-speed systems. Operationally they are the unrolled versions of the conventional rotary motors. See Induction motor, Synchronous motor
Suspending the rotating part of a machine in a magnetic field may eliminate the contact friction present in conventional mechanical bearings. Magnetic bearings may be based on either attractive or repulsive forces. Although well developed, radial magnetic bearings are relatively expensive and complex, and are used in specialized areas such as vibration dampers for large drive shafts for marine propellers. In contrast, the axial versions of magnetic bearings are in common use in heavy-duty applications, such as large pump shafts and industrial drums. See Antifriction bearing