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What is maglev?
Magnetic levitation (maglev) is a relatively new transportation technology in which noncontacting vehicles travel safely at speeds of 250 to 300 miles per hour or higher while suspended, guided, and propelled above a guideway by electromagnetic fields. The guideway is the physical structure along which maglev vehicles are levitated. Various guideway configurations—T-shaped, U-shaped, Y-shaped, and box-beam—made of steel, concrete, hybrid steel/concrete, or aluminum have been proposed.
Figure 1 depicts the three primary functions basic to maglev technology:
(1) levitation or suspension; (2) propulsion; and (3) guidance. In most current designs, magnetic forces are used to perform all three functions, although a nonmagnetic source of propulsion could be used. No consensus exists on an optimum design to perform each of the primary functions.
Suspension Systems
The two principal means of levitation are illustrated in Figures 2 and 3 below. Electromagnetic suspension (EMS) is an attractive-force levitation system whereby electromagnets on the vehicle interact with and are attracted to ferromagnetic rails located on the guideway. EMS was made practical by advances in electronic control systems that maintain the air gap between vehicle and guideway, thus preventing unwanted physical contact.
Variations in payload weight, dynamic loads, and guideway irregularities are compensated for by changing the magnetic field in response to vehicle/guideway air gap measurements.
Electrodynamic suspension (EDS) is an repulsive-force levitation system that employs magnets on the moving vehicle to induce currents in the guideway. Resulting repulsive forces produce inherently stable vehicle support and guidance because the magnetic repulsion force increases as the vehicle/guideway gap decreases. However, the vehicle must be equipped with wheels or other forms of support for "takeoff" and "landing" because EDS systems will not levitate until lift forces are greater then the vehicle weight, which usually occurs at speeds above approximately 25 mph. EDS has progressed with advances in cryogenics and basic superconducting magnet technology.
Propulsion Systems
"Long-stator" propulsion using an electrically powered linear synchronous motor (LSM) winding in the guideway appears to be the best-known option for high-speed maglev systems. It is also considered the more expensive option because of perceived higher guideway construction costs.
"Short-stator" propulsion uses a linear induction motor (LIM) winding onboard and a passive guideway. While short-stator propulsion typically reduces guideway costs, the LIM is heavy and reduces vehicle payload capacity, resulting in higher operating costs and lower revenue potential compared to the long-stator propulsion. A third alternative is a nonmagnetic energy source (gas turbine or turboprop) but this, too, results in a heavy vehicle and reduced operating efficiency.
Guidance Systems
Guidance or steering refers to the lateral forces that are required to make the vehicle follow the guideway. The necessary forces are supplied in an exactly analogous fashion to the suspension forces, either attractive or repulsive. The same magnets on board the vehicle that supply lift can also be used concurrently for guidance or, more often, separate guidance magnets can be used to reduce the chances of magnetic interference.
Three examples of the best-known maglev systems are shown in Figure 4.
Sources:
Text: Web page at http://inventors.about.com/library/inventors/blrailroad3.htm
Graphic courtesy of Transrapid International USA
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