04-08-2012, 12:24 PM
Magnetic levitation
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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 magnetic fields. The guideway is the physical structure along which maglev vehicles are levitated. Various guideway configurations, e.g., T-shaped, U-shaped, Y-shaped, and box-beam, made of 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.
Propulsion Systems
"Long-stator" propulsion using an electrically powered linear motor winding in the guideway appears to be the favored option for high-speed maglev systems. It is also the most expensive because of higher guideway construction costs.
"Short-stator" propulsion uses a linear induction motor (LIM) winding onboard and a passive guideway. While short-stator propulsion 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 sideward 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, which supply lift, can be used concurrently for guidance or separate guidance magnets can be used.
Maglev and U.S. Transportation
Maglev systems could offer an attractive transportation alternative for many time sensitive trips of 100 to 600 miles in length, thereby reducing air and highway congestion, air pollution, and energy use, and releasing slots for more efficient long-haul service at crowded airports. The potential value of maglev technology was recognized in the Intermodal Surface Transportation Efficiency Act of 1991 (ISTEA).
Before the passage of the ISTEA, Congress had appropriated $26.2 million to identify maglev system concepts for use in the United States and to assess the technical and economic feasibility of these systems. Studies were also directed toward determining the role of maglev in improving intercity transportation in the United States. Subsequently, an additional $9.8 million were appropriated to complete the NMI Studies.
Why Maglev?
What are the attributes of maglev that commend its consideration by transportation planners?
Faster trips - high peak speed and high acceleration/braking enable average speeds three to four times the national highway speed limit of 65 mph (30 m/s) and lower door-to-door trip time than high-speed rail or air (for trips under about 300 miles or 500 km). Still higher speeds are feasible. Maglev takes up where high-speed rail leaves off, permitting speeds of 250 to 300 mph (112 to 134 m/s) and higher.
Maglev has high reliability and less susceptible to congestion and weather conditions than air or highway travel. Variance from schedule can average less than one minute based on foreign high-speed rail experience. This means intra and intermodal connecting times can be reduced to a few minutes (rather than the half-hour or more required with airlines and Amtrak at present) and that appointments can safely be scheduled without having to consider delays.