Maglev (transport) – Wikipedia, the free encyclopedia
Maglev, or magnetic levitation, is a system of transportation that suspends, guides and propels vehicles, predominantly trains, using levitation from a very large number of magnets for lift and propulsion. This method has the potential to be faster, quieter and smoother.
There are three primary types of maglev technology:
Electro magnetic suspension
In current electromagnetic suspension (EMS) systems, the train levitates above a steel rail while electromagnets, attached to the train, are oriented toward the rail from below. The electromagnets use feedback control to maintain a train at a constant distance from the track, at approximately 15 millimeters (0.6 in).
In electrodynamic suspension (EDS), both the rail and the train exert a magnetic field, and the train is levitated by the repulsive force between these magnetic fields. The magnetic field in the train is produced by either electromagnets (as in JR-Maglev) or by an array of permanent magnets (as in Inductrack). The repulsive force in the track is created by an induced magnetic field in wires or other conducting strips in the track.
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 or some other form of landing gear to support the train until it reaches a speed that can sustain levitation.
Propulsion coils on the guideway are used to exert a force on the magnets in the train and make the train move forward. The propulsion coils that exert a force on the train are effectively a linear motor: An alternating current flowing through the coils generates a continuously varying magnetic field that moves forward along the track. The frequency of the alternating current is synchronized to match the speed of the train. The offset between the field exerted by magnets on the train and the applied field creates a force moving the train forward.
Stabilized Permanent Magnet suspension
SPM maglev systems differ from EDS maglev in that they use opposing sets of rare earth magnets (typically neodymium alloys in a Halbach array) in the track and vehicle to create permanent, passive levitation; i.e., no power is required to maintain permanent levitation. With no current required for levitation, the system has much less electromagnetic drag, thus requiring much less power to move a given cargo at a given speed.
Because of Earnshaw’s theorem, SPM maglev systems require a mechanism to create lateral stability (i.e., controlling the side-to-side movement of the vehicle). One way to provide this stability is to use a set of coils along the bottom of the magnet array on the vehicle being levitated, which centers the vehicle over the rails by means of small amounts of current. Because the voice coils are not needed to provide lift and there is almost no drag, this system uses less power than other maglev systems: when the vehicle is centered over the rails, it uses no power. As the vehicle navigates a curve, the controller moves the vehicle to a ‘balance point’ inside the curve so that the (magnetic) centripetal pull of the magnetic rails in the ground offset the vehicle’s (kinetic) centrifugal momentum. This balance point varies based on the vehicle’s weight, which the controller automatically accounts for, resulting in zero steady state power consumption.
Pros and cons of different technologies
EMS (Electromagnetic suspension)
Pros- Magnetic fields inside and outside the vehicle are less than EDS; proven, commercially available technology that can attain very high speeds (500 km/h); no wheels or secondary propulsion system needed.
Cons- The separation between the vehicle and the guideway must be constantly monitored and corrected by computer systems to avoid collision due to the unstable nature of electromagnetic attraction; due to the system’s inherent instability and the required constant corrections by outside systems, vibration issues may occur.
Pros- Onboard magnets and large margin between rail and train enable highest recorded train speeds (581 km/h) and heavy load capacity; has recently demonstrated (December 2005) successful operations using high temperature superconductors in its onboard magnets, cooled with inexpensive liquid nitrogen
Cons- Strong magnetic fields onboard the train would make the train inaccessible to passengers with pacemakers or magnetic data storage media such as hard drives and credit cards, necessitating the use of magnetic shielding; limitations on guideway inductivity limit the maximum speed of the vehicle; vehicle must be wheeled for travel at low speeds.
Inductrack System (Permanent Magnet EDS)
Pros-Failsafe Suspension – no power required to activate magnets; Magnetic field is localized below the car; can generate enough force at low speeds (around 5 km/h) to levitate maglev train; in case of power failure cars slow down on their own safely; Halbach arrays of permanent magnets may prove more cost-effective than electromagnets
Cons- Requires either wheels or track segments that move for when the vehicle is stopped. New technology that is still under development (as of 2008) and as yet has no commercial version or full scale system prototype.
An EDS system can provide both levitation and propulsion using an onboard linear motor. EMS systems can only levitate the train using the magnets onboard, not propel it forward. As such, vehicles need some other technology for propulsion. A linear motor (propulsion coils) mounted in the track is one solution. Over long distances where the cost of propulsion coils could be prohibitive, a propeller or jet engine could be used.
Some systems use Null Flux systems these use a coil which is wound so that it enters two opposing, alternating fields. When the vehicle is in the straight ahead position, no current flows, but if it moves off-line this creates a changing flux that generates a field that pushes it back into line.
Power and energy usage
Power for maglev trains is used to accelerate the train, and may be produced when the train slowed (“regenerative braking“), it is also usually used to make the train fly, and to stabilise the flight of the train, for air conditioning, heating, lighting and other miscellaneous systems. Power is also needed to force the train through the air (“air drag“).
At low speeds the levitation power can be significant, but at high speeds, the total time spent levitating to travel each mile is greatly reduced, giving reduced energy use per mile, but the air drag energy increases as a square law on speed, and hence at high speed dominates.