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Our discussions should be as to which maglev technology (or perhaps multiple maglev technologies) we should use for updating our Post Roads of the near future.

Maglev [systems], also called magnetic levitation or maglev,  support floating vehicles for land transportation that are supported by either electromagnetic attraction or repulsion. Maglev trains were conceptualized during the early 1900s by American professor and inventor Robert Goddard and French-born American engineer Emile Bachelet and have been in commercial use since 1984, with several operating at present and extensive networks proposed for the future.

Maglev trains incorporate a basic fact about magnetic forces—like magnetic poles repel each other, and opposite magnetic poles attract each other—to lift, propel, and guide a vehicle over a track (or guideway). Maglev train propulsion and levitation may involve the use of superconducting materials,electromagnetsdiamagnets, and rare-earth magnets.

Electromagnetic suspension (EMS) and electrodynamic suspension (EDS)

Two types of maglev trains are in service. Electromagnetic suspension (EMS) uses the attractive force between magnets present on the train’s sides and underside and on the guideway to levitate the train. A variation on EMS, called Transrapid and used in Germany, employs an electromagnet to lift the train off the guideway. The attraction from magnets present on the underside of the vehicle that wrap around the iron rails of the guideway keep the train about 1.3 cm (0.5 inch) above the guideway.

Electrodynamic suspension (EDS) systems are similar to EMS in several respects, but the magnets are used to repel the train from the guideway rather than attract them. These magnets are supercooled and superconducting and have the ability to conduct electricity for a short time after power has been cut. (In EMS systems a loss of power shuts down the electromagnets.) Also, unlike EMS, the charge of the magnetized coils of the guideway in EDS systems repels the charge of magnets on the undercarriage of the train so that it levitates higher (typically in the range of 1–10 cm [0.4–3.9 inches]) above the guideway. EDS trains are slow to lift off, so they have wheels that must be deployed below approximately 100 km (62 miles) per hour. Once levitated, however, the train is moved forward by propulsion provided by the guideway coils, which are constantly changing polarity owing to alternating electrical current that powers the system.

Maglev trains eliminate a key source of friction—that of train wheels on the rails—although they must still overcome air resistance. This lack of friction means that they can reach higher speeds than conventional trains. At present maglev technology has produced trains that can travel in excess of 500 km (310 miles) per hour. This speed is twice as fast as a conventional commuter train and comparable to the TGV (Train à Grande Vitesse) in use in France, which travels between 300 and 320 km (186 and 199 miles) per hour. Because of air resistance, however, maglev trains are only slightly more energy efficient than conventional trains.

Benefits and costs

Maglev trains have several other advantages compared with conventional trains. They are less expensive to operate and maintain, because the absence of rolling friction means that parts do not wear out quickly (as do, for instance, the wheels on a conventional railcar). This means that fewer materials are consumed by the train’s operation, because parts do not constantly have to be replaced. The design of the maglev cars and railway makes derailment highly unlikely, and maglev railcars can be built wider than conventional railcars, offering more options for using the interior space and making them more comfortable to ride in. Maglev trains produce little to no air pollution during operation, because no fuel is being burned, and the absence of friction makes the trains very quiet (both within and outside the cars) and provides a very smooth ride for passengers. Finally, maglev systems can operate on higher ascending grades (up to 10 percent) than traditional railroads (limited to about 4 percent or less), reducing the need to excavate tunnels or level the landscape to accommodate the tracks.

Besides the costs of construction, one factor to be considered in developing maglev rail systems is that they require the use of rare-earth elements (scandium, yttrium, and 15 lanthanides), which may be quite expensive to recover and refine. Magnets made from rare-earth elements, however, produce a stronger magnetic field than ferrite (iron compounds) or alnico (alloys of iron, aluminumnickelcobalt, andcopper) magnets to lift and guide the train cars over a guideway.

Maglev train systems

Several train systems using maglev have been developed over the years, with most operating over relatively short distances. Between 1984 and 1995 the first commercial maglev train system was developed in Great Britain as a shuttle between the Birmingham airport and a nearby rail station, some 600 metres (about 1,970 feet) away. Germany constructed a maglev train in Berlin (the M-Bahn) that began operation in 1991 to overcome a gap in the city’s public transportation system caused by the Berlin Wall; however, the M-Bahn was dismantled in 1992, shortly after the wall was taken down. The 1986 World’s Fair (Expo 86) in Vancouver included a short section of a maglev train system within the fairgrounds.

Several commercial maglev systems are currently in operation around the world. In Aichi, Japan, near Nagoya, a system built for the 2005 World’s Fair is still in operation. It is about 9 km (5.6 miles) long, with nine station stops over that distance, and reaches speeds of about 100 km (62 miles) per hour. The Korean Rotem Maglev runs in the city of Taejeŏn between the Taejeŏn Expo Park and the National Science Museum, a distance of 1 km (0.6 mile). The longest commercial maglev system is in Shanghai; it covers about 30 km (18.6 miles) and runs from downtown Shanghai to Pudong International Airport. However, Japan has plans to create a long-distance system by the middle of the 21st century that connects Osaka to Tokyo, a distance of 514 km (319 miles). The United States has no commercial maglev trains, but several prototype systems exist or are under construction. 

Pros and cons of different technologies

Each implementation of the magnetic levitation principle for train-type travel involves advantages and disadvantages.


Technology  Pros  Cons

EMS[32][33] (Electromagnetic suspension)Magnetic fields inside and outside the vehicle are less than EDS; proven, commercially available technology that can attain very high speeds (500 km/h (310 mph)); no wheels or secondary propulsion system needed.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.

EDS[34][35]
(Electrodynamic suspension)
Onboard magnets and large margin between rail and train enable highest recorded train speeds (581 km/h (361 mph)) and heavy load capacity; has demonstrated (December 2005) successful operations using high-temperature superconductors in its onboard magnets, cooled with inexpensive liquid nitrogen.Strong magnetic fields on board 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 wheeledfor travel at low speeds.

Inductrack System[36][37] (Permanent Magnet Passive Suspension)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 (3.1 mph)) 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.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.

Neither Inductrack nor the Superconducting EDS are able to levitate vehicles at a standstill, although Inductrack provides levitation down to a much lower speed; wheels are required for these systems. EMS systems are wheel-less.

The German Transrapid, Japanese HSST (Linimo), and Korean Rotem EMS maglevs levitate at a standstill, with electricity extracted from guideway using power rails for the latter two, and wirelessly for Transrapid. If guideway power is lost on the move, the Transrapid is still able to generate levitation down to 10 km/h (6.2 mph) speed, using the power from onboard batteries. This is not the case with the HSST and Rotem systems.

[edit]Propulsion

Some EMS systems such as HSST/Linimo can provide both levitation and propulsion using an onboard linear motor. But EDS systems and some EMS systems such as Transrapid can only levitate the train using the magnets on board, 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 the cost of propulsion coils could be prohibitive.

[edit]Stability

Earnshaw's theorem shows that any combination of static magnets cannot be in a stable equilibrium.[38] However, the various levitation systems achieve stable levitation by violating the assumptions of Earnshaw's theorem. Earnshaw's theorem assumes that the magnets are static and unchanging in field strength and that the relative permeability is constant and greater than unity everywhere. EMS systems rely on active electronic stabilization. Such systems constantly measure the bearing distance and adjust the electromagnet current accordingly. All EDS systems are moving systems (no EDS system can levitate the train unless it is in motion).

Because maglev vehicles essentially fly, stabilisation of pitch, roll and yaw is required by magnetic technology. In addition to rotation, surge (forward and backward motions), sway (sideways motion) or heave (up and down motions) can be problematic with some technologies.

If superconducting magnets are used on a train above a track made out of a permanent magnet, then the train would be locked in to its lateral position on the track. It can move linearly along the track, but not off the track. This is due to the Meissner effect.

[edit]Guidance

Some systems use Null Current systems (also sometimes called Null Flux systems);[30][39] these use a coil which is wound so that it enters two opposing, alternating fields, so that the average flux in the loop is zero. 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 and pulls it back into line. However, some systems use coils that try to remain as much as possible in the null flux point between repulsive magnets, as this reduces eddy current losses.

[edit]Evacuated tubes

Some systems (notably the Swissmetro system) propose the use of vactrains—maglev train technology used in evacuated (airless) tubes, which removes air drag. This has the potential to increase speed and efficiency greatly, as most of the energy for conventional maglev trains is lost in air drag.[40]

One potential risk for passengers of trains operating in evacuated tubes is that they could be exposed to the risk of cabin depressurization unless tunnel safety monitoring systems can re-pressurize the tube in the event of a train malfunction or accident. The RAND Corporation has depicted a vacuum tube train that could, in theory, cross the Atlantic or the USA in ~21 minutes.[41]

[edit]Power and energy usage

Energy for maglev trains is used to accelerate the train, and may be regained when the train slows down ("regenerative braking"). It is also used to make the train levitate and to stabilise the movement of the train. The main part of the energy is needed to force the train through the air ("air drag"). Also some energy is used for air conditioning, heating, lighting and other miscellaneous systems.

At low speeds the percentage of power (energy per time) used for levitation can be significant consuming up to 15% more power than a subway or light rail service.[42] Also for very short distances the energy used for acceleration might be considerable. But the power used to overcome air drag increases with the cube of the velocity, and hence dominates at high speed (note: the energy needed per mile increases by the square of the velocity and the time decreases linearly.).

http://en.wikipedia.org/wiki/Maglev_(transport) 

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