Julia Ottenstein & Mary Carter
Physics, May 2010
*Note: unfortunately we were not able to upload our diagrams from our final paper onto the blog, but see the final paper for our included photos and descriptions.
Magnetic Levitation
Magnetic Levitation has been theoretically possible since the early 20th century, when H. Kammerlilngh Onnes discovered superconductivity in 1911, yet only now is the possibility finally being implemented. Simple, efficient, safe, and with incredible possibility for speed, Magnetic Levitation trains are often considered the transportation of the future. But how do they work?
One of the first obstacles which faced engineers in the development of a magnetic levitation train was the belief that they required superconducting electromagnets. The most powerful magnets are those which are wound around core, with electricity flowing through them (See Figure 2). Superconducting electromagnets are magnets in which the coils are made of superconducting material wound with a cylinder as the core. Generally, electric resistance approaches 0Ω as temperature approaches 0K (See Figure 1), however in superconductors materials, resistance reaches 0Ω above 0K (Nave, “Superconductivity”). The superconducting materials usually used in magnets are niobium-titanium or niobium-tin.
A characteristic of the superconducting family is that it excludes magnetic fields, outlined by the Meissner Effect (Nave, “Magnetic Levitation”). This was discovered in 1933 by the German researchers Walther Meissner and Robert Ochsenfeld. The Meissner effect occurs when the superconducting material has a hollow center. In this hollow center, there is absolutely no magnetic field (See Figure 3) (Vidali 55).
Scientists have used this level of no resistance to make extraordinarily powerful electromagnets, called superconducting magnets (Nave “Superconducting Magnets). These magnets are powerful enough to levitate mice by polarized the water molecules in their bodies (Choi). Though not absolutely necessary, superconducting magnets would reduce the amount of power necessary to operate the trains and reduce the weight of the magnets on board. However, superconductors need to be kept at very low temperatures in order for them to maintain their superconductivity, and the refrigeration system necessary to keep the materials at a low enough temperature is expensive and bulky enough to hinder large-scale implementation of the technology (Vranich 90-91).
The first and most basic aspect of the magnetic levitation train is the levitation. The idea is build off of the principle of magnetism: opposites attract. So, theoretically, if you had two strong magnets and put them north pole to north pole, one on top of the other, the one on top would experience a repulsive force large enough that it would float. The presence of superconducting electromagnets makes it possible to float not only strong magnetic materials, but water even inside living animals, such as mice in an experiment done at NASA in 2009 (Choi “Mice Levitated in Lab). Therefore, the magnitude of magnets necessary to float trains weight thousands of pounds is available, even without superconductivity.
Two systems have been designed by Japanese manufacturers to use this phenomenon on a larger scale—to power trains. The first is the “attractive” or electromagnetic levitation design where a t-rail runs down the middle of the track and the train wraps around the rail, with the superconducting magnets being at the part of the train wrapped around the rail and closest to the rail. These magnets are attracted to the rail, which cannot move because it is bolted to the ground, so the magnet would float up toward the rail. In this model, the force of attraction pulling up works against the force of gravity pulling down and they balance, leaving the train in midair—about 3/8 of an inch above the t-rail. The other model, the “repulsive” levitation design or the electrodynamic model, has magnetic coils on a railbed and requires superconducting coils to be inside the train. The superconducting coils induce a current in the coils on the ground because the superconducting coils are so powerful that they force the domains of the steel rails to align with them. However, the effect only lasts for only as long as the train is passing over the coils, and creates enough repulsion to balance out the force of gravity and cause the train to float, this time about 4-6 inches above the railbed (Vidali 115-116). See Figure 1.
However, once the train is floating, there is still the question of how the train propels itself, if it is not using wheels. Scientists have solved this with the linear synchronous motor or LSM (See Figure 2). In the LSM, the railbed itself, as in the repulsion model, is magnetized. Magnets are set up alongside the train and the train itself contains as a chain of magnets. Say that the front of the train contains the south end of a magnet. Leaving the station, the magnet alongside the track, a little further up, will be the north end of a magnet, and the front of the train will be attracted to it. This same process will be happening down the train, because the magnets are lined up with their counterparts alongside the rails. However, once the front of the train reaches the north pole, its own momentum will push it past this pole to the south pole, which will repulse it and send it towards the next north pole, just a little way ahead. See Figure 2. In this way, the magnets in the train and on the railbed pull it forward and push it at turns, propelling the train without using much electricity at all, except for the starting, maintaining the superconducting electromagnets, and for comforts in the train (Vranich 98). The way that this is set up, it is impossible for there to be collisions between two trains, because the fact that the railbed itself is directing the train—two trains on the same track must be going in the same direction, and cannot catch up to each other because the railbed determines the speed. Therefore, magnetic levitation trains are much safer than current trains and certainly safer than airplanes. Also, it has been determined that there is no chance of harm from the strong magnetic fields, because they release only around .5-1 gauss, while electric drills release a magnetic field of 7.3 gauss and electric blankets release about 100 gauss (Vranich 99, 102-103).
Because of the fact that the train doesn’t actually touch the tracks and there is no friction except air resistance, speeds of magnetic levitation trains are almost infinite. Though magnetic levitation trains have only been tested up to around 150 mph, they theoretically could go at speeds up to 1,700 mph. The limit is decided based on what accelerations humans could be comfortable in or issues with surroundings, not the maximum that the train could actually go. Also, by not actually touching the tracks, it enormously decreases the amount of wear-and-tear the tracks would have, making maintenance of the tracks more cost effective (Vranich 89).
Research on the possibility of creating Magnetic Levitation trains started in the early sixties, and the first models coming out in Germany and Japan in the 1970’s. The first Transrapid Magnetic Levitation train, a German model, was tested in 1970, at a speed of 55 mph. Within 3 years, the Transrapid train tested at 3x the first speed. The latest model, the Transrapid-07 has traveled at speeds of 270 mph, and was even once struck by lightning, from which it suffered no damage. This model is lighter, more aerodynamic and has better controls than its predecessor (Vranich 100-101). The first Japanese model, the HSST design, was tested in 1971, using the attractive model. The HSST-03, a later Japanese model, had a reliability factor of 99.96% (Vranich 106). The United States has discussed on several occasions the idea of putting high speed rails into areas in southern California, Florida, and even Pittsburgh, however, any of these plans are still far from being implemented.