Maglev: the history
original Air-Rail Link maglev system used at Birmingham International Airport, which ran across a 600m track with a levitation altitude of 15 millimetres. 7
Technology
1. Magnetic Levitation and stability
Earnshaw’s theorem states that point charges — a particle assumed to have no dimensions with a charge — can’t be maintained stably in a stationary equilibrium when only electrostatic forces are present. It was initially applied to electric fields, but in the context of the presence of a magnetic field, it proves that it is impossible for a static system to levitate against gravity and achieve stability when permanent magnets are used. 8 For example, if one attempted to put a simple bar magnet on top of another one, sliding and flipping sideways would inevitably be the only possible results. However, there are some exceptions to the
Figure 4: An example of the Transrapid system in Germany utilising servomechanisms
theorem; examples include spinning systems, such as the ‘Levitron’ toys, and diamagnetic materials, which only repel against the material field. The two main types of maglev systems use another method to also bypass this: servomechanisms (used by EMS systems) and the previously discussed induced current (used by EDS systems). In an electromagnetic suspension system (EMS), the electromagnet current is constantly adjusted by an electronic stabilization system (feedback loop) while its bearing distance is being measured. The train wraps around the track and is pulled up while the servo controls maintain a constant gap between the train and the track. In an electrodynamic suspension system (EDS), passive stability is achieved by the
Figure 5: A diagram illustrating the Meissner Effect
moving magnetic fields that create current. Additionally, a type-II superconductor made from a permanent magnet on the train would lock the train laterally, ensuring it doesn’t stray off the track. The principles behind this are the Meissner effect and flux pinning. 9 The Meissner effect explains that when a superconductor is raised above its critical temperature, an increasing amount of magnetic flux penetrates it, while the opposite happens below the critical temperature. The superconductor requires constant cooling to stay below its critical temperature. A phenomenon called flux pinning then occurs where the flux within the superconductor is prevented from moving, thereby creating a pinning effect
7 http://news.bbc.co.uk/1/hi/sci/tech/488394.stm; consulted: 27/7/2023. 8 Gray, T. (2004) ‘Ignorance = maglev = bliss’, Popular Science ; available at https://www.popsci.com/diy/article/2004-02/ignorance-maglev-bliss; consulted: 29/7/2023.
9 Johnston, H. (2011) ‘Flux pinning in action’, Physics World ; available at https://physicsworld.com/a/flux-pinning-in-action/; consulted: 29/7/2023.
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