Initially there was proposal for levitation of electromagnets using repulsive force(EMS) that is to hold two magnets by their reulsion,but the idea was dropped according to some papers but the reason is not clear to me. What is the disadvantage of repulsion over attraction force (EMS)that are being used now?Ofcourse feedback mechanism is used for transrapid why it can't be used for repulsion mechanism?
There are mainly three modes of suspension : the attraction force used in tranrapid and HSST, the propulsion force mode used in superconducting magnetic levitation system and the electrodynamic suspension system like MLX. The superconducting magnetic levitation system takes advantage of the pining force between the superconducting bulks and the permanent magnet guideway and the pining force would provide self-stablized levitation and guidance force. The reason why the EMS uses the attraction force is that the propulsion force is not stable for levitation.
Actually, you should keep in mind that the electromagnetic suspension is always naturally unstable, therefore you will need a feedback system. If you use two magnets as a repulsion system, you will have to control the lateral displacement of one of them. The difference between the atraction and repulsion is, basically, the axis of displacement that you have to control. In the attraction you have to control the vertical axis, while in the repulsive method you have to control the vertical displacement. This principle of repulsive force is being applied in some new products, like the levitating speakers.
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