Magnetic Levitation And Systems Pertaining to Maglev Trains
...ter and therefore the dipoles line up with the external field. Even though paramagnetism is stronger and as a result masks the effects of diamagnetism, it is too weak and is not often noticed. The last type of magnetic material is ferromagnetism. This is the property that is responsible for the strong magnetic property of iron. Ferromagnetism is a result of the alignment of intrinsic spin axes of the electron that constitute the material [1]. This spin is a quantum mechanic property. Simply speaking, the distribution of the electron’s surface charge is spinning about an axis that creates current loops that make a magnetic field [1]. In most elements the spins are in all different directions but in a ferromagnet the spins line up. In theses materials the distances between are small enough that the wave functions overlap and create a coupling force [1]. Unlike diamagnets and paramagnets, which become magnetized because orbital motion of the electrons, ferromagnetism is the effect of the intrinsic spin of electrons. Furthermore, ferromagnetic material remains magnetized after the externally applied field is removed. This effect is called hysteresis. Magnetic Levitation A logical step is to apply the repulsive force of magnets in order to levitate objects. This is a lot more complicated then it would seem. Overcoming the gravitation force is not a problem, equilibrium is not a problem either, but stability is. In fact, there is a theorem that states it is impossible to levitate a permanent magnet [4]. In 1842, Samuel Earnshaw proved it so. There are loopholes in this theorem though. For one it only applies ferromagnetic materials in static conditions. In addition, the theorem only applies to physics in the classical sense. If one would look at magnetic levitation quantum mechanically, they would find that any body sitting on a surface is technically levitating [4]. This property is due to the electromagnetic intermolecular forces and the exclusion principle [1]. Another way to violate the Earnshaw’s theorem is to make the system dynamic. You can use permanent magnets levitate objects by moving them in a way that cancels out the forces that make the object unstable. Electromagnetic suspension is done in this manner. As stated before, if the material is diamagnetic, it expels the magnetic field as stated by the Meissner-Ochsenfeld effect [1]. Another dynamic way to create a levitating force is by oscillating the magnetic field. By oscillating the field, ferromagnetism becomes diamagnetism. Finally, by rotating a ceramic material over a permanent magnet it is possible to make that object levitate. It must be ceramic as to prevent induced magnetic currents, which would reduce the rotational energy [4]. Although Earnshaw’s theorem is true, there are many ways to maneuver around it to levitate objects. Now, two of the methods explained in the preceding text will be further explained as they are related to magnetically levitated trains. Electromagnetic Propulsion The basic principal behind electromagnetic propulsion is that opposite poles attract and like poles repel. Although this is a horribly simplified definition of the actual process involved in maglev trains, it is the governing idea. Both electromagnetic suspension (EMS) and electrodynamic suspension (EDS) have three main components. A large electrical power source is needed to supply enough current to create the magnetic field. Metal coils that line the tracks are used to transport the current and large guidance magnets attached the bottom side or the trains. Figure one shows an illustration of the components of the guideway. Figure 1.) Illustration of maglev guideway for an EDS system The magnetized coils that run along the guideway supply a strong enough magnetic field that allows the train to levitate. Depending on the type of maglev, the train can levitate anywhere from 1 to 10 centimeters [2]. The attractive type of maglev trains is called electromagnetic suspension. This train has magnets that are wrapped around an iron guideway [5]. The magnets attached to the underside of the train are attracted to the iron lifting the train in the +z direction. The EMS system has primarily been researched and constructed in Germany by the Konsortium Magnetbahn Transpid [5]. The propulsion in this system is provided by a long-stator linear synchronous motor or LSM. Because of power limitation in the copper levitation coils the train can only levitate about 1 centimeter [5]. Since this air gap is so Figure 2.) The Transrapid TR07, an EMS Maglev vehicle small it is unable to operate with constant current in the levitations coils. For this reason EMS requires an active control system to meet the safety standards required for mass transit. Currently the Transpid TR07 is in operation form Hamburg to Berlin. The other maglev system currently in production is the electrodynamic superconducting magnetic suspension system. Differing form the EMS, EDS is a repulsive suspension system. It was first created in the United States in the 1960’s [5]. Figure 3.) Japan's EDS MLX01 experimental maglev train On this train there are superconducting coils that are mounted on the bogies of a moving train that induce circulating currents in the conducting guideway [5]. The result is levitation and lateral guidance forces. The EDS maglev trains do not require an active control system, differing it from the German’s EMS train. One draw back of the EDS is that there is no suspension at low speeds so wheels are required until it reaches a lift off speed of about 100 km/h [2]. Although the tires could be considered a safety precaution in case of a sudden energy loss. The propulsion force in the EDS maglev is supplied by a linear synchronous motor [5]. Because of the high B-field generated by the superconducting coils the EDS maglev is able to levitate 10 to 25 centimeters above the track. The reason the train levitation is dependent on speed is due the magnetic fields dependence on current. The faster the train is going the higher the current produced from the guideway conductors resulting in a stronger induced magnetic field by the superconducting coils. The conducting guideways also create a guidance force in the x direction on the bogies that center the vehicle. In addition the circulating current in the guideway produce a drag force in the –y direction that is over come by the propulsion force [5]. Figure 4.) EMS and EDS maglev trains Circuit Modeling of EDS Maglev Although the guideway is comprised of a large number of guideway loops, looking a single loop shows the dependence of lift and drag forces. The model has a self-inductance L and a self-resistance R. The voltage around the loop is induced by the superconducting magnet that is traveling over the guideway. ...