In contrast to electromagnetic suspension systems, electrodynamic suspension systems can provide stable suspension without a feedback system. Imagine a magnet held above a moving flat sheet of aluminum or copper. Following Ampere’s law of induction, an electric field is induced in the moving metal surface that causes eddy currents to flow in closed loops near its surface. The eddy currents in turn set up their own magnetic field, whose polarity, in accordance with Lenz’s law, opposes that of the magnet’s field. Consequently, the magnet is repelled from the moving metal surface, countering the force of gravity. If the magnet is pushed down toward the moving metal surface, the induced currents and the resultant repulsive force increase, restoring the equilibrium position automatically. Conversely, if the magnet is moved upward, the levitating force decreases. Consequently, the system is said to be inherently stable. (Of course, the magnet must be held in position because the induced eddy currents also cause an electromagnetic drag force that tends to pull the magnet in the longitudinal direction along with the moving metal sheet.)
In practice, the stability of a magnetically suspended object is more complicated because any disturbance from the equilibrium position in the repulsive force case results in an oscillation about the equilibrium position. The oscillation may decrease or increase with time depending on whether the net damping force present is positive or negative, respectively. Both active and passive damping mechanisms may be employed to provide net positive damping and ensure stable suspension. In the attractive force case, the feedback system mentioned above provides an effective positive damping force that attenuates any oscillations.
The configuration of choice for most electrodynamic suspension systems places the magnets on board and the electrical conductors on the guideway. One disadvantage of the electrodynamic suspension system is that, because the lift force depends on the speed, mechanical support must be used at rest and at low speeds until the lift-off speed where the magnetic suspension force exceeds the force of gravity is reached. Pneumatic tires mounted on retractable landing gears can provide suitable mechanical support. Support at rest or at low speed can also be achieved with repulsive magnetic forces. This can be accomplished using AC-excited coils interacting with eddy currents induced in reaction rails or coils. However, this method tends to be rather energy-intensive.
A variety of combinations of vehicle-borne magnets and guideway-mounted electrical conductor configurations have been devised for electrodynamic suspension systems. These include the use of continuous-sheet and discrete-coil guideways. Three configurations are illustrated in Fig. 2. In all cases, the guideway mounted conductors are completely passive. The null-flux coil configurations have high lift efficiency and can produce forces in two opposing directions (up and down or left and right).
Both permanent and superconducting magnets (SCMs) can be used as the magnetic field sources. Designs utilizing permanent magnets have been investigated by both American Maglev Technology, Inc. and by Lawrence Livermore National Laboratory. The latter group has incorporated permanent magnets in Halbach arrays, which provide greater field strength at the pole faces per unit weight and substantially reduced stray fields behind the arrays. The principal advantage of SCMs is greater force per unit weight. This is due to the strong magnetic fields that can be produced over large volumes of space and because the SCMs do not use iron cores. Air gaps on the order of 10 to 20 cm are possible with SCMs. Such large air gaps help ensure against contacting the guideway surfaces when guideway irregularities or disturbances including wind gusts and seismic activity are encountered. However, in practical designs, component parts tend to take up at least some of that space, reducing the actual clearance between the outer vehicle and guideway surfaces to 6–8 cm. The major disadvantage is that the superconducting windings must be kept below their critical temperatures to maintain their superconducting states. This requires the use of specially designed cryostats and cryogenic refrigeration systems. Superconducting magnets used in transportation applications are of the ‘‘low-temperature’’ type, which are operated at liquid-helium temperatures. The Japanese have successfully developed efficient cryogenic systems for use with SCMs on their high-speed ‘‘linear motor cars.’’ The newer ‘‘hightemperature’’ superconductors, which can be cooled with liquid nitrogen, are not yet ready for such applications.
electrodynamic suspension systems use eddy current repulsion for guidance as well as lift. Depending on the type of lift and propulsion system being used, and the guideway configuration, either separate magnets or the same magnets can be used for lift and guidance. In the Japanese linear motor car system, the same onboard SCMs provide the magnetic field for lift and lateral guidance and serve as the field sources for the secondary side of the LSMs. Eddy currents induced in the null-flux coils mounted on the guideway sidewalls in this system interact with the magnetic fields of the SCMs to produce both lift and guidance forces. The guidance forces are enhanced by crossconnecting the null-flux coils on opposite sides of the guideway (Fig. 6). Null-flux lift and guidance rely on the same operating principle. That is, the two loops of a null-flux coil are wound in opposite directions so that when a magnet is in the neutral position, an equal amount of magnetic flux links both loops so that no net current flows in the two connected loops and hence, no force is generated. When the magnet is displaced relative to the neutral position, then one loop links more flux than the other, a net current flows in both loops, and a restoring force is produced.
