MAGNETIC LEVITATION

 

Conventional magnetic bearing can be improved by the use of superconducting technology, mainly with reference to two aspects.

The first aspect is stability: when conventional permanent magnets are utilized to levitate some ferromagnetic body, the system is unstable because the force between the magnet and the body is of the attractive type and the lower is the distance of the body to the permanent magnet, the higher is the attractive force, thus an active control of position is required. When a permanent magnet is suspended on a superconducting YBCO (Yttrium Barium Copper Oxide) pellet, supercurrents generate in the pellet when external magnetic field changes and the total magnetic field in the SC pellet is mainly constant in time. Two cases are possible. In the zero field cooling case, the pellet is first refrigerated, when no magnetic field is present then the permanent magnet is positioned: a repulsive force will be present between permanent magnet and SC pellets and stable vertical levitation is obtained. In the field cooling case, first the permanent magnet is positioned, then the YBCO pellet is refrigerated: the magnetic field is trapped in the SC pellet and an attractive or repulsive force between will be generated if distance between pellet and permanent magnet is larger or lower of the initial equilibrium one and stable levitation is again obtained, without any control.

The second aspect is efficiency: when conventional magnetic bearing are utilized, the currents which are responsible of the magnetic levitation force produce energy losses due to the Joule effect. When superconducting magnetic bearing are utilized, no Joule heating is present, if current density is lower than the critical one, instead some a.c. loss mechanisms are presents but dissipated power is always much lower than the dissipated one in a conventional solution.

 

Some of the main applications of superconducting bearing are concerned with the realization of flywheel for energy storage use and the realization of a very fast magnetic levitated train. In the first case YBCO pellets are positioned in the case and permanent magnets are positioned on the moving part of the flywheel. In the second case superconducting coils are positioned on the vehicle and levitation is obtained due to interaction of SC coils on the vehicle with normal coils in the guide way, when the vehicle is moving. Both of the systems have been studied at the Applied Superconductivity Laboratory.

 

Vertical stability of a permanent magnet on an YBCO pellet has been studied in cooperation with CISE (Now CESI) in Milano, by means of a finite element model of the YBCO pellet which was developed at the Department of Electrical Engineering of the University of Bologna. The obtained results were compared with the experimental ones obtained at CESI

Levitation force vs. gap, zero field cooling case

 

A figure-eight-shaped coils electrodynamic suspension (EDS) magnetic levitation (MAGLEV) system, without cross connection was proposed and analyzed. The guideway coils are positioned under the MAGLEV vehicle; they are parallel to the horizontal plane. Two superconducting (SC) coils, on the vehicle, interact with each figure-eight guideway coil. The performances of the system are analyzed by means of the dynamic circuit theory. The currents in the SC coils are supposed to be constant in time while they move as a rigid body, with a constant velocity.

 

 

References:

1.       P.L. Ribani, A. Cristofolini, M. Fabbri, P. La Cascia, F. Negrini, “Modelling of high-temperature superconducting bearings”, Il Nuovo Cimento, Vol. 19D, n. 8-9, pp.1483-1488, August-September 1997.

2.       F. Negrini, P.L. Ribani, E. Varesi, S. Zannella, “Numerical Modelling of High Temperature Superconducting Bearings”, Inst. Phys. Conf. Ser. n. 158, Vol. 2, pp. 1671-1674, (Paper presented at Applied Superconductivity, The Netherlands, 30 June-3 July, 1997).

3.       P.L. Ribani, N. Urbano, “Study on Figure-Eight-Shaped Coil Electrodynamic Suspension Magnetic Levitation Systems Without Cross-Connection”, IEEE Transactions on Magnetics, Vol. 36, n. 1, pp.358-365, January 2000.