4.2 LTS Maglev Ë 89changing magnetic field is produced by a magnet, superconducting coil magnet at
present, on the moving train. Both PM and SCPM can also be used as a suspended
magnets of the EDS system if their magnetic fields are strong enough.
The EDS Maglev can be of various types. In the normal flux system, a flat excitation
magnet system moves over a conducting roadbed consisting of periodically spaced,
independent loops or a continuous plate. In the null flux system, there are double-
layer coils in each side of the “U”-type track. The one-layer coil is unpowered, closed-
circuit inverse pair loops (null flux). Interaction between it and on-board supercon-
ducting magnets has a guidance force for the train. The other layer coil plays the role
of propulsion.
The USA first invented the EDS Maglev concept using a superconducting magnet
in 1966 [2, 3]. The magnetic levitation and guidance forces of the EDS Maglev are
obtained through the interaction of the magnetic field of an on-board superconducting
magnet with eddy currents induced in a stationary metallic guideway.
The low-frequency forms of Maxwell’s equations are used to analyze the EDS
Maglev, and the AC losses of superconductors is very low at this time. This problem
can be dealt with by Ampère’s circuital law and Faraday’s law, and the Lorentz force
law. Ampère’s circuital law means that the magnetic flux density integrated around
a closed loop equals the net current passing through the loop. Faraday’s law of
electromagnetic induction shows the mechanisms by which a changing magnetic flux
generates eddy currents. The Lorentz force law states that a magnetic force results if
there is a current flow in a magnetic field.
When the guideway has a certain resistance, the magnetic energy in on-board
magnets will be dissipated by the resistance of the guideway. The power lossPgives
drag forcesFD.
FD=P
v. (4.1)
For a given decay constantLs/Rs, energy dissipation is smaller with faster speed. The
induced current can only flow in the skin depth of the guideway. The skin depth훿is
훿=^1
휋f휇 0 휎
(^12)
, (4.2)
wherefis the frequency of the induced current,휇 0 is the permeability, and휎is the
conductivity. It can also be expressed as
훿=
k 1
(v휎)12 , (4.3)
wherevis vehicle speed andk 1 is a constant determined by the system.