Magnetic river is an electrodynamic magnetic levitation (maglev) system designed by Fredrick Eastham and Eric Laithwaite in 1974. It consists of a thin conductive plate on an AC linear induction motor. Due to the transverse flux and the geometry, this gives it lift, stability and propulsion. The name refers to the action that provides stability along the longitudinal axis, which acts similar to the flow of water in a river.
Linear motors
A linear induction motor (LIM) is essentially a conventional induction motor with its primary "unwound" and laid out flat. The rotor, normally consisting of a series of conductors wound onto a form of some sort, is replaced by a sheet of magnetically susceptible metal. Due to its good conductance to weight ratio, aluminium is almost always used for this "stator plate". When the primaries are fed current, they induce a magnetic field in the stator plate, which generates forces away from the plate and along it. The simplest way to use these forces to produce linear motion is to arrange two such motors on either side of a single stator plate. That way the lift forces from one motor are opposite of the other, and clamping the two motors together results in there being no net sideways force (it is contained in the stress of the clamp). This is normally arranged in a C-shaped device which is hung above a vertical stator plate. Arrangements of this sort can be commonly seen on many pioneering transit systems from the 1960s, normally running through a slot in the middle of the vehicle floor. By the late 1960s, a fatal flaw in this "sandwich motor" arrangement had been discovered. The stator plate cannot be made of a single casting, as it is kilometres long. Instead, it is made of many smaller plates that are then welded together. The strength of these welds is much smaller than the plate itself, and are prone to breaking in cold weather. When the vehicle passes, any misalignment between the motor and the stator results in enormous forces being generated, pushing the plate back into the center of the motor. These forces may be great enough to break the welds between the plates, or simply deform them. In this case, a motor on a following vehicle can strike the plate, catastrophically.
Single-sided LIM Looking to address the problems found in the sandwich motor, starting in 1967 Eric Laithwaite and his team at Imperial College London began experimenting with single-sided LIM arrangements. In this arrangement there is no corresponding set of magnetic fields on the "far side" of the stator, which requires some other system to be used to create a complete flux path. The team initially considered small plates of soft iron, like those in a transformer core. The size of the flux arrangement, and thus the size of the iron plates required, was a function of vehicle speed, power frequency and the size of the magnets. The size of the magnets is a function of the power dissipation within them, and are therefore a fixed size for any given type of vehicle; larger magnets are needed for higher power levels, which are used on higher-speed vehicles. Thus the only real variable is the frequency of the power supply. At the time, efficient high-power frequency conversion was expensive and heavy, so using standard 50 Hz mains power was the only practical system. Considering these inputs, a single-sided LIM demanded flux "core" about 30 cm deep, which would add enormously to the cost of the tracks.
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