Magnetic river

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.

[1] 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.

[2] 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.

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.

They noticed that by turning the vehicle-mounted rotor side of the motor through 90 degrees, so it was aligned "across" the tracks instead of along them, the flux was able to spread through the entire stator plate, thereby eliminating the problems with depth.

The only downside of this approach is that the vehicle in proper alignment feels both attractive and repulsive forces, meaning that greater energy is needed to provide the required amount of lift.

[6] Tom Fellows of the Tracked Hovercraft team approached Laithwaite to build a model of a maglev system for the upcoming Transpo '72 trade show.

Using the repulsive design he found that the model required a very wide motor, about 25 cm for a track that was to be only 9 m long, so Laithwaite began examining ways to reduce the system size.

This was found to cause the system to become unstable, until someone accidentally hooked up the lift conductors the "wrong way" so the current flowed in the same direction in the two loops.

[6] When Laithwaite hired an engineering firm to build the model, they noted that a 9 m long stack of iron plates would be highly unlikely to survive the journey to the US intact.