Linear Induction Motors (LIMs), which are the basis for propulsion of Elon Musk’s proposed near-supersonic LA-To San Francisco commuter train, are interesting things. I don’t have an opinion about the future of the proposal, but I have some recollections about LIMs and trains. In the late 1960s, I was working for an aerospace contractor that had a government contract to study the practicality of LIM-driven trains. Some of my friends worked on the program and even managed to mangle some boxcars in a slow-motion derailment one day, so I got some education in the technology over several weeks of lunchroom conversations.
Naturally, Wikipedia has a good write-up about LIMs. In concept you take a rotating ac-induction motor, like the ones that have been used in ceiling fans for a hundred years or more, saw a slot in it and flatten it out. (Yes, Tesla invented it. There’s a connection with Musk there.)
What you had in the ceiling fan was a cup-shaped metal object on a pivot. The cup fit into a slot in the stator, which was wound with coils driven by ac. The fan blades were attached to the cup.
What you get when you flatten the thing out is this: The cup turns into a long flat vertical rail and the active part of the motor straddles that. When you excite the windings with ac, you get linear forces between the windings and the rail. The idea in 1968 was to hang the powered part of the motor (It would be the stator in a conventional motor) from the railcar, have it straddle the rail, which would have been the “rotor.” And run the thing from electrified rails or a pantograph.
The research for this project got to the point where there was actually a full-scale test track with real boxcars and stuff. Power EEs will guess that there was a real problem with maintaining an air gap, especially when the track wasn’t straight or perfectly level. But my friends got data, and that was the purpose of the program.
In a Different Century
The Hyperloop has some striking differences. The c20 version envisioned adapting LIMs to the existing railroad infrastructure, with load-hauling capabilities similar to those of a conventional GE diesel-electric locomotive. In contrast, the c21 Hyperloop is essentially a railgun with the load capacity of (maybe) a private car, with compressed air suspension.
Yet the propulsion method in a railgun is still a LIM. Since one of the major challenges in any electric motor is minimizing the air gap, getting the car around curves is challenging. (I can’t help but think of the famous Tehachapi Loop.)
Also, I can’t help making a connection between railgun technology and Mr. Musk’s other enterprise: SpaceX. Maybe when the Hyperloop isn’t carrying passengers between LA and San Francisco, it can be launching payloads to Lagrangian points.
So be sure to check your gate number before boarding.