# Linear Induction Motors detailed question

Hey all,

I’m currently a student in Mechatronics. I’ve always had a big interest in the mechanical components of roller coasters and have a pretty wide breadth of knowledge on them, but came up with something that I’m uncertain of and looking for any documentation or general knowledge on this.

As we know, LIMs use to magnetomotive force to drive the train forward by first attracting, and then switching polarity, repelling the train down the track. Does this polarity switch happen for each car that passes over them (presumably with their own prox sensor), or does this happen on a larger scale over the length of the entire train?

LIM's and LSM's are essentially rotary electric motors unrolled. Electromagnets attract and repel magnets (LSM) or conductor plates that generate a magnetic field (LIM) with a wave of opposing north/south magnetic poles moving down the track. There are a ton of videos on the YouTube that illustrate this.

Jeff - Editor - CoasterBuzz.com - My Blog - Phrazy

Precisely. You didn’t read my question specifically: does the polarity switch for each individual car passing, or for the train as a whole?

I did read your question, I just don't think it's the right question. I mean, these are employed on water rides with metal plates in the boats. The polarity in an AC motor changes by definition of being AC.

Jeff - Editor - CoasterBuzz.com - My Blog - Phrazy

Well that seems like an incredibly silly statement. Wouldn’t it be obvious that Linear Induction motors receive power via a rectifier circuit, and given instructions to fire from the PLC?

It would be foolish to assume that the polarity in the LIM motors switch at the same rate as the AC current coming to them. It has to be much more controlled and precise in order to fire in a sequential manner to bring the train up to speed.

So, asking.... Do the LIMs fire and change polarity as each individual car passes them (creating smaller, more rapid waveforms), or a larger waveform at the midpoint of the train?

Last edited by cmwein,

If you're going to make it silly and foolish, maybe this isn't the best place to ask a question.

The coils in the motors change polarity at whatever frequency the controller dictates. The magnetic coils are wound alternating in a single motor, which are inches apart, not the length of a car or a train (or ProSlide boat).

Jeff - Editor - CoasterBuzz.com - My Blog - Phrazy

And with LIM's in particular (which you seem to be confusing with LSM's) there aren't any stationary magnets on the train. The fins will have the induced opposite magnetic pole wherever the primary coils are creating a change in magnetic field. There's no real need to synchronize anything.

Hobbes: "What's the point of attaching a number to everything you do?"
Calvin: "If your numbers go up, it means you're having more fun."

Too much science! My head hurts...

Life is something that happens when you can't get to sleep.
--Fran Lebowitz

LSM (Linear Synchronous Motor) are the ones that have to flip the polarity. Each car will have a magnet (maybe two) under it. As each magnet passes over each stator, the polarity must switch at precisely the exact right moment, otherwise efficiency will suffer. VFDs (Variable Frequency Drives) can operate rotary motors without any prox sensors, so I would venture a guess that the linear motors don't either.

LIM (Linear Induction Motors) operate on a different principle. The stator has a three phase winding in it*. With the three phases in the stator, it creates the same effect as if permanent magnets were being dragged across the fin on the coaster car. Conductors will oppose the magnetic field that flow through them (flux). They do this by generating electricity. In this case, its eddy currents, which can generate very strong magnetic field. The induced magnetic field in the conducting fin will be attracted to / repealed by the magnetic field in the stator, creating a forward force.

I'm sure I missed some detail there, but that what I can type up in a few minutes off the top of my head.

*The LSM probably uses three phases as well, but its less critical than in an LIM.

The usual explanation for a LIM is that the reaction plate rides on a 'magnetic wave'. Remember the reaction plate is itself not magnetic; it is a conductor, most often aluminum or a copper-cobalt alloy, which is NOT magnetic. The current flowing through the windings of the stator generates a magnetic field that moves along the surface of the stator and induces a magnetic field in the reaction plate. The interesting thing about this is that because the magnetic field in the reaction plate is induced by the moving field in the stator, the reaction plate field will *always* be synchronized with and lagging slightly behind the field in the stator. In fact, the relative motion of the field and the reaction plate is critical: if the reaction plate were moving synchronously with the field, there would be no movement of the field around the reaction plate, therefore no current would be induced in the reaction plate, therefore there would be no responsive magnetic field!

In amusement rides, we commonly see two basic configurations of linear induction motor: single sided and double sided. In a single-sided LIM, the reaction plate is usually backed with a ferrous material which serves to amplify the stator field. You'll find these in applications where the motor has to lie flat, such as on Disney's Peoplemover, or on the ProSlide water coasters. I think this is the configuration Premier used on the Mummy coasters as well. In a double-sided LIM, as we see on the most of the Premier launched coasters, the aluminum fin sits between two stator coil packs. Finally, Intamin used an interesting configuration on the Impulse coasters where there are three fins on the train. The center fin, which may actually be a copper alloy, is flanked by two coil packs. But the coil packs are wound to have two outward-facing sides, so the two outer, aluminum fins are driven by the outboard sides of the stators, and fixed backing plates mounted opposite the coils substitute for a steel backing plate on the fin. That way the outer fins can also be used with permanent magnets for eddy current braking. For propulsion, the whole arrangement behaves as a double-sided LIM flanked by two single-sided LIMs, using only two coil packs.

In case you missed it, I wrote an article about this stuff for First Drop #53/54 about ten years ago; you can read it on my web site at http://www.davealthoff.com/tech/lim.html . That will get you started; then you can stop over at https://pointbuzz.com/content/ask-rideman-magnetic-braking to see how it all works in reverse.

--Dave Althoff, Jr.

/X\        _      *** Respect rides. They do not respect you. ***
/XXX\ /X\ /X\_ _ /X\__ _ _ _____
/XXXXX\ /XXX\ /XXXX\_ /X\ /XXXXX\ /X\ /X\ /XXXXX
_/XXXXXXX\__/XXXXX\/XXXXXXXX\_/XXX\_/XXXXXXX\__/XXX\_/XXX\_/\_/XXXXXX

I suddenly remembered this video...

Jeff - Editor - CoasterBuzz.com - My Blog - Phrazy

I was hoping the video would be this electrical wizard but explaining LIMs...