# LIM Question?

Wednesday, January 8, 2003 2:35 PM

Im building a model of Mr Freeze and its going really well so far. Well I need some answers to my questions really quick. I am purchasing bar magnets (8) and will place 4 on each side of the track. I will superglue or drill a wooden block into the back of the magnets leaving a 1/2 or 1/4 inch airgap. There will be 2 sets of magnets on each side. I am going to place wires onto the magnets leading to a curcuit breaker leading to a plug. I need suggestions on how to apply power to the magnets? For the trains that will run on the track will have aluminum foil attached to it. Could you please leave any suggestions.

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Wednesday, January 8, 2003 4:14 PM
I think you don't understand how an electromagnet works.

If you want electromagnets, you need to wind a coil of wire around a metal core and apply current to the coil. One problem is that doing that will produce a magnet with a pole at each end and not much of a field in the middle, as the field will be at the center of the coil rather than around the edge where you want it.

Before you electrocute yourself, get to a library and do a little research on electromagnets.

A safety tip from...
--Dave Althoff, Jr.

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Thursday, January 9, 2003 11:39 AM
Besides aluminum isnt attracted to regular bar magnets. You need some kind of Iron core to it.

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RACE FOR THE SKY!!
TOP THRILL DRAGSTER

*** This post was edited by Ultimate Coaster on 1/9/2003. ***

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Thursday, January 9, 2003 7:40 PM
Ultimate Coaster...
The reason aluminum or copper are generally used for LIM reaction fins is that it isn't attracted to the magnets. You don't want the reaction fin to stick to the magnet, so you make it out of something non-ferrous. But you want something conductive because you want to induce (remember the I in LIM stands for "Induction") a current in that reaction plate. The idea is that the fin itself won't interact with the magnet, but the induced magnetic field in the fin will react to the magnet.

--Dave Althoff, Jr.

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Thursday, January 9, 2003 10:05 PM
RideMan lays the electromagnetic smackdown yet again.

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James Draeger
-Captain Sarcasm

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Friday, January 10, 2003 8:55 AM

This is a bit off topic, but:

Do the magnetic brakes work the same way: Induce a current in the brake fins which react with the magnets which slow the train down?

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Be polite and ignore the idiots. - rollergator
You must be this dumb to ride Viper. -SFGAdv.

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Friday, January 10, 2003 9:16 AM
Yes, except magnetic brakes don't use electromagnets. The motion of the conductive fin through fixed magnets creates the induced currents and mangetic fields.
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Friday, January 10, 2003 9:39 AM
I am making an LIM at my desk today out of paper clips and white-out. It doesn't seem to be working well. It's still M3G@-1337 though. r0x0r.

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A day is a drop of water in the ocean of eternity. A week is seven drops.

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Friday, January 10, 2003 11:11 AM

RideMan said:
Ultimate Coaster...
The reason aluminum or copper are generally used for LIM reaction fins is that it isn't attracted to the magnets. You don't want the reaction fin to stick to the magnet, so you make it out of something non-ferrous. But you want something conductive because you want to induce (remember the I in LIM stands for "Induction") a current in that reaction plate. The idea is that the fin itself won't interact with the magnet, but the induced magnetic field in the fin will react to the magnet.

--Dave Althoff, Jr.

Good point. But help me with this: So basically the electricity comes off the magnet and pulls it rather than the magnet itself? And also, do the LIM's pull the train and then when it gets too close the LIM shuts off to avoid pulling the train backwards and slowing it down or is it that the LIM produces energy in one direction so it pulls the train towards it then pushes it away (I dont think thats possible, but I dont exactly know everything about LIM's anyways). And finally, would it be possible to create a magnetic fin that would be maybe positively charged, then have the LIM be negatively charged on one side so that it would pull the train forward and then have the other side be positively charged so that the LIM would push the train away (push it forward, away from the LIM)? (Like if you hold the N pole of a magnet to the S pole it attracts. The coaster could be the N pole, and the 1st half of the LIM the S pole, and then the other end of the LIM the N pole so that it both pushes and pulls the train to allow for double the power).

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RACE FOR THE SKY!!
TOP THRILL DRAGSTER

*** This post was edited by Ultimate Coaster on 1/10/2003. ***

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Friday, January 10, 2003 12:01 PM

Jim Fisher said:
Yes, except magnetic brakes don't use electromagnets. The motion of the conductive fin through fixed magnets creates the induced currents and mangetic fields.

Okay. Let's try to get a little more detail...

The fin goes through a constant magnetic field so there are little eddies of current induced in the fin, right? These in turn create their own magnetic fields which align with the field created by the static magnet. How does this slow down the train? Does the electrical resistance in fin dissipate the energy from the electrical current?

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Be polite and ignore the idiots. - rollergator
You must be this dumb to ride Viper. -SFGAdv.

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Friday, January 10, 2003 12:02 PM
Hmmm...How to explain this...

The first basic thing to remember is that if you pass a conductor through a magnetic field, you induce a current in that conductor. That is, a current moves through the conductor. For this to happen, the magnetic field and the conductor have to be moving in relation to one another. In the case of a LIM, initially, at least, it is the field that is moving...this is done by switching coil phases. Ideally this is done using 3-phase windings because that way you can easily control the direction of field movement. It is a bit like the chaser lights you see decorating roller coaster tracks.

When a current moves through a conductor, it generates a magnetic field around that conductor. That field will interact with the moving field that generated it in the first place, in fact it will tend to lag behind that field just slightly. It's not a real efficient process, but it works. Imagine there is a beach ball in the middle of a pool. One way you can get that ball without jumping into the water is to start pulling water towards you. By doing that you set up a current which will slowly drag the beach ball in your direction. It's a bit like that.

The nice thing about this is that all you really need is this moving electromagnetic wave, and the reaction plate will 'surf' along the motor. You don't have to worry about polarities because the polarity is constantly changing along the LIM and along the reaction plate. The bad thing about it is that there are more efficient ways to do the same thing.

It is the LSM, the linear synchronous motor, that uses permanent magnets and does the synchronous, precisely timed push-pull thing. It's more efficient than a LIM, but it also is harder to make it work because the timing is critical.

Jim Fisher talked briefly about magnetic brakes; I think it is worth noting that on Cedar Point's Wicked Twister, there are three fins on the train...as nearly as I can tell, two outer aluminum fins and a center copper fin. Where there are motors, there are two boxes with the windings in them, one on each side of the copper fin, and two steel plates, outboard of the two outer fins. That way the center fin is the reaction plate for a double-sided LIM while the outside fins are the reaction plates for a pair of single-sided LIMs to the outboard. Those same outer fins are also used as the magnetic braking fins once the magnetic braking calipers drop into place.

--Dave Althoff, Jr.

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Friday, January 10, 2003 12:03 PM

Ultimate Coaster said:
And finally, would it be possible to create a magnetic fin that would be maybe positively charged, then have the LIM be negatively charged on one side so that it would pull the train forward and then have the other side be positively charged so that the LIM would push the train away (push it forward, away from the LIM)? (Like if you hold the N pole of a magnet to the S pole it attracts. The coaster could be the N pole, and the 1st half of the LIM the S pole, and then the other end of the LIM the N pole so that it both pushes and pulls the train to allow for double the power).

This was discussed in a previous thread. In short no. Magnetic monopoles don't exist. Also, by conservation of energy, this can't work. If you want a more elaborate explanation I can give it, but there is always search.

-----------------
Be polite and ignore the idiots. - rollergator
You must be this dumb to ride Viper. -SFGAdv.

*** This post was edited by ApolloAndy on 1/10/2003. ***

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Friday, January 10, 2003 2:35 PM

Im totally lost. I thought I knew alot about LIM motors but I guess I dont. So to make a working LIM creating you dont actually use magnets?sry if this sounds crazy but 1st off what is a coil? you need to wind a coil of wire around a metal core and apply current to the coil. How would you do that?

Could you list the materials that i would need to create one and the steps in a more lamen term lol. Thanx alot Rideman

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Friday, January 10, 2003 3:47 PM

Its not really in Laymen terms, you kinda need to be an enginner to figure it out, like what the different wording means and such.

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So you believe that you are studying us, then kindly explain why you are the ones trapped in your seats.

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Saturday, January 11, 2003 6:38 AM
Hey Rideman, would what i am doing work for my science project. I took a model train track, and put a piece of aluminum siding down in the middle of it, about a centimeter wide. Now, i am gong to plug in the track, and have a current run through the steel rails on it. Now i am going to have a little car with steel wheels, that are attatched to a coil. Will it run?, what i am going to test it which wire around the coil runs the the best speed, or how many coils make the best speed.
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Saturday, January 11, 2003 8:40 AM
Thanks Rideman and ApolloAndy, those explanations helped alot!

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RACE FOR THE SKY!!
TOP THRILL DRAGSTER

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Tuesday, January 14, 2003 3:07 PM
Im in soo much trouble, Sci fair is in 1 week and im still lost. Ok so do i wrap a magnet around copper wiring or do i need to use a magnet? Is there any way to hook up my LIM or electromagnet to an electrical outlet instead of a battery because I want more power lol, i might post my pics of my model Mr Freeze later
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Wednesday, January 15, 2003 8:30 AM

STAY AWAY FROM THE WALL OUTLET!
DON'T MESS WITH ANYTHING OVER 12 VOLTS UNTIL YOU HAVE A BETTER UNDERSTANDING OF ELECTRICITY!

I would suggest that you look for a fall back option. I'm a mechanical engineer with 30 years of industrial experience. I work with rotating induction motors on a regular basis. I've built many rotating DC permag motors virtually from scratch.

With this experience, I would allow myself at least a month to put together a working model LIM.

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Wednesday, January 15, 2003 8:39 AM

Den said:
"I am making an LIM at my desk today out of paper clips and white-out. It doesn't seem to be working well. It's still M3G@-1337 though. r0x0r.
"

haha.....my laugh of the day.

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Jus' gimme de light!

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Wednesday, January 15, 2003 5:07 PM

Unless your project is specifically about LIMs, i'd say cheat a little. If otherwise you've got a working coaster model, it'll be a showstealer without a true LIM launch anyway. Make a little bit about how the LIM launch works, include the price of a commercial coaster launching lim set and claim cost is why you're using a rubber band to launch the model. No one is going to be offended that a kid isn't dropping a cool million dollars on their science fair project. Not that it'd actually cost that much to do, but who knows that?

If your project is about the LIMs and you're this lost at this stage in the game, i'd ask your teacher if you can change topics, cause you're dead. I build a brushed LSM model that worked _most_ of the time moving one very minimal cart consisting of a magnet, a motor brush and some balsa. If you're looking to launch into a real model, it'd take me at least a week. a real LIM would take longer.

Use a rubber band.

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