Maximizing eddy currents- looking for tips

[salukikev]

Hello,
I'm looking to create resistance using magnets and eddy currents. I've never attempted this before except during the usual experiment of observing a neodymium magnet's fall slowed & contained by a copper tube.

I'm required to mock up this experiment as part of our statement of work for this project.

I'd like to know in general terms what factors are effecting the resistance of the system, and more specifically how to maximize (and/or adjust)the effect.

In this way, I could minimize costs by using the smallest & cheapest magnets/materials to perform my task.

Some more specific questions I have include:
1. Pole(s): Would it be best to stick to a simple 2 pole magnet for this purpose, or would multiple poles be beneficial? Should multiple magnets be arranged in a simple N-S-N-S-N-S type configuration?

2. Magnet Spacing: What is an optimal spacing (if any) between magnets to maximize resistance? (eddy currents)

3. Tube spacing: To what degree does clearance between magnets and the ID of the tube effect resistance? What would be an appropriate clearance to maximize effect but allow otherwise free travel?

4. What materials (tube & magnet type) is best suited for a cost/effectiveness balance?

5. Should magnet size (volume & accordingly, budget) be more devoted to larger diameter or thicker disc?

6. What is the best way to reliably adjust the resistance of the system?

Thanks very much for any help or suggestions to finding the answers to one or more of the above questions!
-Kevin

 

[salukikev]

Well, I forgot to mention that I'm now using a T-shaped conductor rail that is 1/8" thick. I also set the magnets into the C-channel via slots and they are held back by a thin section of polypropylene inserted into the C-channel.

I thought those magnets were a good representation of something practical, but I may go ahead and scale them up a bit just for the sake of demonstration. They are the 6.1lb maximum pull magnets listed here:

http://www.mcmaster.com/#5848K73

I'll try some of the other tips you mentioned- I haven't got to take apart any hard drives yet today, but also wanted to ask about thickness- do you think I should thin that rail down more? I may also try using a copper rail if you think it would make a noticeable difference.

 

[salukikev]

Here are a few pics of two rigs I made up. The one you've seen before but now has an adjustable iron bracket.

The other is the C-channel I described with cut outs for the magnet(s) and/or steel. This rides on a 1/8" aluminum rail.

 

[IRstuff]

I should mention that eddy current brakes are generally not particularly effective at low speeds, since induced current is proportial to the change in flux.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[salukikev]

Hi!
Just wanted to update you guys with recent results. I'm pretty happy with the cannibalized hard drive solution we came up with- thanks to IRstuff for that!

Not only do we get shielded high-strength magnets with appropriate pole arrangement, but we get a steel bracket available to adjust spacing with high precision and rigidity.

Here are some new questions regarding that setup:
1. All of these magnetic assemblies are arc-shaped magnets as they are made to act on the arm of a hard drive and the coil positioned between them. Where might I find a similar design/dimension/specification magnet in a non-radial form factor? (rectangular). The mumetal feature seems to be helping as well, so I'd like that same arrangement on a rectangular design if possible.

2. We have some 1/8 thick 3/4" wide copper bar arriving today, we will be trying that instead of aluminum. I'm figuring about .01" clearance per side for our shuttle. I wonder how much this figures into our resistance. I also wonder if adding features to the rail (holes/slots/zigzag perforations) would positively affect our resistance by causing more magnetic field "ripples" in the eddy currents. Any feedback on that theory would be appreciated! As a free bonus- adding some features like that would help the rail to lose some of the generated heat.

3. I know that the substantial copper content is going to drive the price up for a product like this. I wonder if we had a copper coating on aluminum or something similar if we could get a benefit and reduce cost. I'm still also trying to narrow down the optimal thickness. I may take it to .0935" I think that is about the minimum that we could tolerate structurally.

Anyway, thanks so much for the help! Some of these recent experiments have surprised me, I'm starting to believe this
magnetic brake approach may actually wind up viable after all. In any case, I'm glad to have something that demonstrates the effect.
-k

 

[prex]

2)Any cut or perforation in the conductive rail will make smaller the braking effect. Don't do that.
3)Coating aluminum with copper is no use. The optimal thickness of the conductor rail for such small magnets is probably close to the minimum structurally acceptable.

prex

http://www.xcalcs.com

: Online engineering calculations

http://www.megamag.it

: Magnetic brakes and launchers for fun rides

http://www.levitans.com

: Air bearing pads

 

[IRstuff]

To add to Prex' comments, the objective is to have as MUCH current as possible, since it's the current that's sucking away from the kinetic energy. Slots, etc., increase resistance, hence, they are not desirable.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[dgallup]

I still don't see how having a velocity dependent resistance is acceptable for exercise equipment. Most machines try to create a constant load regardless of velocity. What sort of machine is this?

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[salukikev]

Hi dgallup- sorry for the delayed response!
This is an accessory for aerobic exercise, it won't require much physical resistance- which is really the only reason this magnetic option is still even being considered.

I've enjoyed learning about this application, and I really appreciate the help I've received on the forum. I continue to study up on the specific dynamics of eddy currents and how we can improve this. Currently, I am trying to work out ways that I can conveniently vary the resistance. Of course this means either moving the magnets apart in some manner or moving the cradle assembly away from my rail. The latter has so far turned out to be the most practical approach in terms of effectiveness, with the main reason being that changing the position of one magnet relative to the other requires a great deal of force. Further, that required force is on a dramatic gradient whereas I would like for it to be a consistent feel.

The downside of the "whole cradle" approach is that I'm trying to fit this whole system within a handle component. If I'm moving the whole cradle out of the way of my rail, it means my handle has to be 2x wider in that direction to accommodate both the rail and the magnetic cradle- this results in a pretty huge handle component.

I wonder if I could do something with arrangement of the magnetic poles to improve my magnet separation without reducing my resistance. I do have space to slide one magnet down relative to the other. I can do this with no increase in handle size, but again the force required to actually pull one magnet out of the field is pretty inconvenient.

Thanks for any ideas, and thanks also for any suggestions as to a magnetic engineering firms that I might contact to discuss this in more detail. I've been calling around trying to find a good applications engineer to speak with, but haven't really succeeded yet.
-kevin