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Maximizing eddy currents- looking for tips

Maximizing eddy currents- looking for tips

(OP)
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

RE: Maximizing eddy currents- looking for tips

If you do not define a goal for your device, most of your questions won't have a definite answer.
If it's just to make a demonstration of a braking effect, why find an optimum set up? (and optimum with respect to which parameter? braking force, speed,...?)
If it's for a usable device (a brake) then the magnet-in-tube is for sure not an optimal, even not a good, configuration.

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

RE: Maximizing eddy currents- looking for tips

(OP)
Thanks for the replies!  I was trying to keep the inquiry general, but I will elaborate on the goal.  We are essentially trying to create a low cost piece of exercise equipment, and although I expect that I am in agreement with the majority of this forum that magnets/eddy currents aren't the right choice for this task given the cost/performance goals, it is part of our statement of work to prove that theory as effectively as possible, so I am obliged to do so.
Further, I readily admit that I've never really had cause to experiment with eddy currents in a similar application, so I'm starting from scratch.  

To Prex's questions:  Optimum to braking force.  I want to optimize as best I can to demonstrate that the minimum required magnets/configuration will be cost prohibitive when compared to more practical solutions.  It is for a useable device, and if magnet-in-tube isn't optimal or good, please suggest what is.
My guess is that shuttle on rail is closer to optimal.  That is to say- a shuttle sliding up/down a beam.

One definition I can clarify is that in every considered version of this, the motion will be linear, not radial as in Foucault's Disk, although it is a useful link, and I'd like to thank dgallup for citing it.

Hope that helps to clarify my goals, and I thank you for any further information!
-kevin

RE: Maximizing eddy currents- looking for tips

dgallup has it right;  High Flux and low resistance.  I once made an eddy current brake using a linear motor magnet rail and a copper bar.  That might be the least expensive way to show the force that can be made [and the cost to do it].

RE: Maximizing eddy currents- looking for tips

Yes, the shuttle on rail is the most effective configuration.
You put on the shuttle a horseshoe shaped yoke made with even a single magnet and soft iron, then you let an aluminum flat fastened along the rail pass in the yoke gap.
Dimensions, one or more magnets, length of rail, mass of shuttle, etc., it all depends on your budget and whether you want a small gadget or something to show in a conference room.
To adjust the braking force you let the alu flat slide out of the yoke gap.

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

RE: Maximizing eddy currents- looking for tips

The force you get is going to be velocity dependent so probably not a good choice for exercise equipment.  

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.
 

RE: Maximizing eddy currents- looking for tips

(OP)
I received my magnet(s) today- and built a very low friction shuttle from slick polyethylene about 4" long.  I arranged the horseshoe magnet as described and mounted the shuttle to a .25" thick 1" wide aluminum bar.  
This is one of the sample magnets I used:
http://www.mcmaster.com/#5841k22
From the package insert- the pole arrangement has N & S poles oriented directly out the bottom of each horseshoe leg, and so curving to form a crescent.  I presume this is standard.

The space between the legs is .25" at the tips- the legs curve slightly, so I only had to dremel a tiny flat on each leg for clearance.

So far, the resistance is very small- I will be experimenting more tomorrow when the rest of my materials arrive, but any further comments/suggestions would be appreciated.  I realize I will likely require an unreasonable amount of velocity to achieve significant resistance.

As always, thanks for any help!
-k

RE: Maximizing eddy currents- looking for tips

We cannot see your magnets on McMaster, a password is required.
Please show us a quoted sketch of your assembly.

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

RE: Maximizing eddy currents- looking for tips

(OP)
Actually I'm pretty sure that link will work just fine.  The part # is there at the end of the URL, and McMaster's site will highlight any part # in the catalog when added to the end of the URL like that.  It's not in my cart or anything.

I'll send a pic tomorrow AM of the test rig(s) I have set up though.  Its pretty basic at present.
Thanks!
-k

RE: Maximizing eddy currents- looking for tips

I was able to see that link from another PC.
You won't go far with Alnico magnets and that type of horseshoe is very small (though this depends on the weight of your shuttle).
Also the orientation of the poles is not good for inserting the alu bar between the poles. You should try by putting the alu bar in front of the poles (the magnet being sideways with respect to the movement), then installing a piece of iron on the other side of the alu (fastened to the shuttle), leaving a minimum gap to let the alu pass in.
This setup will give much more braking force, but I doubt it will be sufficient for your goal.

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

RE: Maximizing eddy currents- looking for tips

(OP)
Ok, here are two test rigs.  I had originally started the green plastic design before arriving here.  Obviously, it is based on the magnet-in-tube design, and allows for adjusted spacing and pole orientation along the threaded rod.  We used cheap ring magnets we had on hand which are fairly strong for the price (we were/are still trying for maximum bang for the buck) and they move inside a 1" ID aluminum tube.
I'm not really pursuing any of that anymore but including it anyway.

The Red plastic design illustrates the captured aluminum bar and horseshoe magnet(s).  I'll try the arrangement you described next.

I know these are going to be weak and inadequate results- hopefully I've collected enough visual proof at this point to satisfy our obligation to nix this magnetic brake idea.

Thanks for the help!

RE: Maximizing eddy currents- looking for tips

Both of your configurations do not appear to be anywhere close to optimum for getting the flux into the metal.  In the case of the horseshoes, the peak flux is on the face of the show, which you have pointing at the other horseshoe.  The only flux that's getting into the slide is some fringing fields.

Ditto the ring magnets; peak flux comes from the flat faces of the rings, which you have pointing at other magnets.

If you have some old hard drives, you might contemplating cannibalizing a couple for their magnets.  They're configured nearly ideally for your flat rail application, since they have both N-S poles on both faces, and they have back-iron on the opposite face.  And, they are very strong for their size.  For your tube application you'd need to arrange flat magnets in a ring, where the flat pole faces are facing the tube wall.

TTFN

FAQ731-376: Eng-Tips.com Forum Policies
Chinese prisoner wins Nobel Peace Prize

RE: Maximizing eddy currents- looking for tips

(OP)
IRstuff- thanks!  The tube was supposed to be just a quick mockup before I even really started taking the idea very seriously- it was based on the simple magnet drop experiment, but I have a much better understanding of this effect now that I've invested some research.  I understand now that there is no good reason to pursue the tube design, but it was here at my desk so I thought I'd include the photo.

The horseshoe is closer to a "realistic" design, but again, I was just trying a few random arrangements to see if/how they effect the result.  As you noted- the magnets shouldn't really be facing, but I was considering that arrangement for the ability to adjust resistance (in this case cancellation).  

I like your hard drive idea, and I do have a collection available for parts.  I'll let you know how that goes!
Thanks!
-k

RE: Maximizing eddy currents- looking for tips

IRstuff:

I never realized that. Within arms reach I have 7 HDD magnets stuck to my light fixture from dead drives through the ages. Sure as snot, none will stick to the fixture by the mumetal side.

On a side note, they keep getting smaller and smaller as technology advances.

-AK2DM

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
"It's the questions that drive us"
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

 

RE: Maximizing eddy currents- looking for tips

IRStuff: You are correct.  With the backiron/mumetal, the magnets become stronger on the active face.  The assembly becomes a magnetic circuit, making it easier for the flux on the back faces to travel from north to south pole.

RE: Maximizing eddy currents- looking for tips

(OP)
Hi guys,
Just checking in and wanted to thank you again as I'm finally getting some results.  I do have a quick question about my current setup.

I have a set of strong bar magnets.  They are .25" wide x .1" thick x 2" long.  I have them set into either side of a 5" length of aluminum C-channel.  This makes up the "shuttle" component (along with some friction reducing details).  

This shuttle rides along another aluminum rail, and I can feel faint eddy currents acting on the shuttle when rapidly moved on the rail.

My question is in regards to both magnet orientations and possibly implementing an iron insert.  

Currently magnets are attracting each other.  I can mount them so they are repelling, or replace one with a comparable iron bar.

Any comments on the best arrangement?

Some colleagues have volunteered some broken/obsolete hard drives to dissect tomorrow, but I wanted to post this question for feedback as well.
Thanks!
-k

RE: Maximizing eddy currents- looking for tips

MagMike:  Thanks for the feedback.  My thought/question was whether there was any noticeable difference between straight iron or steel back-iron vs. mumetal.  My thought was that since normal steel allows a considerable amount of flux to "leak" out of the far side, but a mumetal backing would constrain the flux more that there would a slight increase in frontside flux.

salukikev: As for the slide, I thought someone had suggested essentially using copper, since it has higher conductivity.  You might try getting some copper bus bars over the aluminum slide and see if there's an increase in braking.  Supposedly, the regenerating brakes on the Prius do a pretty good job of saving wear and tear on the actual mechanical brakes.

TTFN

FAQ731-376: Eng-Tips.com Forum Policies
Chinese prisoner wins Nobel Peace Prize

RE: Maximizing eddy currents- looking for tips

You  still have quite small magnets for a conductor rail 1/4" thick. You could try with half that thickness so as to reduce the gap between magnet poles. Check also that your alu rail is with pure (99.5% or 1050) aluminum, not an alloy, and of course copper would be better, but it is costly.
If those magnets are magnetized along the .1" thickness, you can pile them putting one onto the other so that they attract. Then take a mild steel flat 2"x2"x1/4" and bend it to form a U with a space of about 3/4" between the legs. Then put 2 piled magnets inside one leg and 2 more on the facing leg: this is your horseshoe. The alu rail will pass in the gap of about 1/4" left between the two magnets.

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

RE: Maximizing eddy currents- looking for tips

(OP)
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.

RE: Maximizing eddy currents- looking for tips

(OP)
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

RE: Maximizing eddy currents- looking for tips

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

RE: Maximizing eddy currents- looking for tips

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?

----------------------------------------

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.
 

RE: Maximizing eddy currents- looking for tips

(OP)
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
 

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