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

 

[prex]

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

 

[dgallup]

You are going to want the highest flux density magnets and the lowest resistance conductor to maximize force. See Foucaults Disk for another version:

http://physics.kenyon.edu/EarlyApparatus/Electricity/Foucaults_Disk/Foucaults_Disk.html

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]

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

 

[sreid]

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].

 

[prex]

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

 

[dgallup]

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.

 

[salukikev]

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

 

[prex]

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

 

[salukikev]

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

 

[prex]

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

 

[salukikev]

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!

 

[IRstuff]

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
Chinese prisoner wins Nobel Peace Prize

 

[salukikev]

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

 

[IRstuff]

One of the zehr cool things about those magnets is that the backing iron is mumetal, i.e. magnetic field shielding; I'm tempted to think that this would make the exposed magnet face fluw stronger.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[analogkid2digitalman]

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"
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

 

[MagMike]

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.

 

[salukikev]

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

 

[IRstuff]

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
Chinese prisoner wins Nobel Peace Prize

 

[prex]

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