Magnet strength

[Greystoke]

Hi, I'm a newby from Port Elizabeth, fiddling around with some designs. Still not sold on using magnets. Rather use electro-magnets.

Can anyone tell me what the field strength of the currently used magnets should be, so that I can make realistic (electrical) copies.

[windgat]

Hi Tarzan, welcome! The rare earths are amazingly strong, sorry I give you any figures. I know the field is strong enough to penetrate 5mm mild steel.

I have some 50x25x12.5mm grade N35 (24 of them!) and if you can suggest some sort of test (lifting power?) I can do to determine field strength I would be happy (and interested) to do that.

[Greystoke]

OK, thanks
I'll think about the test. That would be interesting.

Regards
Cor

[windgat]

Good, I look forward to trying it!

[Greystoke]

Hummm,
Studied this issue over the last few days, and . . . no way I can do this in any simple way. Not even to get an approximate estimate. Don't have the equipment.
Perhaps I need to do the old thing . . .trial and error.
I stil don't want magnets fying around.

[windgat]

Surely we can come up with something cunning!? Lets hear some ideas please!

How about putting two magnets each side of two 8mm aluminium plates. That is, a magnet on the bottom, one ali plate, then the second ali plate, and on the top another magnet. Then suspending the top plate from its four corners, and suspending weight from the bottom plate (or pulling it down with a spring scale) until the magnets separate?

Or maybe setting something up so that a torque wrench can be used to measure the required separation force?

[Greystoke]

I think I

[windgat]

Interesting. Calibration seems to be a tricky issue in that setup... Also, how many others could duplicate your system accurately?

Everyone(?) has access to a scale, so how about the following setup, where two non magnetic plates, say 10mm plywood or perhaps better aluminium, separate the magnets. The four bolts stop the lower plate falling too far. To use it, add weight to a hook below the lower plate until the plates separate (and the lower one is held by the bolts. Then weigh the lower plate and added weight, to get a 'holding weight at 20mm' calibrated measure. How does that sound?

[Greystoke]

I can do this test. I have a little lab-jack with which I can move the lower magnet up and down with precision. However . . . how do you translate the attraction distance into a flux density? Remember that I need no of windings and DC current to make a coil wound magnet of similar properties.

... Also, how many others could duplicate your system accurately?

I need to build a test set first and do some measurements. I rather like the light alu rotor and the cheap transformer, although building this thing may be a bit more difficult.

[Greystoke]

BTW: You make very neat drawings. How do you do that?

[Greystoke]

Another question:
How are these magnets polarised? North to South lengthwise?

[windgat]

Hi Greystoke. The idea was to just add weight to the lower plate until the plates separate. Hmmm - I suppose you could use a jack too, attached to a large spring (fish) scale, which is attached to the lower plate. However, I'm hoping to do something that some guy in the middle of rural Africa could easily duplicate, which is why I'm trying so hard to make it need minimal technology.

Drawing is done with a full 3D design package, so I could use it to produce precise drawings for others to copy once we agree on a method.

The magnets of this shape are usually polarised towards the large faces, not lengthwise. For wind generators you want the flux coming off the rotor plates at a right angle (normal to the plane of rotation). So in my drawing they would be attracting (both magnets the same way, either both North up, or both South up).

Converting to flux density... not sure. Perhaps once you have done the weight test, you could use magnetism formulas to work out the flux density required to to produce a force equal in magnitude to F=ma for the weight of the lower plate at separation?

[Greystoke]

. . . . However, I'm hoping to do something that some guy in the middle of rural Africa could easily duplicate, which is why I'm trying so hard to make it need minimal technology.

I understand your objective.
On the other hand . . the correct way to measure magnetic flux is to move the magnet through a coil and measure the induced electric charge in the coil. I'm still looking at a simple way of doing that. The force of attraction is a bit more difficult to convert. But . . . I'll give it a try (back to the books )

[windgat]

Correct it may be... I guess precise measurements of everything are then needed? Purity of the copper, average distance from magnet (surface? centre?) to coil, speed of rotation, influence of iron left on transformer, temperature... I don't know too much about this all, but intuitively it seems like the cut away transformer approach has lots of complex sub-problems to be solved before you arrive at a figure for flux.

I have some 8mm aluminium plate offcuts, so maybe I'll look at making something out of that along the lines of the drawing I posted.

[Greystoke]

No, no . .
Nothing as wholesale as that. All I want to find out is what size coil must I make to make something comparable to your magnets. And there's nothing wrong with this cut-away transformer . . . if I know the number of windings and the permeability of the metal, I can work out the flux exactly.

Look,
I'm interested in constructing a possible alternative to your (very impressive) design, for my purpose only, and - of course - I will share the experience. I was only asking for some info.
I'm interested in this concept, and I think its worth trying.

[windgat]

Aha I see. I was relating it to the question I sometimes get 'How many coil windings do I need to get a certain voltage?' When I try to think how to work that out I get lost in the things that need to be measured accurately before a calculation can be done. Sounds like you have some working knowledge of magnetism formulas - look forward to seeing how you work it all out!

[Greystoke]

Well, I'm a bit rusty, but I still have all my old notes and books, so . . yes I can (should be able to . . ) work it all out.

[windgat]

Look forward to seeing/learning...

Here is the result of the weight separation test. I put a bunch of bolts into the lower container, and topped up with water until the wooden plates separated. The plates are made from 10mm marine ply. The four bolts are stainless steel (non magnetic), and the 'strings' fishing nylon. I leveled it off and locked the nylon in place with a dab of epoxy before starting the test. I used two 2"x1"x0.5" magnets, nickel coated, grade N35.

The total weight at separation was 2873g (bottom platform 63g, lower magnet 121g, added weight 2689g).

Magnet strength test

Magnet strength test

It would be great to hear from others out there what their magnets rate at - it would mean perhaps we can verify the strengths our suppliers claim. I can supply detailed drawings for making the identical test jig if anyone is interested.

[Greystoke]

Amazing!
A simple test. Well done.

Now to work out what that means for my benefit. I'm gonna need some time for that.

[Greystoke]

I knew this wasn't going to be easy.
The formula to work out the force between two coil wound magnets only applies to magnetic dipoles that are far appart in relation to the size of the coils. As the dipoles get closer, the formula gets pretty warped.
Anyway . . . I still used the formula to give me an idea of how many windings carrying 1 amp

[windgat]

Wow. Thats a lot of copper! BTW I have added technical drawings of the test jig I used to the 'Drawings' page of the website.

Lowering the top magnet... ok, that would need something to keep the top magnet perfectly level. Also something to stop them smacking into each other (which tends to shatter or at least chip the magnets).

But it seems I don't understand how that setup improves things. You would end up with a distance and a weight (i.e. force), by varying the distance and keeping the force constant (weight of lower magnet). The jig I made varies the force (lower weight) while keeping distance constant (at 20mm) - so don't you also end up with a force and a distance? I don't understand why the formulas would be simpler in one case vs the other. Whats more, the relation of force to distance is exponential (or something like it - not linear is what I really mean) so in the 'lowering scenario' there will be a very narrow super-sensitive range over which the distance measurements would be needed - physically difficult to do accurately. What do you think?

[Greystoke]

The force to distance relationship is cubed (to the power of "3") but only at distances large in relation of the size of the dipole. Measuring at a close distance gives you answers that do not comply with general engineering formula. In other words: You get the wrong answer. At close distances only large computers can work out the sums.
So . . I need an answer at a larger distance. (And I garantee you that the answer will be different.)
This is important windgat. The answer will give you a more realistic (and accurate) idea of the strength of the magnetic dipole.

As for your concerns . . . perhaps you could put the top magnet on a plank, and similarly put a screen over the bottom magnet to prevent it from jumping around.

[windgat]

Ok, I see. How far apart do they need to be to make the formulas work well? In my current rig, the distance between the edges of one magnet N to S is 12.5mm, and the distance between the nearest faces of the two magnets 20mm. I could easily pad the space between the wooden plates to give more distance...

[Greystoke]

Distance to size should (idealy) be greater than 10:1, or around 200mm apart. This has no pratical application. Its just a limitation of simple magnetic theory. At close distances the flux density becomes influenced by the shape of the magnet itself (among other things).
In my estimate, your rig will stop working at a separation distance of

[Greystoke]

OK Windgat,
I think I'm hot on the trail. Been reading all night (no sleep), and I think that the result of your test could be used to determin the expected (ordered) grade. Need more time.