Magnet strength

The spinning discs that hold the magnets, and magnet issues

Re: Magnet strength

Postby windgat on Mon Jun 23, 2008 11:41 am

Great!! There power of science lies in its ability to predict (while of course acknowledging those awkward details like unpredictability inside singularities!)

So here is a challenge - can your working out predict what the separation weight will be of the same test rig if the distance is increased from 20mm to 30mm? (for further than that I need to use longer bolts)
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Re: Magnet strength

Postby Greystoke on Mon Jun 23, 2008 12:27 pm

Humm,
The trouble with your magnets is that they are oblong. How am I going to account for that in formulas that are based on circular circumferences? Can you not buy some longer bolts, or maybe use some plastic cable ties?

I've collected a lot of info on Neodymium Magnets, and I'm trying to get my calculations to agree with some of the info supplied. So far, little fun. But I'm sure I'll get there.
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Cor
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Re: Magnet strength

Postby Greystoke on Tue Jun 24, 2008 6:27 am

OK, I worked it out.
The separation force at 30mm will be
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Cor
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Re: Magnet strength

Postby windgat on Tue Jun 24, 2008 3:01 pm

At 30mm, 1055g+63g+121g = 1236g. Not bad! And there may be measurement inaccuracy - its not clear exactly when the plates separate as the strings are not exactly equal length. At 50mm, between 255g and 340g (uncertainty percentage is higher with smaller weights).

So it seems you have those equations whacked, even for non circular faces?

BTW no need for extra long bolts - I just added some spacers between the magnets.
Attachments
P6240769b.JPG
Spacers in magnet test
P6240769b.JPG (12.65 KB) Viewed 10660 times
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Re: Magnet strength

Postby Greystoke on Tue Jun 24, 2008 3:15 pm

Wow :!:
At least I got something (approximately) right. Thanks for that windgat.

Well . . . . studying the subject I got convinced that there is no way in which I can replace these magnets with coil-wound equivalents. :oops: So . . I dropped that idea.

I'm still battling to get my formula approach to agree with some of the information given on the internet. I mean: what use would this be, if I can't correlate the test result to the grade of the magnet?

I've been on this for days now (enjoyed it) but . . . . need to take a break.

Catch you later
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Cor de Wit
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Cor
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Re: Magnet strength

Postby windgat on Tue Jun 24, 2008 3:24 pm

Yes its been fun, thanks! Be great if you scanned/photographed your workings out sometime and posted them here!
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Re: Magnet strength

Postby Greystoke on Tue Jun 24, 2008 3:28 pm

Oh yes I will. But at the moment I'm not sure if they're ready for peer-review. Just need to do a bit more research.

See you.
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Cor
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Re: Magnet strength

Postby Greystoke on Sun Jul 06, 2008 6:57 am

I finally managed to compute the force between two rectangular magnets using numeric integration on an Excel spreadsheet.
But I'm disappointed . . . . the results differ from windgat's measurements by more than 10% - mostly upwards. Last time I was - in fact - more accurate using a "fiddle factor". :(
The trouble is that I'm having to transform the magnets into a coilwound equivalent of similar dimension for the formulas to work. However, I can't figure out how "life-like" that actually is.

Hmmmmm . . . . . back to the books. :roll:
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Cor
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Re: Magnet strength

Postby Greystoke on Thu Jul 10, 2008 9:59 am

I think I got it right :!:
I (mathematically) replaced the magnets by a combination of elementary mini magnets, each with their own magnetic moment and then calculated the forces between each of them. I then summed them up using Simpson's numerical integration method, and I got:
Force between the two magnets:
@ 20 mm - 2873g
@ 30 mm - 1100g
@ 50 mm - 275g

Next is an attempt to calculate the volts induced when using windgat's constructions.
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Cor
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Re: Magnet strength

Postby Greystoke on Fri Jul 11, 2008 9:06 am

I managed to calculate the flux density between the magnets that are
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Cor
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Re: Magnet strength

Postby Greystoke on Sat Jul 12, 2008 8:54 am

Well,
Seeing that you guys are so enthusiastically interested - :mrgreen: - I am going to try out a novel way of generating elctricity, viz:

Image

A conductor moving in front of the magnet will cause the generation of eddy currents that will create a magnetic field opposed to the magnet's field.
Using two slotted discs rotating on either side of the magnet will therefore cause field fluctuations inside the magnet from maximum to (almost) zero weber, and back again.
These field fluctuations will be picked up by the coil surrounding the magnet in which - as a result - a voltage will be induced.

What I like about this design is that you dont fling pieces of steel around. The moving parts are very light. What I envisage as a disadvantage is that the slotted discs might get very hot as a result of the eddy current losses.
But a trial will tell all that.
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Cor
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Re: Magnet strength

Postby windgat on Sat Jul 12, 2008 5:15 pm

Greystoke wrote:I managed to calculate the flux density between the magnets that are
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Re: Magnet strength

Postby windgat on Sat Jul 12, 2008 5:18 pm

That sounds interesting... both magnets and coils stationary - definite advantages to that.

But perhaps the eddy currents will take up much of the mechanical energy, and leave little behind for the coils... I look forward to seeing results of a test!
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Re: Magnet strength

Postby Greystoke on Sun Jul 13, 2008 6:03 am

The losses can be minimized by using a better conducting material. Either a thicker Al disc or . . . maybe a copper disc, and, at some time in the future . . . a superconductor. :mrgreen:

So,
Where can I buy these magnets windgat?

PS: I'm "sexing up" my calculations. They're in an excell spreadsheet. I'll post an address where you can get it.
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Cor
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Re: Magnet strength

Postby Greystoke on Sun Jul 13, 2008 7:35 am

Hi windgat,

I
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Re: Magnet strength

Postby windgat on Sun Jul 13, 2008 10:00 pm

Greystoke wrote:So,
Where can I buy these magnets windgat?

There are available here: http://windpower.org.za/products/index.php?main_page=index&cPath=4

I have 24 50mmx25mmx12.5mm N35 nickel plated here that I would be happy to pass on at a good price. If thats a size you could use tell me how many you would like and I will work out a figure. Otherwise tell me an ideal size, and we can pool another order to get good pricing again.
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Re: Magnet strength

Postby windgat on Sun Jul 13, 2008 10:17 pm

magnet pointsb.jpg
Diagram for magnetic force spreadsheet calcs from Greystoke
magnetic force.zip
Magnet force spreadsheet from Greystoke
(214.88 KB) Downloaded 548 times
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Re: Magnet strength

Postby Greystoke on Mon Jul 14, 2008 6:42 am

Hi windgat,
Thanks for uploading that file. Still don't know what my problem is. I'm still working on some more explanatory notes. When I'm ready with that I'd appreciate if you updated the new file.

As for the magnets . . . they seem OK. Doesn't really matter if only for a trial. May have to be a bit more particular later when I build the first model. Right now I only need two. (Still have to think of a set-up :?: )

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Greystoke
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Cor
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Re: Magnet strength

Postby windgat on Mon Jul 14, 2008 9:50 pm

The board software seems to like .zip and not .xls, so it might work to zip before uploading. I also increased the max attachment size which should help.
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Re: Magnet strength

Postby Greystoke on Tue Jul 15, 2008 6:21 am

You're right :!:

Here's the address: <A HREF="http://users.iafrica.com/c/co/cordewit/ME/magnetics.zip" > Magnet computations </A>

PS: Why can't I post links :?:
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Cor
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Re: Magnet strength

Postby windgat on Tue Jul 15, 2008 4:15 pm

Links are disallowed to avoid spamming (i.e. sites can't get Google search credits by being linked to in this board)
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Re: Magnet strength

Postby Stefan on Sat Aug 16, 2008 12:00 am

Any new results on the experiments? Sounds interesting :) Got to love innovation
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Re: Magnet strength

Postby windgat on Sat Aug 16, 2008 10:12 pm

No new results... you are welcome to make the test rig and post more data!

I am curious to verify the claims suppliers make about magnet grade - pretty much taken on faith otherwise. So if you have two magnets and are in Cape Town you are welcome to make a plan to bring them over and 'calibrate' them.
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Re: Magnet strength

Postby Stefan on Sat Aug 16, 2008 11:26 pm

If I get any magnets that you havent tested. I will be sure to bring them over!
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Re: Magnet strength

Postby windgat on Sun Jun 14, 2009 10:47 am

I am trying to establish a relationship between number of winds, magnet strength, rpm and open circuit voltage.

To do this I need a measure of magnets strength in a single figure. So I am thinking of multiplying the volume by the grade. But maybe the grade is not 'zero-based'.

Another way of putting my question is: is a N42 magnet stronger (i.e. will produce more voltage) than a N35 by a factor of 1.2? (42/35)
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