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Question about magnets
By Bach On, Section Homebrewed Electricity Posted on Sat May 24, 2003 at 10:03:32 AM MST
Bad design, I guess....

I've got a source on failed hard drives so have about 16 of the crescent shaped magnets out of these. These look very similar to the #31 magnets at Forcefield. (I can get more as hard drives in our school system fail.) My only cost is the sweat of getting them out of the defective drives and listening to my wife complain about the mess I make while doing it. SOP.
I'd say these magnets are about an inch and a quarter in width and maybe a half inch from the bottom of the curve to the top. They are slightly thicker than a quarter. They come on a bracket with holes that make it easy to attach them to a surface without damaging the magnets. They are NdB, I believe, with a nickle plating. One will easily hold a large set of vice grips or a carpenter's hammer. Getting two of them apart requires quite a bit of effort and they'll bite you pretty hard if you get a finger between them.
The issue is that they have a N and a S at either end of the crescent shape. Imagine looking at a flat piece of unused chewing gum. One end is N and the other is S.
I bought some #20 coil wire at a motor rewinding shop and made up several coils with perhaps 100 turns of wire. (#20 seemed a good compromise size.) These coils are perhaps 2 inches long and just slightly wider than the magnets (maybe an inch and a half.)The coil resistance measures at about 4 ohms.
I mounted 8 of the magnets on a plywood disk eight inches in diameter. It has a hole in its center. I put a bolt through it and put it in my hand drill. I spun the drill up and tried holding one of the coils up as close as possible to see what kind of meter readings I'd get.
The bottom line is that the results weren't all that good. A few millivolts was about the best I could get - even when the gap was very close. I'd guess the drill is turning at maybe 2 or 3 hundred RPMs. Something seems to be wrong. So I'm back to the drawing board.
Has anybody successfully used these magnets in a wind genny? If so, how did you arrange the coil to magnet arrangement? And what kind of results did you get? I figured to use a two blade prop, perhaps at 48-60 inches (total).
As always, any help and advice is appreciated.
Maybe I'll get used to the new board. But I'm still in a steep learning curve.
Bach On!

Question about magnets | 9 comments (9 topical)

Possibilities (none / 0) (#1)
by windstuffnow on Sat May 24, 2003 at 11:03:56 AM MST

Just a thought here and I really don't know for sure but don't those magnets have 2 poles on one side? That being the case then the coils would need to be wound 1/2 the width of the magnets and each of those magnets would be considered a "pair" of magnets. I vaguely remember playing with a set from some old drives and it seems to me this was the case. I could be wrong.

Ed

Have Fun! Windstuff Ed

Yes... (none / 0) (#2)
by DanB on Sat May 24, 2003 at 04:47:11 PM MST

Ed's got it.... when they are crescent shaped - they have opposite poles right beside each other. Like a pair of magnets... so you might want smaller coils! Best bet - build your magnet rotor (sounds like you have!) and then test single coils till you find something you like.

[ Parent ]

Thanks (none / 0) (#3)
by Bach On on Sat May 24, 2003 at 05:16:45 PM MST

My original plan was for 8 magnets and 8 coils. I understand that the magnets have N and S on the same face - towards the coils - just at opposite ends. The backing plate helps to focus the flux towards the coils. Am I correct there?

DanB, or somebody. How wide is the flux field of each pole (N-S) with these magnets? Should the coil be only as wide as the magnet width, or even smaller (or larger)? My understanding is that a N pole should cut across one leg of a coil while the S pole is cutting across the other leg of the same coil. Is the highest flux point right at the ends?

Could I double the number of coils and get better results? The magnets are evenly spaced along the edge of the circular rotor (the top of each magnet is at the edge). There is perhaps an inch of space between each magnet. That's not quite enough space to put another magnet, but there is space for a one inch coil.

Since I can get more magnets, could I place another magnet below the others (closer to the center of the rotor disk)? I could make the coils thinner but a bit longer. Magnet polarity would be the same. Then two Norths and two Souths would be going across legs of the same coils at the same time. Does that make any sense?

Thanks for the responses.

Bach On!!!
- - I'm not superman, but I am very dense! -

More magnet spacing possibilities (none / 0) (#4)
by Electric Ed on Sat May 24, 2003 at 08:35:11 PM MST

As the "other Ed" said, treat each of these magnets as if it was two.

Try to detect where the poles are located and mark them, then lay out your magnet spacing so that the poles are the same number of degrees apart.

Than design coils that have the "legs" the same number of degrees apart.

http://www.electric-ed.com/images/Coils4.gif

Electric Ed

Magnets (none / 0) (#5)
by RayW on Sun May 25, 2003 at 05:35:23 AM MST

Break the magnets in half, this would seperate the poles. Does anyone think this would work???? RayW.

Theoretical question??? (none / 0) (#6)
by Bach On on Tue May 27, 2003 at 09:50:56 PM MST

Ray,

Honest! I'm not trying to be a wise acre. I really did give this suggestion some thought.

Let's assume that we have a magnet 1 and half inches long x 3/4 inches wide and maybe a 1/8th of an inch thick. Half the magnet is South and half is North. The opposite poles are at the long ends, though they attract along the upper face too. There is a metal backing plate, so flux on the lower face is negligible.

Now, if you break these suckers in half, aren't you still going to have two poles per piece? I've never heard of an all North magnet or an all South magnet.

So, by breaking these magnets in half, wouldn't I just be reducing the area of the magnet that excites the coils? Since the area of the magnet is one of the varibles in determing what kind of power a genny will produce, maybe this isn't the best plan of attack?

Or have I missed something?

Bach On
- - I'm not superman, but I am very dense! -
[ Parent ]

Breaking magnets (none / 0) (#7)
by Electric Ed on Wed May 28, 2003 at 06:36:49 AM MST

"Now, if you break these suckers in half, aren't you still going to have two poles per piece? I've never heard of an all North magnet or an all South magnet."

You are absolutely correct. There would be nothing to be gained by doing this.

Electric Ed

[ Parent ]

pole location (none / 0) (#8)
by John on Wed May 28, 2003 at 03:17:17 PM MST

I think that you missed something. On your hard drive magnets the poles are aligned through the thin section, not the length. Separating the two poles won't reduce the pole face area and it will allow for spacing between magnet poles on the rotor.

John

[ Parent ]

how many poles... (none / 0) (#9)
by troy on Thu May 29, 2003 at 10:44:52 AM MST

A better way to think of the hard drive magnets, is that they are really two magnets joined at the hip. If you break them in half, you get two complete functional magnets with a N pole on one flat side and a south on the opposite flat side. The hard drive magnets are not polarized over the length, like a bar magnet, but rather, "two" magnets "stuck" together and polarized on the flat faces.

best regards,

troy

[ Parent ]

Question about magnets | 9 comments (9 topical)

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