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flux density v/s surface area
By kitno455, Section Mechanical Posted on Sat Apr 16, 2005 at 04:55:41 PM MST
when building a pma, what sort of magnet shape works better

i was thinking a bit about coil and mag sizes, and i wanted to see what folks ideas are regarding the shape of the mags, and what affect that has on the coil size/placement.
basically, lets say you had a dual-rotor alt, and you had a certain 'volume' of mags. they could either be big, flat disks, or tall, thin cylinders (poles on the flat faces on both)
now, mechanical attachment issues aside, the big difference would seem to be increased flux density over a smaller surface on the cylinder mags, and lower flux density over a larger surface on the disk mags. the rotor could be smaller diameter (and thicker) on the cylinder unit, and the coils could be physically smaller, less wire for same number of turns, lower resistance, and smaller stator size.
how does this affect the current the unit is capable of? any other affects i am missing?
allan

flux density v/s surface area | 14 comments (14 topical)

Re: flux density v/s surface area (3.00 / 0) (#1)
by picmacmillan on Sat Apr 16, 2005 at 11:49:13 AM MST

my limited understanding allan is that the idea is for the magnet to cross the coil perpendicular, being optimum...this displaces the electrons in the coil quickly..for instance, the side of a rectangular magnet will cross the path of the coil all at once from leading edge to trailing edge., sending the electrons flying in a perpendicular direction.....if the magnet is wedge shaped , then the coils should resemble the magnet shape when done. therefore again, the magnet will cross the coil sending the electrons perpencicularly, and the whole side of the magnet will cross the coil displacing the electrons quickly...i have been told that round magnets are not optimum as they will cross the coils and not get the same action from the electrons..i have one genny with round magnets though, as money dictated what i could buy...just my 2 cents..others can explaing it better than i and in more detail i am sure...good luck....pickster
http://www.frecklefarmloghomes.com

Re: flux density v/s surface area (3.00 / 0) (#2)
by kitno455 on Sat Apr 16, 2005 at 01:29:02 PM MST

ok pick, that makes sense. what if i was talking then about two rectangular mags, one that was 2 x 1 x .5, and the other that was 2 x .5 x 1? (really the same mag with the pole on a different face)

now, both mags have a 2 inch side that hits the coil first, but one has a 2x1 pole surface, and the other has a 2x0.5 pole surface. what would that look like for output i wonder?

allan

[ Parent ]

Re: flux density v/s surface area (3.00 / 0) (#3)
by windstuffnow on Sat Apr 16, 2005 at 02:41:09 PM MST

The ones with the most surface area will give the most output.

Have Fun
Windstuff Ed

Have Fun! Windstuff Ed
[ Parent ]

Re: flux density v/s surface area (3.00 / 0) (#4)
by nothing to lose on Sat Apr 16, 2005 at 07:14:03 PM MST

"The ones with the most surface area will give the most output. "

I would agree with that, but is he also asking about thicker ones at the same time?
So you have a larger surface area (2x2) at 1/4" thick Versus smaller area (2x1) at 1/2" thick?

I think that might be what he meant as I first read it. I don't know the answer to that though. Larger area weaker flux or smaller area but much stronger flux????
.
nothing to lose

Spelin and tpying are my strong points, not electronics.
[ Parent ]

Re: flux density v/s surface area (3.00 / 0) (#5)
by kitno455 on Sat Apr 16, 2005 at 08:22:04 PM MST

exactly what i meant. the volume of the mag stays the same. i did some silly tests with vizimag, but i dont really know how to use it yet. making the mag thicker and smaller increases the flux density over the mag, so i wondered if that might be useful in an genny.

ed seems to know his stuff, but i wonder why what he says is true? why does the stronger 'wall' of flux from the thicker magnet not provide the same output as the wider area of flux of the shorter mag?

allan

[ Parent ]

Re: flux density v/s surface area (3.00 / 0) (#7)
by windstuffnow on Sat Apr 16, 2005 at 09:24:08 PM MST

I think Johnlm laid it out pretty good with the numbers for comparison. I've found that the more surface area of flux tends to wind over smaller stronger magnets. It also holds true in the formula:

NARBP*2

where:
N = number of turns
A = area in sq meters of 1 magnet
R = revolutions per second
B = Flux in Tesla
P = total number of poles on 1 disc

adding anything to the formula increases output, as well, taking anything from the formula decreases output.

On another outlook, if you increase the number of magnets using the smaller ones you'll increase the surface area and be able to utilize the extra strength of the smaller magnets. In the end you've sucessfully increased the magnetic surface area and are using the smaller magnets more effectivly.

Aint' that just too much fun? So many possibilities !

Windstuff Ed

Have Fun! Windstuff Ed
[ Parent ]

Re: flux density v/s surface area (3.00 / 0) (#6)
by johnlm on Sat Apr 16, 2005 at 08:37:48 PM MST

The magnet as Allen described; 2" X 1" X 1/2" will have the following lines of flux in free air (not on a steel plate, which would increase the numbers). This is for a ceramic magnet with Br=3950, a Neo with a Br of 12300 would be corespondlingly more.

On the 1/2" X 2 " face at a distance of 2 mm from the mag there will be 8943 lines of flux.

On the 1" X 2" face there will be 12898 lines of flux at 2 mm from the surface.

As you can see the surface area has a more significant influence in the equations than the thickness.

Source magnetic field strength equations from a magnet company catalog.

Johnlm

Re: flux density v/s surface area (3.00 / 0) (#8)
by kitno455 on Sat Apr 16, 2005 at 09:51:25 PM MST

wow, john, ed, et al, thanks alot. i was looking for a relationship between area and thickness, re: flux, and i guess i got it. john, is the formula used specific to this companies mags, and/or the mag shapes?

basically, i have a boatload of curved ceramics about the size of dominoes from auto starters. they have poles on the big face. i came up with a way to build a magnetic circuit that concentrates the flux noticably over a smaller surface. i wanted to know if this more complex construction (or any other construction with thick mags) was worth it.

so why are so many gennies mentioned here using 1/2 inch thick mags? big rotor too much trouble?

hmm, this really means i gotta start scrounging for more dead hard drives....

allan

Re: flux density v/s surface area (3.00 / 0) (#9)
by Flux on Sun Apr 17, 2005 at 01:28:59 AM MST

No , in general the flux concentrator ideas are not worth the effort.The only possible case where it is of benefit is with ceramic magnets in a motor conversion type alternator where you may be able to use the magnet material more effectively.

With air gap alternators there is no advantage and most likely you will increase leakage flux considerably and loose out.

You can't increase total flux, that is what generates power. Fiddling with flux density is only of benefit in nearly closed iron circuits ( very small gaps)

Flux

[ Parent ]

Re: flux density v/s surface area (3.00 / 0) (#10)
by nothing to lose on Sun Apr 17, 2005 at 07:24:38 AM MST

" boatload of curved ceramics about the size of dominoes from auto starters."

Starters?? What kind are you getting mags out of? I have been getting them from junk motors, heaterbolwers, radiator fans etc.. but had not seen any in starters here.

I got a couple pretty strong ceramics I was playing with. Was very surprised how far the flux travels and how strong it was. I held a steel nail length wise (point away from the mag) and was tapping it to a tin can. Real far away from the mag it did nothing of course just tap tap tap. Still pretty far when it started getting sticky, was far away when it got holding fairly well, and was not very close at all when the nail jumped off my finger and flew!
I geuss that was a very scientific test to see how strong a ceramic actually is :O

Actually, without expensive test equipment I am looking for a way to compare various mags I get, hopefully with some accuracy.
After the little test I discribed above I am now thinking maybe a sliding wood frame to hold the magnets, a wood ruler to measure the distance, wood pivot bar with a steel bolt in the end. Connect a spring to a scale and the bolt. Slide the magnet frame to various distances like 1", 1 1/2" etc.. and read the pull on the scale. Still not great but may help to group used mags in sets of near equal power. Getting used mags from junks motors of variuos types and other sources, I really wonder if they are actaully all the same just because they look the same. I mean I am sure some will use low grade mags and some high grade, and just because they measure the same size the 2 will still not have the same strength.
I know I have a couple ceramics that are pretty darned close in all measurements, one is pretty weak the other is strong, I can tell that just by sticking them to the fridge. They look the same, but one is barely a fridge magnet to hold notes, the other I could use as a pickup tool if I want.
I suppose one is a real magnet, the other maybe half magnet and half filler. The problem is they look the same to me. Maybe they were once both the same and one just got really weak for some reason? Too hot maybe?
.
nothing to lose

Spelin and tpying are my strong points, not electronics.
[ Parent ]

Re: flux density v/s surface area (3.00 / 0) (#11)
by kitno455 on Sun Apr 17, 2005 at 09:18:18 AM MST

japanese 4 and small 6 cylinder cars, and some smaller, recent fords. you can tell at a glance cause they have a black painted central tube that is about 2.5 inches in diameter. there are two different lengths, the common one has 1.75 inch long mags, the other is 2.5 IIRC.

mine are from mazda, nissan, mitsubishi, ford, all the same. dont even bother with toyota, they have the same starter, but the mags are glued in. i got the long ones out of a nissan quest minivan, it was the same too, just a longer armature and mag tube.

6 mags per starter, they are bar mags that have been machined curved. no difference in strength between front and back. really easy to get out, you need a 12 mm and an 8 mm socket. remove the big wire from the stator and the two bolts from the end, and tap the thing on your sole till it comes apart.

on the other hand, i got some mags out of a VW robert bosch starter, they are similar length to the short ones, but a .25 inch wider and slightly thinner. the case is harder to open, and they dip the magnet tube with the mags in it (yuck). but they have a really neat feature- they are magnetized in a curve. the inner surface is much stronger than the outer.

how did i test this? you and i think alike. i clamped a piece of wood to my work table. i taped a known mag to the bottom of the wood. i put the test mag in the bottom of an upside down plastic grocery bag, and stuck it to the known mag. then i clamped a pair of locking pliers to the handles of the grocery bag, and started sticking mags to them till the weight was too much. i tried this will all my mags, different directions, different stackings of mags, steel backers, etc. not as good as a fish scale, but close, and no friction on the guides, and handles any size mag without mods.

allan

[ Parent ]

Re: flux density v/s surface area (3.00 / 0) (#13)
by nothing to lose on Mon Apr 18, 2005 at 05:10:26 AM MST

"japanese 4 and small 6 cylinder cars, and some smaller, recent fords. "

Thanks, I have not tried any of those, mostly just older stuff and supposedly American (imports anyway).

Have you tried spinning the starters to see if they produce anything or how much. Just curious.
.
nothing to lose

Spelin and tpying are my strong points, not electronics.
[ Parent ]

Re: flux density v/s surface area (3.00 / 0) (#14)
by kitno455 on Mon Apr 18, 2005 at 07:20:14 AM MST

no, i figured they would not do much, since they have a single turn of really heavy wire on the armature, so i did not bother. however, if you want to try, get the toyota starters. they have ball bearings....

also, these things are gear reduction, they have a small half-plastic planetary unit, so if you use them directly, you will find the armature spins a bit faster than the bendix (output gear).

also, now that i go back and look, the ford mags are slightly wider than the japanese mags, and seem to be magnetized in a curve like the bosch ones (which are wider still).

allan

[ Parent ]

Re: flux density v/s surface area (3.00 / 0) (#12)
by johnlm on Sun Apr 17, 2005 at 07:47:53 PM MST

The equations will work with any brand or type (Neo, Ceramic Alnico etc) of magnets, as long as you know the Br rating of the magnets. The catalog only gives equations for round (disc) magnets and rectangular magnets, and the lines of flux can be calculated at any distance from the pole face with the magnet mounted on a steel plate or not. Its interesting the equations for square / rectangular magnets are more complicated than for disc magnets. I suppose you could use the calculations for curved magnets with the distance from the pole face being an arc at some distance from the face. I entered the formulas (fairly lengthy formulas) into a spread sheet so that I can do what if entrys of all the variables to see the outcomes. The catalog is a 2Mb pdf doc at www.magnetsales.com

To the folks at Otherpower: I am not promoting nor have any affiliation or interest in this company. They just had some nice application literature in their catalog.

Johnlm

[ Parent ]

flux density v/s surface area | 14 comments (14 topical)

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