Author Topic: Sourcing ceramic magnets for Piggot's 2F  (Read 4654 times)

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TDC

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Sourcing ceramic magnets for Piggot's 2F
« on: December 15, 2014, 08:58:30 PM »
Hi folks, my first post.  Great forum! I hope I can contribute. I've been interested in this stuff since I built a small Savonius as a little kid.......  40 years later I'm finally going to build a real machine!

Does anyone know a source for the 50 x 50 x 20mm ceramic magnets used in Hugh Piggot's new 2 meter machine?  The closest thing I've found in the US is 2" x 2" x 1" at magnets4less.  Close enough? I imagine I could dial it in with magnet gap?

Mary B

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Re: Sourcing ceramic magnets for Piggot's 2F
« Reply #1 on: December 16, 2014, 02:11:22 AM »
I don't think you will see enough of a flux difference to make any big changes needed, tweak air gap so it runs right and go

Flux

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Re: Sourcing ceramic magnets for Piggot's 2F
« Reply #2 on: December 16, 2014, 03:38:03 AM »
Yes it will have a bit more flux with 2 x 2 x 1 magnets. In theory you could reduce turns a bit but far simpler to keep things as they are and increase the air gap to get the speed back up. Not easy to make winding changes without lots of experience.

Anyway there are several grades of ferrite which may cause more variation than the extra thickness.

Flux


kitestrings

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Re: Sourcing ceramic magnets for Piggot's 2F
« Reply #3 on: December 16, 2014, 06:20:20 AM »

CraigM

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Re: Sourcing ceramic magnets for Piggot's 2F
« Reply #4 on: December 16, 2014, 09:06:44 PM »
I found it's fairly easy (but slow going) to cut ceramic magnets using a wet tile saw with a diamond blade. Magnet4less.com has a sale on 4" x 6" x 1" ferrite magnets for $7 each. Cut the magnets into a wedge shape and you'll get much more magnet material into a given area.
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dnix71

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Re: Sourcing ceramic magnets for Piggot's 2F
« Reply #5 on: December 16, 2014, 10:08:37 PM »
I noticed that AlNiCo magnets are never mentioned for use here. They are physically the toughest, but easily demagnetize if handled incorrectly.

Has anyone tried using AlNiCo magnets with a smaller rare earth behind it to maintain the magnetism?

joestue

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Re: Sourcing ceramic magnets for Piggot's 2F
« Reply #6 on: December 17, 2014, 12:22:15 AM »
Has anyone tried using AlNiCo magnets with a smaller rare earth behind it to maintain the magnetism?
http://www.gitam.edu/eresource/Engg_Phys/semester_2/magnetic/magnetic_files/develop_BH.gif

if you can find SM-Co magnets for half the price of neodymium (per kilogram), then they would be useful.
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TDC

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Re: Sourcing ceramic magnets for Piggot's 2F
« Reply #7 on: December 17, 2014, 07:06:52 PM »
Thanks for the replies.   2" x 2" x 1" it is, grade C8, $2.81 from magnets4sale.com   Hugh Piggott's design calls for 50 x 50 x 50mm ferrite Y30BH C8.  It sounds like he also has prototype 3 & 4 meter designs with ceramic/ferrite.   https://www.youtube.com/watch?v=xq7C_5ywqd0

TDC

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Re: Sourcing ceramic magnets for Piggott's 2F
« Reply #8 on: December 17, 2014, 09:59:59 PM »
About the air gap, in this post, 3rd from the top, http://www.fieldlines.com/index.php/topic,145700.108.html, Chris speaks of an 18mm air gap with a 3mm gap between stator and magnets. I was thinking air gap IS the gap between the stator surface and the magnets? Can someone clarify?

hiker

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Re: Sourcing ceramic magnets for Piggot's 2F
« Reply #9 on: December 17, 2014, 10:12:41 PM »
post some picts as you build...  :}
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Flux

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Re: Sourcing ceramic magnets for Piggot's 2F
« Reply #10 on: December 18, 2014, 04:44:31 AM »
The air gap is the distance between magnets in an axial machine, the gap between magnets and stator is a necessary evil to provide mechanical clearance.

A magnetic circuit consists of a magnet, bits of iron and an air gap where the business is done. Iron such as the rotor discs has a low reluctance and needs little in the way of magnet to force flux through. Air is what needs so much magnet to get a decent gap flux, its reluctance is low ( about 1/1000 that of iron).  The materials you put in the gap such as resin and copper are essentially non magnetic and behave as air.

Alnico has a much higher remenence than ferrite ( it is similar to neo ) but the coercivity is very low and is easily demagnetised. What defines a magnets usefulness is the BH product and the shape of the demagnetising quadrant of the BH curve. Alnico has a very low BH product. Ferrite is considerably higher and has a straight line demagnetising curve so is much more difficult to demagnetise. Neo has a BH product way higher than ferrite and at low temperatures also has a straight line curve.

Samarium cobalt is not as powerful as neo but not far behind and has a straight line curve up to a few hundred degrees. Being non corrosive it is in many ways the best material to use but has been prohibitively expensive but I see that this is changing.

For conventional iron cored alternators, alnico is not very satisfactory, the stabilised flux level is little more than ferrite, but it may work in air gap machines with a large air gap.

The main reason for replacing Alnico and its many better relatives was the high cost and cobalt shortages pushed the price way up to the point that ferrites were developed (largely for the loudspeaker industry ).  It's unlikely you will make a useful machine from Alnico unless you find some big blocks cheap. Mixing alnico with neo is not much use because of the shape of the alnico demagnetising curve, you are adding basically a large air gap to the neo and the end result will be ineffective. Mixing magnets is not really useful but you can get some results with ferrite and neo because the ferrite will run back up its demagnetising curve and not a recoil line as for magnet steels,

Flux