A bunch of questions regarding radially magetized ring magnet.

[PolaczekCebulaczek]

I'm interested with this kind of magnet geometry (for my generator project but not OU) and have some questions:

1.Is such magnet even possible to make at home? how to magnetize a ring like this?
2.Would such magnet loose it strength overtime?
3.How field lines would look like?
4.Compass inside the ring, would compass point in any fixed direction?
5.What if compass is not positioned in exact center of the ring, would it make any difference?
5.Can I induce current in coil when ring rotates? (coils inside/outside, near edge of ring)

[gyulasun]

Hi,

Some answers:

1) Of course I cannot know your technical knowledge and means, no offense but I do not think that an average experimenter could make such magnets at home.  I mean in general, for any magnet shape.
2) The direction of magnetization is not known to define long term strength stability for permanent magnets that are manufactured correctly.
4) Yes, but the length of a compass needle would be important to consider, say the length of the needle is less than the thickness of the ring: I may be wrong but it would stay put in axial direction when put in the exact center (axle) line of the ring
5) Then it would turn towards the direction of the stronger pole at an angle which would come from the difference between the attract-repel forces ruling between the needle_ends and the inside pole of the magnet wall.
6) Well, not much at all because inside and outside the flux does not change much when such ring magnet would be rotated

Imagine your ring is placed flat onto the surface of the table as if it would be a cake and you cut it up to some pieces radially as cakes are normally cut.  You may wish to consider assembling such ring from some pieces of arc or trapezoid magnet shapes that has the correct and same direction of magnetization. Of course you need to create a strong hardware, a hoop, to keep the repel pole magnets next to each other in a safe position. 

I mention this because arc shaped or trapesoid magnets are not rare nowadays, after some search I found these:

http://www.greatmagtech.com/product_cat_list/Arc-Neodymium-Magnets-c21616/0/45.html and this shape for example:
http://www.greatmagtech.com/products_info/Arc-magnet-with-countersunk-hole-generator-magnet--335493.html 

Or   https://www.magcraft.com/arc-magnets 

Of course you can also search with such keywords like 'arc' or 'trapezoid' or 'special' shaped magnets.
I have no any affiliation with any of the manufacturers... 

There would be another option for you. I know that most manufacturers gladly make such magnets for you but the cost can go skyhigh for the so called custom orders they force the buyers into by saying to pay for special tools etc.  At least  this has been so for years but maybe you can have some luck by seeing these free sample offers (probably for relatively small sizes) and the shipping should only be paid for.  See this link where even a single quantity, 1 piece radially magnetized ring magnets are offered:
https://www.alibaba.com/showroom/radial-magnetization-ring-magnet.html 

They call such magnetization direction for rings as radially magnetized or even uni-pole magnets.

Perhaps it would be worth for you to choose a few offers and write them with your needed magnet sizes, perhaps they would be willing to make it for you a single piece for free or at a minimal price and you pay only for the shipping what you can select as 'by Sea' too. 

Gyula

[PolaczekCebulaczek]

Hi,

Some answers:

1) Of course I cannot know your technical knowledge and means, no offense but I do not think that an average experimenter could make such magnets at home.  I mean in general, for any magnet shape.
2) The direction of magnetization is not known to define long term strength stability for permanent magnets that are manufactured correctly.
4) Yes, but the length of a compass needle would be important to consider, say the length of the needle is less than the thickness of the ring: I may be wrong but it would stay put in axial direction when put in the exact center (axle) line of the ring
5) Then it would turn towards the direction of the stronger pole at an angle which would come from the difference between the attract-repel forces ruling between the needle_ends and the inside pole of the magnet wall.
6) Well, not much at all because inside and outside the flux does not change much when such ring magnet would be rotated

Imagine your ring is placed flat onto the surface of the table as if it would be a cake and you cut it up to some pieces radially as cakes are normally cut.  You may wish to consider assembling such ring from some pieces of arc or trapezoid magnet shapes that has the correct and same direction of magnetization. Of course you need to create a strong hardware, a hoop, to keep the repel pole magnets next to each other in a safe position. 

I mention this because arc shaped or trapesoid magnets are not rare nowadays, after some search I found these:

http://www.greatmagtech.com/product_cat_list/Arc-Neodymium-Magnets-c21616/0/45.html and this shape for example:
http://www.greatmagtech.com/products_info/Arc-magnet-with-countersunk-hole-generator-magnet--335493.html 

Or   https://www.magcraft.com/arc-magnets 

Of course you can also search with such keywords like 'arc' or 'trapezoid' or 'special' shaped magnets.
I have no any affiliation with any of the manufacturers... 

There would be another option for you. I know that most manufacturers gladly make such magnets for you but the cost can go skyhigh for the so called custom orders they force the buyers into by saying to pay for special tools etc.  At least  this has been so for years but maybe you can have some luck by seeing these free sample offers (probably for relatively small sizes) and the shipping should only be paid for.  See this link where even a single quantity, 1 piece radially magnetized ring magnets are offered:
https://www.alibaba.com/showroom/radial-magnetization-ring-magnet.html 

They call such magnetization direction for rings as radially magnetized or even uni-pole magnets.

Perhaps it would be worth for you to choose a few offers and write them with your needed magnet sizes, perhaps they would be willing to make it for you a single piece for free or at a minimal price and you pay only for the shipping what you can select as 'by Sea' too. 

Gyula

Thank you such much!

well, it's quite tricky to magnetize a ring magnet like that using coil or another magnet(I don't know how...yet)
Is this a correct picture of field lines?

[gyulasun]

....
Is this a correct picture of field lines?

Hi,

Yes, for a six-pole radially magnetizatized ring it is correct.
The N and S poles alternate within the inside area (or space) as seen in the drawings here
for instance: http://www.aomagnet.com/radial-ring-magnets-c-19/  while for a unipole ring there is a single pole
(say South) in the total inside diameter area or space of the ring and the opposite single pole (the North)
on the total outside surface and nearby space of the ring.

Here in this link you can see some more field line distributions that includes the fields of unipole
ring magnets too (unfortunately their picture is rather small and blurry):
http://www.jjmagnet.com/f_products1.htm

In this pdf file you can see a clear picture on the fields of a unipole ring magnet as compared to a multipole ring:
http://www.aicengineering.com/files/2014328143328.pdf I copied this picture from the pdf file, see attached.   

Finally here is a large sized hence rather expensive (OD=70 mm) unipole radially magnetized ring magnet
with its nearly homogeneous South field inside and all North field outside:
http://www.materialhandlingzone.biz/product/aomag-uni-pole-radial-oriented-magnetization-neodymium-large-ring-magnets-diametrically-magnetized-od-70-x-id-62-x-20mm/   

Gyula

[phoneboy]

@ PolaczekCebulaczek, this was just an untried method I had come up with to try for a design I had.  I had read about people making weak magnets by heating iron and allowing it to cool oriented in the earths magnetic field, so I wondered if I could do something similar with a ferrite ring.  The idea was to make a two part mold as if i were going to cast a part.  You would need some wood, sand, sodium silicate, a co2 source, a ferrite ring magnet, and copper wire for the flat coils.  Once the form was made for the ring and the coils,  the magnet would be placed in an oven and the temp raised to past its curie point.  The ring would then be placed back in the form and the coils energized while the ring slowly cooled, hopefully reorienting the domains and creating a reasonably strong radial magnet.  See old vizimag pic for ref.