Magnetizing a Steel Ring

NOT TRUE! One pole can be the center, with the other pole being the periphery. The magnetic field lies would come out one or both sides of the center and loop around to enter the periphery (or vice-versa)

A magnet may have poles, but the magnetic field is still a closed loop. There is no such thing as a monopole magnet.

The stuff is kinda fuzzy, but I am reminded of Maxwell's Equations - the foundation of electromagnetism. One of the equations is Gauss's Law for Magnetism. It is as follows:

This states that the surface integral of the magnetic field is equal to zero. Or stated in the differential form:

Which says that the divergence of the magnetic field is zero.

In human being terms (I'm not saying that electrical engineers or physicists or not human - that has not been proven yet) Gauss's law says that all magnetic fields that leave any enclosed area are equal to the magnetic fields entering the area.

Basically, what goes out must come in, and vice-versa. There is no monopole that 'emits' a field outward.

Now think of a bar magnet. The field lines go out through one pole, loop around and come back in through the other. But they do not end! They continue unbroken through the bar magnet coming out again through the first pole.

If you understand some calculus and electromagnetic theory, try looking on wikipedia for Maxwell's Equations and/or Gauss's Law for Magnetism:

http://en.wikipedia.org/wiki/Maxwell%27s_equations

http://en.wikipedia.org/wiki/Gauss%27s_law_for_magnetism

Back to the original question, loop your wire around the loop with the wire parallel to the loop - or make a loop out of the wire. When current flows around the loop a magnetic field will then go through the center of the loop of wire and curl around the loop. To find the direction of the B (magnetic) field, grab the loop of wire with your right hand with your thumb pointed in the direction of the current flow. Curl you fingers around the wire putting them through the inner part of the loop. Your fingers show the direction that the magnetic field points through the loop as it passes through the inside of the circle. (I hope I explained that well. . .)

Good luck!

Who said anything about a monopole?

My first full sentence was:"One pole can be the center, with the other pole being the periphery." That's two poles!

Perhaps instead of center and periphery I should have said inner circumference and outer circumference.

I see I left the 'n' out of lines...

For such a magnet to be useful, a magnetic core would be required both inside and outside the ring, as well as across at least one end to complete the magnetic field circuit.

The world is three-dimensional, and magnetic fields in free space add linearly, so of course you can have a field that is radial and pointing outwards; this would be in the central plane of the toroid. See my post #25 for more details

(P.S. If #1 had been my own posting, I would now go back and give myself an off-topic rating - and it wouldn't be the first time I had tried such a thing)

Seems someone else has saved me the trouble.

Were it possible for me to re-do Post #1, I would have replied with illustrations and dispensed with the ambiguity of words which, apparently, seem to have confused a number of folks. I know that a magnetic field can be radial in the plane of the toroid, but it cannot be radial everywhere. I wrote Post #1 prior to emc_c's post and prior the OP's subsequent reply which clarified what he had in mind. Hindsight is 20/20.

Point taken. I must admit I read the original posting as simply asking how you could avoid the magnetic field being everywhere perpendicular to the axis of the toroid.

I read the OP's initial post somewhat differently. He asks, "If I wanted to magnetized (sic) a steel ring so that the magnetic field extends out ward from the torois (sic) how would I do this."

and

"So how would I magnetize the steel ring so the field extended outwards?"

Because magnetic fields are closed loops, at some point the field must be parallel with the toroid surface and so cannot be everywhere "outward" (in the OP's own words). His two questions imply (to me, at least) that he is speaking of a field topology that more describes one of an electric field which can extend outward (or conversely, inward) everywhere. Post #1 attempts to point out these distinctions.

If you need a not too strong magnet you can use the method in

above sketch:

Usually the coil is dimensioned for the full saturation of the iron circuit and supplied by a loaded capacitance. It gives a very short very high current pulse. The gaps have to be computed for minimal losses so that the whole possible flux will pass through the region where the ring is. The radial thickness of the ring should not be too important since the permeability of the magnets is near to the one of air and it is as the gaps will be bigger.

If you want to have alternate magnetic polarities on the periphery then the circuit can be modified. This is basically used to magnetize the PM encoders.

I know that some will make negatives comments but the method although it has its limits is broadly used in the industry.

Depending on the ring's reluctance & grain structure and how strong a magnet you need, you would have to construct a low reluctance charging fixture with one pole fitting closely in the ID of the ring. The post from that pole would have to have a very large coil wound around it to generate the charging field. The other end of the post would have to be connected to the other pole (a hollowed out cylinder) which would fit closely around the OD of the ring. No part of the fixture can touch or come close to the ends of the ring as this would shunt the charging field intended to magnetize the ring.

Good luck!

You received lots of good information, from the Engineers who participated in this Blog. Some in conflict with others, but all well intended.

So now I offer this little bit of observation. First you did not indicate just how large of magnet you wish to create. You just wanted to learn how to make one. Some say that you cannot do it others say that you can. "I say that if you would like to get one, on the cheap, get your self a discarded speaker from a boom box, like the ones that those who are trying to out do each other with the bass frequencies". On the back end of the speaker you will find a circular magnet, usually having a metal disk attached to the surface that you can see. The magnets are usually from 1/4" to 3/8" thick and may be as large as about 3 1/2" in dia.

These magnets are usually glued in place. Take a sharp flat chisel and place it where the magnet attaches the speaker frame. Strike it sharply with a hammer. The magnet will usually separate from the speaker with one sharp blow. Make sure that you cause the magnet to fall onto a soft area like a folded up blanket. These magnets will break if permitted to fall onto a hard surface like concrete. The metal disk that seems to be permanently attached is usually held in place by the magnets energy. It can be pried loose with a sharp chisel or knife edge. Often you will find that the disk has a centering pin that is as large as the center hole in the magnet to prevent it from slipping around.

I have several of these. I am trying to get 12 of them for the purpose of constructing a 3 phase generator for use as a wind turbine, or attaching to a small ICE that will be fueled with syngas produced by burning wood cubes/chips in a home brew gasifier. I am also salvaging wire from the inexpensive box fans that only last about a season, the ones that can be purchased from Lowe's or Home Depot, or other discount stores for about $10.00. This wire is consistent in size and can be re-insulated simply by creating an automatic varnish applicator made from a saturated cotton ball held in place so that the wire receives a fresh coat of varnish as it is rewound to the dimensions and lengths needed to create the parts for the generator.

I am tracking these items so as to show folks that with a little effort, they could provide for their electricity just as I will do for minimum costs. I am also salvaging the electronics from discarded Computer monitors,TV's Micro wave ovens and anything that looks as if it may contain electronics that would be useful in construction of the inverters for the conversion of said energy for house hold use and battery maintenance.

Everyone should have some kind of hobby. Creating something useful and needed from other folks discarded items is one of mine. One can learn a lot with out spending the retirement check for the materials to do so.

TooMuchFun

Hi,

there seems to be a basic misunderstanding:

Europium assumed the field lines coming out of the toroid all over the inner, side and outer surfaces. This cannot be realised, any magnetic assembly has 2 or (n x 2) poles.

Others assumed that the magnetic field lines have to enter at the inner side of the toroid and to leave at the outer side.

NickName proposed a simple working solution to achieve this. I would add another coil at the left side, this helps but is not essential.

Draw a field line around coil and through the toroid. Measure the length = field-line-length.

You will need a lot of current to magnetise:

Steel: forget, not at all a good material, used from 1900 to 1930 as FeW4 (iron with 4%tungsten)

Ferrites (material most often in loudspeakers and simple motors, cheap) will need 300 to 500 A per millimeter of field-line-length.

Samarium-cobalt magnets need 1.5 to 3 KA/mm

FeNdB - magnets need 4 to 7 KA/mm!!! to get fully magnetised.

This is not a very good idea to do this for a single shot.

Better: take bar magnets and glue these on top of a steel ring (or any nonmagnetic material not too thick). Grind the faces to match without a big air gap. You will need 8 to 16 magnets to make a good ring.

Watch out to mark at the sides with ink or else or by grinding to the appropriate angle: at mounting there will be unexpected forces to overcome that will misorient some of the magnets if not controlled in orientation or secured by screws or clamps.

if ready, grind outer periphery if roundness is important.

Glue onto this assembly an outer ring if necessary.

Big companies do the same procedure as most attempts to magnetise rings end up with fragmented rings.

And some with fragmented coils.

The pulse of magnetising current has to be maintained for around 20 milliseconds, else the inner parts of the magnets will not be magnetised.

RHABE

"Europium assumed the field lines coming out of the toroid all over the inner, side and outer surfaces. This cannot be realised, any magnetic assembly has 2 or (n x 2) poles."

Hi RHABE,

I don't recall ever having made that assumption. Of which toroid to you speak? The conventionally-wound toroid (one having radial windings) or my example in which the current circulates in the ring itself? The latter configuration produces a simple dipole, of course. An example fundamental to most (if not all) undergrad physics courses.

Later in my post I explain that magnetic fields are dipolar, that is, a magnetic field has two poles as opposed to an electric charge which can have one. I am not explaining here the most general case, but most fundamental one: A magnetic field cannot have fewer than two poles. An electric field can. A foray into field configurations of arbitrary complexity would not answer the OP's question and would not fit the context of the question. More likely it would obfuscate the issue and so my reply is to the OP's original question only and the scope of my reply is restricted exclusively to that context.

Does this clarify things a bit?

Hi Europium,

one more misunderstanding, my fault.

I wanted to express that your statement " a ring-shaped electromagnet where all the field lines radiate outward, I have some bad news for you: you can't" is absolutely ok.

In my understanding a toroid is the ring - nothing else.

"Now consider a toroid" and if I am right now, the meaning was a toroidal winding around a ring. Here came the confusion.

So if you think about a ferromagnetic ring - as in the other posts are referred to - this to magnetise inside in, outside out, why not.

RHABE

Here you go, theory, practice and a photograph of the results. The point of this paper (by a friend and colleague) was that you could tweak the design of a common mode choke to yield just the leakage (differential mode) inductance you needed by varying how much of the core was covered by the windings. Your application is different, but you can see that a smaller percentage of the core being covered yields more leakage flux, which is what I believe you were after.

That is definitely the kind of field I want, where the field is present inside and around the toroid. How far apart do the windings need to be to create this field? Is the core made out of specific material? How much current was needed to create that field since the number of windings was reduced? Are there two coils on the core, one for the right and one for the left or is there one loop that is not wound around the core? The picture is a little grainy and fuzzy so I cannot make it out completely.

Is there a website with that information and pictures so I can read it more clearly?

Thanks a lot for the help.

Stephen

Re this: "Is the core made out of specific material? How much current was needed to create that field since the number of windings was reduced?" As I noted in my reply, the specific application described in the paper was quite different than yours. But the geometry factor (core coverage vs field leakage) won't change. How much current you will require will depend on the specific core material you choose and how much field you desire.

Re this: "How far apart do the windings need to be to create this field?" I can't answer your question specifically, but you can do exactly what the paper author did. He laid a thin sheet of acetate over the core winding, sprinkled the iron filings and looked at the pattern of field lines. You don't even have to go for the acetate since you aren't going for a photo. Any old sheet of typing paper will do nicely.

You can start with just a few turns at the midsection, observe the pattern, add a few more turns, observe the pattern, and iterate till you get the pattern you want. Once you have that, you can scale up the core and wire dimensions for the field and current desired/necessary.

The key here is that the amount of leakage correlates with the winding coverage. Once you have that nailed down, the rest is just scaling.

Im afraid you couldnt get that form MF in the air. its simple that the core will concentrate the magnatic line into its own body. the more u value the core has the less line leak outside is, unless you apart the core into two parts and make them into special form such as shade of saddle etc.

or you use only coil without any core in it. you will have large leage mf.

[oh, refer to e-man's power resistor, haha.]

why do you wish to get such mf?

C'mon, its just the same as two bar magnets bent into half-circles and the north poles placed together. If they are properly permanent magnets, the field due to the two together will simply be the sum of the two separately.

It's bound to work with electrical excitation as well - as supported by the practical observation of the iron filings!!!

Hi,

if you want to have this field then simply take a bar and wind a coil around.

This i s a dipole field with its axis along one diameter of a toroidal coil with a somewhat special winding: one half opposite to second half.

Would be better to wind without gaps in right and left side and let some unwound region up and down, so that the magnetic field is going out into air in the upper region, making a loop in air and coming back from the down-region.

Inside the ring the field in the right side is opposite to the field in the left side.

Simplified calculation by hand can be done but not with accuracy.

Accurate calculations require something like magneto-CAD.

So first of all think once more if you really want to have a dipole field and why to use a ring and a winding to generate this. (A bar would be simpler).

There is only one motivation of using a similar arrangement - but this requires another coil geometry, if you want to change the direction of the dipole.

RHABE

If I left a gap on the top and bottom would the field wrap around the coiled regions as well? Would iron fillings show the field all around the toriod?

If so then leaving a gap at the top and bottom would probably give the field I want, like the field in emc_c's post.

Thanks.

Stephen

If you leave a gap at top and bottom, then for sure you don't need a toroid. As someone said previously, you then just have two curved bar magnets.

It would certainly help us to help you if we really knew what field structure you want, or better still what you are trying to accomplish. Can you provide a sketch, drawing, or photo?

quit right, a sketch or photo can explain all, try to put out.

for example this illustrations, can say more thant words do.

where A, BC is wire or coil. between coils is a uniform mf.

the two lines mf is shown above according to same color. very simple

Perhaps this will help: try reversing the currents in one of the coils so that you have an opposed quadrupole. You will that, in the plane that is half-way between the coils, the residual field is radial.

where is quadrupoles?

why not illustrate for details? lazy to draw?

I'd generally prefer not to answer in kind, but in this case it seems appropriate: rather than reverting to rudeness, would you not prefer think for yourself? (Quadrupole just means "four poles", and there a plenty of references on the web, as well as large sources in central Europe and Bradford)

Usually, I would usually simply refer you to Wikipedia - but in this case it only shows a parallel quadrupole, and what they call a magnetic quadrupole is in any case something rather different (although it does approximate a quadrupole in the central region).

Incidentally, you get the same field pattern when you hold a bar magnet perpendicular to a superconductor (unfortunately, you have to hold it like that, because the configuration is not stable)

I know quadra...

neednt offer so many references.

why not illustrate for your words?

a simple illustration can match lots of words.

any physist and engineer will use this tool in his lecture. even a sketch.

words, somtimes might cause confuse. right?

I was impressed you used illustration in cloud show thread. you seem to be able use it.

I wonder, why occur a super conductor? what miracle will take place when I do that?

why suddenly concern about quadra...? does it has some thing to do with toroid mf?

or planer form mf?

I think the statement I made about extending magnetic fields was confusing. What I meant is that I would like the field to be present outside of the toroid instead of strictly inside which is what happens if you coil wire into a toroid. If I coild a wire all the way around a toroid the field would not be present around or inside the ring. Is there a way to magnetize a toroid so the magnetic field is present around and inside the ring??

Thanks

Stephen

Stephen, if you have access to a scanner, would you please sketch specifically what you have in mind, scan the sketch(es) and post the pix here? Show the ring from the top and the sides and show the field lines where you want them in each view. Hand sketches would be just fine.

Tnx,

-e

The field depicted in emc_c's picture is the field configuration I would like.

Wow! That is not at all what I interpreted from your original post!

"magnetized a steel ring so that the magnetic field extends out ward from the torois"

The above is what I pictured. This implies that the entire outer surface is one pole. There must be an opposite pole somewhere, and the only place where that other pole could be, without distorting the outside field, would be the inside surface.

The other item that was not clear was whether you want, as I assumed, to magnetize the piece of steel permanently - that is, make a permanent magnet. The current required to make a permanent magnet is significantly greater than the current required to temporarily magnetize a piece of soft steel.

Finally, if emc-c's field structure really is what you want (a N pole at one side/end and a S pole at the opposite side/end), then why use a toroid, where much of your field does no good? What are you trying to accomplish????

Dear friend, Im afraid your picture is wrong. it cannt be done.

just see the #5 thread, he wrote

Gauss's Law for Magnetism is as follows:

in the differential form:

This mean the magnetic field is a unresouce field. but in your situation, the integratial will be 34 ( I count). its impossible.

but you can get bipole from up and down side of the toroid. if you hope to get the inside cycle direction mag field, pls add a rod in the center of the toroid. once you used dynamic meter. you will see it.

beside,

that is, make a permanent magnet. The current required to make a permanent magnet is significantly greater than the current required to temporarily magnetize a piece of soft steel.

this may not be right. in fact some soft iron has same B value as hard magnet iron. at the same volume station, they can be same current. only differnet is soft iron has less coercive force than the last.

mc-c picture is a filter (I guess, I cannt see the pic clearly) either for common mode or different mode to eliminate interference from input port. the core , generally is ferrite. nonmetal material. there will be less leakage amount or mf.

if the top thead wish to get the shape, he can refer to what I said above. or in fact. an empty coil can be done.

regards

The 'S' with your integral symbol, or the circle of the symbol, refers to a 3-dimensional surface. The drawing shown was 2-dimensional and incomplete. If you take any 3-D surface, the integral is indeed zero.

Here is the same, but more complete drawing, with a required side view to show the field return paths.

This is essentially the same as the drawing shown by nick name in post 2, without the coil, and as indirectly pointed out by TooMuchFun, is essentially the same structure as a common loudspeaker magnet structure. In many loudspeakers, the central core is actually the magnet, while in others the outer ring is the magnet, but in all cases the end result is a field pointing either radially out or radially in.

In my interpretation of the OP, the poster was requesting a field extending [radially] outward from the ring, and this does it! Or have I missed something?

Hi,

this is exactly the form and magnetisation that had and have torquers in dry-tuned gyros, better on the outside, still better inside and outside.

Best 4 rings. Operated with coils like these:

RHABE

well, no matter how you stay in 2D or 3D space, the law is right. if you drawed the plane pic as you drw now at normal view, I think it would be right.

the front pic you forgot a large outside cycle. if I were your teahcer, I certainly judge your work as wrong, and give you 0 score. of cause ths time its right. 100 score.

if you wish to get a uniform mf, the gap would be very smalll, just like you said in speaker and dynamic mic structure. as well as electdynamic meter etc.

if you wish to get a large area uniform mf, thread #22 give you a good solution. or same I said above.

are you a machine engeer? can draw a wondrful 2 view picture.

A field directed radially in the plane of the toroid is perfectly possible. One might say that the picture is oversimplified; however, apart perhaps from the omission of the internal field, it is no worse in than the equivalent drawings commonly used for bar magnets. (These too ignore the divergence of field lines orthogonal to the plane of the drawing - which is why it is impossible for a single drawing to indicate both the correct field strengths and the directions at all positions).

Sow how might it be achieved?
Ignoring the toroid for now, the simplest* way to create a plane radial field more-or-less as drawn is to place two disc magnets (magnetised along their axes of rotation) with their north poles contacting.

Which leads directly to how you might electrically magnetise the toroid to give this field structure: you need two coils, preferably with internal and external diameters corresponding roughly to those of the toroid. The axes of the coils should coincide with that of the toroid, and they would be placed touching (or almost touching) the surface of the toroid. The coils should be energised so that their axial fields are opposed.

*For the same amount of magnetic material, the field would be larger if you assembled the toroid from outwards-facing bar magnets - but this is both more complex and not relevant to magnetising the toroid.

try to point out which one is right? and draw your pic to explain. why the outsid of the toroid has a uniform divergence line is possible?

Consider the field direction caused by an ordinary cylindrical magnet in a plane that is orthogonal to the axis of the magnet, and about 1/4 of the length of the magnet outside its North pole. Obviously, the magnetic field will be radially symmetric. Also, the magnetic field has field vectors cross the plane, but they have a radial component in the plane. Now place an identical cylindrical magnet so that is is exactly reflected in the plane. If we consider the situation at the surface of the plane, the components of the fields along the axis of the magnets will cancel, but the radial components in the plane will reinforce each-other. So what is left in this plane is a purely radial field.

The divergence is not uniform - it only appears so from the direction of the field lines on the plane. The divergence in the axial direction is what allows the constraints to be satisfied.

(Short of inviting you to a demonstration using two bar magnets, a sheet of paper and some iron filings, that's the best I can do.)

I'm away from home this week, so I can't go out and do it, but make those cylindrical magnets very short, and you have the two discs of your earlier post. Cement one disc with its North pole on the center of a piece of paper, and the other disc with its North pole on a similar-sized piece of transparent plastic. Place the paper one magnet down on a horizontal surface, and sprinkle some iron filings on it and take a picture looking straight down. You'll have a picture of the radial field. Due to the angle of view, the axial component of the field won't be visible.

To be rigorous, now place the plastic one, magnet up, on top. It must be brought pretty straight down to avoid pulling the iron filings sideways. You will now have the rigorously radial field. I doubt that there would be any obvious difference between the one-magnet picture and the two-magnet picture.

In any case, I like both of your posts.

Thanks, and it is indeed no different in principle. The only reason for the alternative description is that it sometimes makes things clearer if we can separate the various effects (I hope).

Having worked with magnetic brushes in the past I believe the pictures will be slightly different, as the filings in one case will "chain" at an angle to the surface; but it could be quite a subtle effect.

too few information,

don't talk so much, maybe your competitors can hear your ideas,

better stay as you are, if you are not working, then no mistake.

see below about magnetisation of an iron ring (soft magnetic material, has a magnetic field as long as there is current flowing in the coils)

and about magnetisation of permanent magnet rings: axial or radial magnetic field.

RHABE