Force/Flux between two magnets

That's a great question. I assume you have deliberately omitted the other end of each of the two magnets top and bottom. Clearly the soft iron will offer a very low reluctance path to conduct the flux from the south pole of the top magnet to the north pole of the bottom magnet. However it would not appear to have any direct effect on the field between the two magnets. I would guess that by closing the magnetic loop from top to bottom it would increase the field strength between the two magnets but I can't explain why and am now waiting for someone clever to give scientific explanations. It is necessary to use keepers when storing magnets, or their field strength will decay, so something is happening but not in a free energy sort of way.

You could build this and measure the field strength in both cases.

I assume you have deliberately omitted the other end of each of the two magnets top and bottom

I havn't omitted anything Red is the N pole Blue is the S pole each magnet has both poles shown...there is no sleight of hand or trick.
It is a straight forward honest question.

Del

You missed my point, by indicating N and S only once in your diagram someone out there might work on the basis that they are monopole magnets. (Yes I can see you have used blue and red, but from some of the comments I have read on this forum, such things are not always clear). I needed to make this point to clarify my reasoning.

Magnetic monopole! Magnetic monopole! Magnetic monopole!

..............only £50 each! Roll up! Roll up! <Cough>.

Can I buy a copy of the unified theory as well!?

Congratulations Del.... you've yet again asked a question that nobody seems to have a decent answer for

I wonder whether this thread will top the 100 posts mark??

John.

OK, I'm just a hacker here, but I have a few questions. Are they horizontal or vertical, do we still have gravity in play? In the first case, with the wood, there seems to be nothing holding those magnets from quickly snapping together. But the iron cage seems to be another story. Would the magnets not "stick" to the iron, unless they where close enough to each other to attract , wait a minute, you didn't say, if the magnets where attached to the wood and iron. That would change everything, and I'd have to think really hard to figure it out, or perhaps do an experment,

Arrrrgggg! You people are driving me crazy!

Arrgghhh. More overthink.

The magets are attached to the wood and iron respectively. Gravity is irrelevant..assume what you want.

It's a straight forward question...no catches.

Picture them a little chunky round magnets like these if it helps..

Del, by my thinking if the magnets are mounted on wood their independent fields will be curved around the magnet so reducing the field to the other magnet?

With the magnets on iron the other side of the magnet is fed through to the other magnet so making the field between the two greater...

Moving the magnets closer together will, of course, increase the field intensity...

On the other hand I could be totally wrong on this and I will be watching this for a physicist to comment!!

John.

the magnets are mounted on wood their independent fields will be curved around the magnet so reducing the field to the other magnet.
Aha...that makes sense to me.
But how much is the effect, just an edge effect? A few percent or a BIG difference...?
You've only answered half the Q so you don't get the tin of tuna prize yet...

I wish I could get my graphics tablet to work so I can draw!!!

What I'm trying to say is that I think that the field around one magnet will reduce the field towards the other magnet when using wood... think of a picture of a bar magnet with the field curving around the magnet?

with a metal holder the fields of the two magnets make the attrraction much greater between them....

One of my toys as a kid was a steam engine that could drive a very simple generator, which consisted of ONE magnet with its poles connected to metal arms that formed the outer magnetic field... obviously if one pole is not fixed to the magnet there was no current generated...

Oh dear.... I'm going in a circle!! head spinning.... must go and lie down in a darkened room....

http://www.lessemf.com/dcgauss.html or make your own and measure it - if it's a small effect in comparison to the wood only version you won't see much of a difference.

OK, I have a gauss meter on order, and am currently machining the soft iron, soon, a trip to Home Depot for the wood, I'll take the magnets out of my furnace motor, all I ask is 24 hours...

"Is the force between the two pairs of magnets the same or different?"

The force between the two magnets is the same and different. By creating a circuit with metal as shown the open magnet ends will produce a higher field density without interference from the directed fields. Field lines follow a path of least resistence. By how much? Got me there, but enough to matter in most applications. The fascinating (and frustrating) thing about magnets and magnetic fields is that the data changes considerably with the slightest change of a variable.

I believe, and maybe I'm wrong on this cause I've had a few beers... the iron will be stronger because you're basically creating a larger magnet by attaching the N to the top and the S to the bottom of your piece of iron... however the wood... well... its just a block of wood so the fields wouldn't be magnified or anything... as to how much the difference is? sorry I'm too lazy to go get my books on this one.

Again I could be totally wrong, so this is just an opinion.

Hi Del

I can't believe that after 14 replies no one has addressed your original question - the force between the two magnets.

This is a perfect place for an experiment. No Gaussmeter required.

Get 4 ( or more) identical extension springs (take some pens apart if you don't already have a selection of springs). Drill a couple of holes through one side of each U, a bit larger than the springs. Make sure the spacings are the same in the iron and the wood. Glue a spring to the magnets at each hole location, with the magnet centered over the springs. Feed the springs through the holes, and glue the other end of the springs to a flat piece of wood. when the glue is dry, Bring in the other magnet, and measure the resulting gaps. I've only included one drawing, since both the wood and the iron should be identical for a fair comparison. Tell us your results!

Nice pic. But won't the 'gap' be a break in the 'magnetic circuit' which I'm told is a 'bad thing'?

Oops! cwarner7_11 and Electroman are correct of course.

So how about this: Cut the U at its very center and add a hinge (brass hinge and screws to avoid nearly all magnetic effects. Put an extension spring on the outside, mounted as shown with sleeves to ensure a fixed screw length. To be at all scientific, this would all have to be machined to ensure correct centering, spacing, etc. There would,of course be a small air gap at the hinge, and it might take some experimenting to find the appropriate spring stiffness/screw length, but it would answer the original question.

Dick

I forgot to mention: The magnets from PM motors are usually curved, making it hard to mount them. I'd suggest magnets taken from old hard drives (or just go out and buy some - most surplus places have a variety), although many of the HD magnets will break while attempting to separate the magnet from its pole pieces. Any computer repair place has got to have stacks of dead drives.

Unless you are using EXTREMELY strong magnets, I doubt that you will have a gap to measure when using the soft iron frame...

DK, its not just the gap which isn't going to work as the magnet will be attracted to the metal, overcoming the springs!

But also because the metal U frame is so close to the opposing poles it is going to attract the field from the magnets causing an incorrect reading... i.e. the U shape should leave a large gap for the magnets to only attract each other...

The metal frame is going to act like a flux concentrator, so the effect of a metal frame will increase the force between the magnets - but by how much??

What?! Do you mean laden or unladen magnets?!

Del - wanna buy some magnets? I've just pinched them out of a free-energy device some bloke was hawking round Benfleet while he wasn't looking. Look - it's packed up now

<wheeze>.

That is truly some phenomenal winding on those bolts...er um... sophisticated electromagnets. Does the fellow you "borrowed" these from have an above unity coil winding machine?

This link may be helpful.?

→http://info.ee.surrey.ac.uk/Workshop/advice/coils/mu/

Is that looking down on the magnets, like they are all resting on a flat surface for example?

Or is it a side view, where the top magnets are above the others, and only the lower magnets are resting on a flat surface (and supported by the normal force of that surface)?

It doesn't matter, imagine they are floating in free space ... or tomato soup if it helps...

Tomato soup would help a lot. I'm very hungry <gurgle>.

Hang on. I'll see if I can find someone who gives a damn <splutter>.

How does one write-down the sound made by a finger flipping back-and-forth over one's lips...?!?

28 posts, and nobody has mentioned the definition of "Ferromagnetism"...?

The iron (being ferromagnetic) when placed into the magnetic circuit, will align the multitudinous magnetic domains within itself, thus magnifying the overall field present in the circuit...

"Yes" ~ sort-of, in one sense, a case of "free energy"... more like a synergism. (As opposed to the wood side, which has NO effect on the overall magnetic circuit ---

Yeah --- I'll beat Del to the punch on this one... "no effect... unless it's IRONWOOD!"

I thought that's what I said... except for the word 'ferromagnetism'?

John.

'Sorry', electroman... there was no offense intended... by the time I got down to your post (26) I was simply scanning for "Key Words", that would have been indicative of the definitive meaning to which I made reference.

O.P. asks: "If different, by how much?" I neglected to add that more data is needed to answer THAT question; now we must get into "the exact grade/nature of the iron... it's permeability, etc etc. Even the GEOMETRY of the iron will have an affect upon the overall magnetic circuit."

Best regards to all ~

No offense taken Tom.... and as you rightly say to estimate the forces involved is going to take a lot more information, I guess...

In the original thread Del was answering this question with regards to a dc pm motor... as the rotor - stator air gap is so small and the suggestion was to put a shim between the magnet and the metal outer case... if the shim was only 0.05 mm thick but it reduced the rotor - stator air gap by half - would the shim have a positive or negative effect on the motor's performance???

John??

The soft iron (via ferromagnetism) provides a magnetic flux path connecting the north pole of one magnet to the south pole of the other magnet. This arrangement behaves as a single magnet with twice the flux density.

Yeh, I see that the soft iron concentrates the flux around the outside, which icreases the flux density in that path ...but how does it icrease the flux density in the open path?

My impression is that the overall flux from the pair of magnets is the same...the iron concentrated the lines of flux around the outside.... I just don't know..thats why I'm asking (I'm still )
Del

Yeh, I see that the soft iron concentrates the flux around the outside, which icreases the flux density in that path ...but how does it icrease the flux density in the open path?

Hi Del,

I admit feeling uncertain about my answer. We do agree that the soft iron provides a path inside which magnetic flux lines are more densely concentrated (same number of flux lines, but concentrated into a smaller cross-sectional area). But you adroitly question what effect this has on the density of flux lines between the other two (open) ends of the magnets. I still intuitively think that the presence of the soft iron connector should increase the magnetic force. But I need to give it more thought, and also talk to some of my physics colleagues. Then hopefully I can decide on the reliability of my magnetic "intuition" (which might consist of accurate subconscious knowledge and/or mistaken subconscious belief).

Del,

I've been thinking this through. If you consider that the soft iron would be essentially turned into a magnet, then you just need to think about the magnetic poles and how the field would be oriented. If the metal bar were straightened (not over analyzing just rearranging the configuration for illustrative purposes) we would have a north and a south pole on opposite ends and opposite sides. Basically a quadropole magnet. The field should cancel itself out in the middle of the bar.

Now bend it back into the original configuration. The portion that should have any effect would be the middle of the bar. The field cancels at that point. So my perspective is that there would be no apreciable difference between the two. I think the force between the two magnets in both configurations would be about the same.

Now if we over-analyze things and we're moving sub atomic particles generated from the nuclear reactor during our previous electromchanical post, that's a different story.

Thanks, nice answer. A tad light on the quantifiables... but the most cogent so far.

So to sumarise

Lower field in the wooden circuit buit not sure by how much?

Ah...but now if we get closer to the origin of this question....

What if we take another set up with soft iron and slip a piece of thick paper between the magnet and the steel at the top and bottom and compare it with the first soft iron set up?
The magnets are slightly closer...but the soft iron link now has a small gap?

What would be the overall effect?

Del

Hi Del and others,

to continue my post 45:

if you think magnetic resistance R_mag=L_mag/(A_mag*µ_rel)

(any resistance is proportional to length and inverse proportional to area and conductivity)

in analogy to electric resistance (replace µ by Κ)

or hydraulic (viscous flow) resistance,

there can be no doubt that distributing the resistance in a circuit can be a single resistor or split into two or more, as the total resistance is kept constant: nothing will change. So putting paper or plastic beneath the magnets will not change anything!

So please past a sketch of the motors cross section, may be the conclusion is possible with this.

In an electric generator (regardless if DC or AC) there is a generator-resistance and a load-resistance. In a short circuited generator the short-circuit-current will flow (provided the fuse doesn't blow) that is limited by the generator-resistance.

In a open loop (no current) electric generator the voltage will be at maximum.

In-between the total of internal and external resistance will determine the current.

Similar in hydraulics and in magnetics:

Irrespective how you distribute the air-gaps in magnetics, if the total length is constant, nothing will change.

The magnet has the role of the generator (including source-resistance), the air-gap the role of the external resistor.

Iron, steel and other soft ferromagnetic materials have a very high magnetic conductivity (µ>>1), or very low magnetic resistance (1/µ<<1). So we can neglect these ferromagnetic parts for calculating "resistance".

So the equivalent two extremes to short-circuit current and open-loop-voltage are residual flux density (at field-strength = 0) and coercive field-strength (flux-density = 0)

Half way in-between is the optimum working point.

In principle the following has to be done:

1.:
H_mag*L_mag + H_air * L_air = 0 (valid only inside the air-gap, outside Hair is not constant and the ring-integral over H*ds has to be solved.)

2.: B_mag*A_mag = B_air*L_air

3.: B_air = µ₀ * H_air B_mag = µ₀ * µ_rel * H_mag

remember that µ_rel is a function of H_mag !

H = field-strength (amps/meter), B = flux density (volts*second/square-meter or Tesla)

4. eliminate air-terms and get an equation B_mag = - k * H_mag
(K is the slope of the shearing-line)

5. plot the magnetisation curve of the selected permanent magnetic material in the 3rd quadrant and take the data of the intersection of shearing line and magnetisation curve: this is the working point of the magnetic material.

6. Recalculate by above equs. to yield B_air and H_air

7. Estimate relation of stray-flux to useful-flux and de-rate the calculated B_air by the ratio of useful flux to total flux. (Typically to only 20 to 40%)

8. Change geometry to optimise.

9.Add the copper windings in the air-gap and calculate force per current .........

Good luck.

It will take some time to post a tutorial here.

RHABE

I edited your picture Del, perhaps the answer should be more clear now. You were totally missing your TIME COILS and just maybe you need a flux capacitor from a delorean also and maybe a "shake 'em up" flashlight.

Sorry guys, the free energy device that was posted recently and that goofy time machine, have had me laughing for days. I have been looking for any excuse to capitalize on that combined ubsurd logic in an off-topic post!

I'll finish this post yet...if my head'll quit spinning and shaking from that "simplified analogy" (apologies-in-advance).

O.P.: "Is the force between the two pairs of magnets the same or different? If different, by how much? Assume these are magnets like you'd find in a small PM DC motor. No catch ... I just want to understand."

So, we aren't talking about a motor ... just a couple magnets like you'd find IN such a thing... and, set-up like down below.

1) In "real-life" magnets, here on planet earth, lines of magnetic flux will ALWAYS form a closed loop. PERIOD.

2) Reluctance, which is the force opposing the presence of a magnetic field, is negligible in iron (compared to vacuum, air, WOOD).

[This is why the gap around electric motor rotors, armatures, etc is kept to a minimum ... otherwise "0" efficiency]

The lines of flux around the magnets on a wooden frame ... UNLESS THAT GAP BETWIXT RED & BLUE IS ONLY A MILLIMETER, more-or-less ... will simply "go-round" between each magnet as though the other wasn't there.

Those magnets on the iron frame ; because the iron is very permeable, and will thus concentrate magnetic flux very well within itself, will exhibit a magnetic circuit similar to the right-hand side above.

Since the permeability of air is extraordinarily high, the lines of flux that exist directly between the magnets will "BULGE" significantly. There will be no straight lines between them, because the air exhibits no tendency whatsoever to "contain" the flux.

Insofar as the quantifications asked for in the O.P. ~ refer to my earlier post. Every single aspect of the circuit ... the iron (its constituents... whether it was cut from rolled stock, or billet, or forged, etc), the magnets' strengths, the geometries involved (including each specific dimension), EVERYTHING plays a part.

Set up those 'experiments' vewwwy kehfuwwwy...

Well ..yes.. the origin of the question is from a discussion about motors, so this is indeed a simplifiction. Maybe if we assume the gap between the two magnets is a fe mm or there is an iron rotor wound with wire between them...it should only effect the magnitude of the effect not the fundamental physics.

If you want to get closer to the real life situation, maybe conside my previous post where we compare two set ups, both with soft iron but one with a shim of paper between each magnet and the soft iron.

The original discussion/argument (here) is about the effect of shimming the curved magnets in a PM DC motro (mabuchi type) in order to get the magnets closer to the rotor...however this introduces a gap into the magnetic circuit. I've seen it done (and done it) with no detriment to performance (and some claimed improvement) others said it would make the performance much worse or wouldn't work at all.

Del

PS. To sum up...the real problem is reconciling practical experience to theory..

Maybe a small piece of paper isn't a 'bad' enough gap to have too detrimental effect on the magnetic circuit??? After all pot cores and the like are often made of separate pieces?

Confuse a Cat

Del

"Hey" there, Del ~

replying to (your) Posts # 48, 50, AND this one (51):

Weaving-my-way back thru the "Original Threads", your post at:

http://cr4.globalspec.com/comment/195740/Re-speed-control-of-PMDC-motors

regarding the use of PAPER shims got me "worried"...

The response from RHABE at:

http://cr4.globalspec.com/comment/197547/Re-speed-control-of-PMDC-motors
saying:
"If you used a soft magnetic shim then I agree that this will boost the motors magnetic flux density giving more torque per current" ... was right-on-the-money, as per my previous post in THIS thread.

The paper shims will DETRACT from any "hoped-for" performance-gains, whereas metal shim stock will assist in providing those gains [See Rosenberg's "Electric Motor Repair", and/or "Electrical Machines and Their Applications" (John Hindmarsh) for all the 'Gory-Details'...]

The (freebie) sketches above should be worth the proverbial "ten thousand words"

Best Regards ~ [and... "like-your-style!"]

"PS" to self ~ B4 somebody screams "liar!" at me, for the statement about "a millimeter, more or less"...

Yes, there will be SOME lines of flux jumping the gap from one to the other at a greater gap... enough to even pull 2 magnets in close proximity smack-against one another.

Point-is, tho', even when very close together ... whether weak or strong, some lines of flux will still take the full trip round-about itself (each magnet), N-S.

Once the reluctance reaches a certain point ("a very small gap", we'll say), the flux sees no difference between this small gap and the "full-round-trip". Thus, the majority of the flux (or at least a "goodly-portion") takes the normal path that it would have taken in free space.

Amendment finished...

ok the problem will be that the metal will make the feilds stronger but will allow for static transfer on the case the wood will allow the feilds to pass and not let the power gain as we did this test. we use the plastic due to the structure and the fact we can cut the feilds down on the outside of the unit. wood will wllow it to pass depending on the type used. the more dense the wood the less feild on the outside. if you want to sheild it use some mumetal or permalloy to stop the feilds in the case.

as for power there are still the alignment of the Vortexes on the Neo's to put into the factor. these will acount for the feild strenght. also you have to remenber that you need to have good magnets as if there not charged right there will be a problem with the feilds on the magnets ( i want to get a sensor pack for my computer soon) this way you can get real time data on the fly.

Hi Del,

I don't have an answer for you but if you google "magnetic field software", you'll get a bunch of websites that offer magnetic field visualization software in 2D and 3D. I pulled this picture off one site.

Some of them are actually free...

no, not free energy, just free for downloading

ok, I'm clearer now. I made the mistake of assuming the lines of force would cross the gap in the diagram, I understand they don't. Let's put in a piece of soft iron to simulate the motor rotor, with an appropriate air gap at each end and assume that this diagram represents a slice through the curved pole pieces. If a metal 'C' piece is used around the outside compared to a wooden 'C' piece, will there be much of a difference? Intuitively it seems there must be, this magnetic connection is normally provided by the motor case, I imagine.

Oy vey! Such a simple question. So much my head hurts! I think the apparent (or measured) attractive force will be reduced when the soft iron is present. Essentially you have created a third magnet (albeit a weak one) which serves to pull the two magnets apart, thus reducing the apparent force. Now go back to watching March madness on BBC and don't do this no more.

Wow, I had not thought of this way. But your specific and reasonable explanation of the "third magnet" pulling on the two original magnets sounds much more convincing than my vague intuition that predicted the opposite result. So I will flip-flop on my prediction! (well at least I warned everyone up front that I was uncertain)

Don't give up so fast. I'm only about 52% sure of my answer. In fact, I originally thought there would be no difference. I'm now trying to decide between building a model of this or learning Quickfield so I can model it or (more likely) taking the lazy path and waiting for somebody who can actually calculate it to give an answer.

By the great Zeus! You are an evil fiend. In the last hour, I've switched my opinion twice. I can find a good argument for all three answers. It would be so simple if we had a magnetic monopole to measure force with (Quick, I'm gonna start a thread on why we need monopoles). I hope you sleep well tonight - I won't.

Curse you, demonic wizard! You fooled me, not once, but three times.

So, here's what I think. BTW, I mentally substituted electromagnets for permanent magnets to help me think better. Plus, I went over to the National Aviary and retrieved my neuron. So, the reluctance of the circuit with the soft iron is much less than the reluctance of the wooden circuit. Thus, for a given number of turns and Amps, I get a bigger B, and thus a much bigger force in the gap. Thus the attraction goes up. We'd have to add a few dimensions, but I'd guess maybe a factor of 2 or 3. Just a guess, mind you.

Sorry to mislead you, Sven. It was the devil (cat) that made me do it.

Now, I'll go back and see if the other question makes any sense. Then, I'll take my neuron back; they just put fresh Glenfiddich in the beaker and I hate to waste it.

Hi,

thats it, you are right.

If you think the soft iron to have "low magnetic resistance" th result is self evident.

If you add electromagnetics you will have to change the equation of ΣH_i*L_i

to ΣH_i*Li = n* I (n is number of turns, I is current)

RHABE

(Glenfiddich is a good idea to test once more after a while. I just got from the distiller my first trial of peach and plum brandy, needs some ageing, will be very good next year.)

When in doubt, use explosives... It's the answer to all of life's tough questions!!!

Forget Moonbeam. We're putting you under the care of Hacksaw Reynolds.

Thanks TVP45. I enjoyed the roller coaster ride as I too switched my opinion back and forth. Glad to hear that your conclusion, seemingly more solidly based in reason than mine, agrees with my original intuition. Now if we could just get verification from an actual experiment.

I thought of a test: sandwich a thin digital weight scale between two permanent magnets so that the magnets (opposite poles facing) exert a measurable force on the scale. Now use a soft iron flexible chain to connect the outward-facing poles of the two magnets. The scale reading should increase if your conclusion is correct. I have the magnets but not the scale. Any CR4 members want to try this experiment?

Interesting idea! I have the magnets and a scale/balance, but... Although the outside of my scale is plastic top and bottom, there is too much steel inside. The magnets attract the scale with either polarity. Anybody got a totally non-magnetic scale?

Interesting idea! I have the magnets and a scale/balance, but... Although the outside of my scale is plastic top and bottom, there is too much steel inside. The magnets attract the scale with either polarity. Anybody got a totally non-magnetic scale?

Regarding the proposed experiment, a person could try placing a non-magnetic spring (instead of a weight scale) between the two permanent magnets. The magnets' force on the spring would cause the spring's ends would move closer together in direct proportion to the force exerted by the magnets (Hooke's Law). Next, using a flexible chain of soft iron, connect the outward facing poles of the two magnets, and note what happens to the compressed spring.

I believe that is essentially equivalent to the experiment I suggested in post #34. Your chain would, I think, only enable a qualitative answer, not a quantitative one. Your soultion would of course be much faster than mine...

I believe that is essentially equivalent to the experiment I suggested in post #34. Your chain would, I think, only enable a qualitative answer, not a quantitative one. Your soultion would of course be much faster than mine...

Yes, I see now that my proposed test is very similar to what your schematic shows in message #34. The messages were coming fast and furious at that time, so I must have skimmed your message without consciously absorbing it. Sorry, my oversight. One detail still bothers me, both in regards to your proposed test and mine: they both include air gaps in the supposedly continuous soft iron path connecting the two magnets (your version has a gap at the hinge, my version has gaps between each link of the flexible chain). But if we completely eliminate the gaps, then how do we achieve the flexibility in the soft iron path that allows the two magnets to move closer together? (in order to measure the magnetic force of attraction between the two magnets, we must give them the freedom to move at least slightly towards each other). Does anyone see a way around this topological conundrum?

Yes! use strain gauges, then you can leave the metal as it is and just measure the strain.

John.

Good idea, but I believe it would only be valid if you made the wooden and metal frames of appropriate thicknesses such that both had the same stiffness. This of course would mean thicker wood, giving the strain gauge a longer lever arm for the wood... And we would have to have some way of zeroing the strain gauges, then adding the magnets (I guess we could just cement them in place, although it would be preferable to have some way of adding and removing them repeatedly.

Since my construction of #34 would have identical air gaps for both wood and iron, I believe it would be valid...

Dick

Isn't there something you could do with a wire, a current source, and an amp meter that would give you a result that you could plug into some function? Wouldn't that be a lot easier?

Ok guys...we seem to be getting no quanitative answers. Here is the picture of the sort of motor (attached to gearbox) I am talking about... 2C shaped magnets, I've drawn a white line indicating (sort of) the position of one shim.

My experience is that adding the shims certainly doesn't make performance worse and some RC Car guys believe it improves torque.

I'm not sure how much the 'magnetic circuit is disturbed ..as
a) the ends of the 2 'C' magnets are pretty close together.
b) The shim isn't going to comletey disrupt the flux...(after all if you insert a paper shim between a magnet and a pice of steel, the steel doesn't suddenly fall away from the magnet.

However how much improvement is gained by bringing the magnets closer to the rotor??? Which effect is more significant or are they equal and opposite?

I dunno.... Anyone care to offer an explicite opinion?

Del

OK, that helps, I think. So, the question is not whether the upper "C" attracts the lower "C" more/less/same but whether there is a greater/lesser/equal attraction force at the rotor?

This totally changes the picture!

Why don't you eliminate the wood/steel question entirely: use steel shim stock instead of paper. Then you have no gap on that side of the magnet. High production motors must have large enough gaps that the sum of all variables never allows the rotor to touch the magnets. When you can take the time to optimize a single motor, reducing the rotor/magnet gap (without introducing another gap elsewhere) should increase torque/Amp.

Hi Del,

please take a paper clip or any other ferrous part and measure around one magnet

how many poles are magnetised.

These magnets come in many different magnetisations.

After sintering the Ba,Sr,Pb-Oxide - Fe-oxide mixture the raw magnets are magnetised with a big pulse of current in a coil of a few turns.

This coil can be so that magnetisation of one magnet is only one direction: radial in or out or multiple pole one after the next in .. out ... in ... out.

So to have a chance of stray flux estimation the number of poles in a magnet is needed.

And: the foto is not totally clear: is this a rotor with windings inside iron, both moving? Or is there a fixed nonmoving iron part inside the moving coil?

The first type has usually a small airgap the other type has a much bigger airgap: the windings plus the inner and outer distance to allow rotary movement despite imperfections and thermal distortions.

So the first type is usually dimensioned with an airgap much smaller as the magnets radial length thus giving a flux density near the remanent flux density of the magnet.

Whereas the moving coil motor (all iron and magnet in stator) usually has an airgap of same radial length as the magnet giving a flux density of near 50% of remanent B.

In these (second type) magnets the shearing line should have a slope of -1, and the areas of magnets and coils are nearly equal, so the length of both shall be equal to get best performance.

As this cannot be achieved in the first type and as the ferrite magnets are pretty cheap the magnets in the first version deviate from this optimum condition and can generate (by copious use of magnetic material) a flux-density that is nearly twice as high as in the optimum layout. With a magnet thickness that can be handled easily and 2 to 5 mm more diameter than in an optimum version.

So optimum is once more relative.

RHABE

You're question would be so easy to answer if you included the concept of phlogiston.

Been reading this one from the side for a few days, was looking for one of my "old" uni texts on electromechanical energy conversion until I saw two things.

Those being the mention of Mabuchi motors and the picture of the motor.

Del, please remove a magnet from the motor and confirm where N and S are. I think the "C" shaped magents may be N at one end and S at the other.

Hint. If the magnets can be interchanged, then they are probably "end polarised" rather than "face polarised".

If they are end polarised, then the effect that you are seeing with increased torque probably relates to the inverse square of the distance between the magnet sets. (The paper makes the stator magnets closer to the rotor magnets and so the attraction goes up.)

I'm pretty sure they are 'face polarised'.. I shall pull 'em out later to check.
( But it's cold and snowing...and I'd have to go out to my garage...and my toast has jsut popped and the kettle boiled )

Del

Here we are..the magnets are indeed 'Face Polarised'.

Del

Hmmmm when I saw the picture of the motor I thought I bet the magnets have the poles at each end of the 'C' section... giving a four pole motor.

Now you've said they are definitely face polarised, I'm back at square one...

I still think that moving the magnets towards the rotor, maybe halving the air gap, will only be a tiny bit offset by the shim increasing the magnetic path round by a tiny amount... In other words there should be a noticeable improvement...

But Idon't have the theory only a gut feeling.

John.

Now that I understand the construction (at least for the next 21 seconds), when you shim it, the total air gap remains about the same (it'll fringe differently, but...) so the reluctance (and the magnetic induction) should stay about the same. You are getting closer to the armature and the attraction should go up as 1/x (my vote) or 1/x² (a previous vote). In either case, the improvement should be slight and probably not measurable by ordinary means.