lab electromagnet from scratch

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Rich Feldman
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lab electromagnet from scratch

Post by Rich Feldman »

We've had occasional chatter here about using "large" NIB magnets to generate axial magnetic fields in disk-shaped spaces, as used in cyclotrons.
Among other contributors, George S. has posted some pictures, Chris B. has posted simulations and inside-out variants, and Richard H. has talked about how to safely position a pair of magnets onto a flux yoke. viewtopic.php?f=15&t=7236&p=51755

Now I'm about to get my hands dirty doing it the old-fashioned way, using many thousands of ampere-turns driven by wallplug power.Good chance to explore and demonstrate practical scrounging and DIY construction ideas for people with cyclotron dreams and tight budgets.
(The cyclotron lab at Houghton College uses a 6-inch commercial magnet from GMW, which weighs over 1,000 pounds and lists for about $30,000.)
Another end result is a touchable demonstration of how many watts of electricity it takes for a resistive magnet to match a high-energy rare earth magnet of a given size. (The dimensional scaling of electromagnets, as with motors and transformers, makes I^2R losses become relatively smaller as the size goes up.)

My current plan is to start with a pilot scale unit. Three-inch coil assembly ID and pole diameter. Will aim for a 1 tesla field in a 1 inch gap, with a sensible trade-off between electric power requirement and the mass of copper or aluminum conductor (whose product is invariant if average turn diameter and material are held constant). Likewise, the voltage-current point will be determined by economics of power supply electronics and the cross-sectional areas of scroungeable conductors. H(air) x length(air) is about 20 kA; the only electrical benefit of small pole diameter is reduction of average turn length, voltage, and power. But steel mass can come down almost as the 3rd power of pole diameter, and 2 local dealers want a whole 60 cents/lb for used steel.

I'm pretty comfortable with the application of Ampere's Law to electromagnetic circuits.
So today my questions are about the permanent magnet options.
1. Can anyone give us the dimensions and measured field strength of an actual PM solution?
2. How does the field strength vary with gap length and magnet length, as shown on 2 axes in attached drawing? (There must be tutorials for industrial designers out there! )
3. Has anyone here bought N45, N50, etc. magnets and verified the purported energy product by magnetic measurements? It seems so easy for vendors on ebay etc. to overrate their product a bit, as with the optical power of lasers marketed to kids who want to burn stuff.
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Chris Bradley
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Re: lab electromagnet from scratch

Post by Chris Bradley »

Rich,

You'll find that I've answered and/or already given leads on how to determine those questions, but I'll go over them again here, and see if I can describe how I look at this problem, step by step.

However, just a mention on your electromagnet design first - depending on what you want the field to do, you can also use focussing poles. So if the windings wrap around a pole piece where it has a diameter D, but the piece then necks down conically to a diameter D/2, then you will increase the field level by x4.

But focussing a field to increase the strength with conical poles is not used for cyclotrons (excepting to shape the fields at the edge) because if you crunch the maths, you'll find that it makes no difference to the energy of particles you can accelerate - for a higher field strength it is only in proportion to an increase in the required centripetal force for a given particle velocity/energy. So instead using a focussed field only makes the applied frequencies higher and the engineering for the cyclotron itself smaller, which may be advantages or disadvantages depending on your objectives.

I am not convinced that there is an optimised way of balancing total conductor mass versus power like you said. Whenever I have done the calculation, whatever you do to increase the conductor thickness you reduce turns in direct proportion. All that does is give you control on the voltage/current combination, but not the power.

Your questions -

1. If you want actual measured values on my inverted yoke, then do the search and I'm sure you'll find them. The results tallied with the predictions that Maxwell SV gave me, close enough (which is a freeware student version of Maxwell which I recommend to you to do your analyses).

2. I will talk in 'practical' expressions here fit for an amateur builder, rather than explicitly scientific: A PM magnet has a certain amount of 'magnetic energy' locked into it. Magnetic fields can do no work, so that energy is never 'taken out' of the magnet. All that happens is that the magnetic energy in the magnet is re-distributed around it, according to the relative permeabilities of material in that space.

A given magnetic field in a given space [including the space of the magnet] *is* a given energy. The energy density is defined as B^2/2u (where B is the magnetic flux density [Teslas], u is the permeability). Now imagine you build a yoke like the ones you show here, with a gap of two inches and around one inch wide. Say you attach a magnet of one inch thickness and width to one of the poles so that there is a one inch gap left. What is the field strength?

OK, so imagine how the magnetic flux circuit flows around the yoke. There will be the same flux all around, it's like current the same all around. But now imagine the different parts are like resistors, and the air gap is the bigger resistor where the 'current' does the 'work'. So the magnet has this fixed energy that it will 'share out' to wherever its flux flows. In the metal yoke, the u, permeability, is very high, maybe 10,000's [relative permeability], so for a given B (which we are aiming to work out what it is) the magnetic energy in the yoke is very low. Whereas in the air gap, the u is 1, so the energy of the field in the gap is very high. Remember the B is the same all around the circuit. It's a magnetic flux density. It's like a superconducting current flux (it does no work). Let's say the flux path in the yoke is 10 inches and the u is 10,000. So that means the air gap energy is 99.9% of the total magnetic energy in the loop. We'll ignore the yoke from now on (of course, you can only do this if the yoke is made from high u material).

3. So now we examine the magnet's performance. Answering Q3, you can be sure that neodymium magnets you buy are rated for around the 1.1 to 1.3T range, and that they are like that, because it is simply down to the way that they are made and the nature of the materials. Let's say it is 1.1T, so it's energy density is “1.1T per magnet volume”. Now, IF that 1 inch gap was completely filled with the magnetic flux from the magnet as it flowed around that magnetic circuit, and there was no stray fields bulging outwards, then that'd mean the field level would be approx 0.55T, because that is simply the same volume as the magnet in the same space, and most of the energy of the circuit is focussed in the air gap and the magnet.

It's 0.55T because the permeability of the magnet itself is around 1, pretty much the same as the air gap. It's a bit like a current source with a big internal resistance. It has its own 'u'. The 1.1T 'current' that you see in magnet specs is equivalent to the 'short circuit/maximum current', so to speak. So whatever you do to make the magnetic flux flow around as smoothly and as unobstructed as you can, the magnet itself will limit how much energy density can flow into any air gap because it has only so much energy to 'give out', some of which it 'needs' to support the fields within it, itself.

So if you made the gap 2 inches for a one inch high magnet, the field would be ~0.36 T (ignoring field fringing effects). But if you put two one inch magnets on top of each other and a one inch gap, then there would be '2 units' worth of 1.1T/per inch of flux path now 'flowing' through 3 inches (viz. the two magnets and the one inch air gap). So now the flux would be ~0.7T.

This is saying that the flux through the magnet itself changes, according to the materials around it. A '1.1T magnet' only actually has 1.1T flux in it when it is clamped into a high permeability yoke with no air gaps. If you take a direct measurement of its surface, you might measure the fields as they 'short circuit' directly back into the magnet, and they don't do this evenly, depending on the magnet geometry. So if you had a 'perfect' magnetometer which did not interfere with the field itself, you'd find 'dead spots' on the surface of the magnet where the flux line is heading straight out of the magnet on some wild, long distance path through free space.

Let's go back to the one magnet scenario with one inch air gap. The bulging fields might double the volume if the magnet is one inch wide across a one inch gap. This would result in a, possibly disappointing, ~0.28T for those expecting to 'see a 1.1T field'. If you close that gap up, the relative percentage of 'bulge' to actual air space filled with a field would come down progressively until it can be almost neglected. Say you have a ¼ inch air gap. So now the 'one inch's worth of magnetic energy is being spread around 1.25 inch of permeability = 1 (we are still ignoring the energy in the rest of the yoke, because it will be relatively so low providing it is a high permeability). So now we'll get 1.1T x 4/5 = ~0.9T.

To hang some real numbers off this, sintered neo magnets are around the 300kJ/m^3 range (~N38). If we want to estimate the flux in a yoked air gap of 10 cm^3 total volume, and the total magnetic material in the yoke is 20 cm^3 of 300kJ/m type, then the total magnetic energy in the circuit = 300kJ x 0.00002 = 6J.

We use the formula E = 6J = B^2/2u, where u = 4x10^-7 pi. So B = sqrt{(6 x 2 x 4x10^-7 pi)] /0.00003} = ~0.7T.

Note on units: The energy content of magnetic materials is often quoted as Mega Gauss Oersted MGO. I believe the conversion is 1 MGOe = ~7.95 kJ/m^3. 'N38' means 38 MGO, so by definition, for example, N45 would be 357 kJ/m^3. If the energy content of a neo is 357 kJ/m^3, that means it is N45, and if it isn't then it isn't an N45!

(Just for completeness, here are the temperature ratings too: H = 120C, SH = 150C, UH = 180C & EH = 200C. Now you know what 'N45 SH' actually means!)

I've not seen discussions on magnets in such simple terms like this. I hope it helps serve to 'demystify' the workings of permanent magnets!
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Re: lab electromagnet from scratch

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Thanks for the fast and detailed response, Chris.
Looks like we share a delight in teaching. In presenting apparently complicated subjects as simply as possible (but, as Einstein said, no simpler than that).

0. Regarding the tradeoff between electromagnet conductor mass and power: If you double the coil cross-sectional area without increasing the average turn length, then you have halved the electrical resistance. So same current with half the voltage, if the doubling were of the wire area. Or use twice as many turns of the original wire gauge, so 2x the resistance and half the current at same voltage. A point to take home: for given coil diameter and ampere-turn requirement, voltage depends on wire gauge and not on the number of turns.

As we reduce the amount of copper or aluminum to save money, the cost of power and power supply stuff goes up. Power density and cooling technology requirement go up as the square. Inspired by the interest of young Noah H., I recently held forth on this subject in another forum. http://4hv.org/e107_plugins/forum/forum ... ost_153043

By the way... It's well known that aluminum has a specific electrical conductivity twice that of copper, and enjoys an even greater edge in conductivity per unit of cost (I'll say dollar). In electromagnets that makes the coils more bulky & forces yoke to be longer & heavier. I found some studies, ranging from the 1910's to 1990's, exploring metallic sodium as an electrical conductor. It stands far ahead of the pack in conductivity per dollar of metal. The more recent study had it filling polyethylene tubes, and investigated things like fire hazard when breached. The older one had Na filling long steel pipes, and studied things like how to make and break joints in the field.

1. I will go back and read your inverted yoke thread more thoroughly. Browsing the archives is more troublesome on the new platform, because of broken links and new names. George Schemermund now appears to be delta9. As an alternative to learning Maxwell, do you know if FEMM comes with realistic permanent magnet material models?

2. Your tutorial about permanent magnet energies was great. Shows why rare earth magnets are necessarily made in various lengths. Since it's time to get up to speed on permanent magnet materials in "circuits", I will try to make a picture with a BH curve and a gap-dependent load line. They'll intersect at some operating point in the second quadrant. Might be used to illustrate why Alnico magnets can be partly demagnetized by disassembly of the yoke system.

3. I confirmed your conversion factor of 7.95 kJ/m^3 per MGO -- the exact value in SI units is 25000/pi, the reciprocal of 4e-5 * pi . An example point would be 1e4 gauss (= 1 T) times 100 Oe (= 7958 A/m).
Let me rephrase my original question 3. We know that N45 material means the BH curve reaches 45 MGO (around the knee in demagnetization quadrant). Can probably find that BH curve on some OEM website. Suppose we bought such a magnet from ebay vendor X, and suspected that he was selling lower grade stock as N45 -- what confirming measurements have been made by forum readers?
You talked about magnets sold with specifications of surface field strenth (or lifting power), and showed how those can be derived from BH curves and geometry.

Time to quit blabbing while the sun shines.
Regards,
Rich
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Re: lab electromagnet from scratch

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I couldn't imagine a more interesting an concise discussion related to Rich's questions than that supplied by Chris. I actually sold a video tape back in my Tesla days that I did called "minimal magnetics", following much of Chris' explanations with demos.

Permanent magnetic technology, the making and the doing, is a rather black art and the fabulous, tell all, book, suitable for a real permanent magnetics engineer, is Moskowitz's book "Permanent Magnet Design and Application Handbook". (Expensive $211.00 Amazon) and might be out of print, though Lindsey books once carried it.) My copy cost me $75.00 back in 1992. There is a huge gap and different world view between permanent magnets and electromagnets. Yes, the actual "magnetic circuitry" is the same, but they are different worlds when it comes to the making and the doing.

I would be stunned if you could put a full, continuous tesla in a 1" air gap, electromagnetically, with a 3" electromagnet. Pulsed, yes, you could do that and perhaps even water cooled intermittently. A tesla is no mean feat in the world of electromagnets of any significant gap.... and 1 inch is a significant gap! 1 inch gap in a magnetic circuit is like a 10 megohm resistor in an electric circuit. Their are analogies between them but you can overdo such analogies.

Good luck with your efforts.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
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Re: lab electromagnet from scratch

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I appreciate the credit, Richard, thanks, as it took a while to write because I was aiming for it to be as useful to others as I could make it.
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Re: lab electromagnet from scratch

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I had a period of magnetic enthrallment. Spent $1000.00 in 1997 for a first quality Bell gauss meter with both axial and transverse probes. I recently found, at a hamfest, a Bell calibrated test standard block for a transverse probe of 1.0003 k gauss. Wow! It cost me $2.00.

You just have to learn to "see at sight" at these events.... A term I coined, meaning that you need a wide net of stored visual experience so that you just don't glance over a table of offerings, but key to specific mental images of goodies you have seen in the past.

Magnetism, especially permanent, is certainly incredibly fascinating and a form of potential energy that is not extant in and of itself in nature and, like all electromagnetism, light, radio waves, etc., has its origins with true innate potential energy of primary particulate charge....charge that has been placed in motion. Charge in motion can create all the energy we see and use, but no magnetic/electromagnetic scenario can create nascent charge. Dyanmic-kinematic magnetism can cause already extant charge to move about, however. Interesting stuff for sure.

I keeping with my past posts, lIght and all EM radiation is a secondary force and not a primary nascent energy/force in the universe.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: lab electromagnet from scratch

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Sorry to have invited a permanent magnet discussion, in OP of a thread whose title is about ELECTRO magnets.
(Thanks to Chris's non-scary introduction, I now have a fair understanding of permanent magnet circuits. It was easy to follow up by reading websites of respectable PM vendors. For example, there are well-commented BH curves with load lines here, http://www.kjmagnetics.com/blog.asp?p=t ... um-magnets in discussion of why max operating temperature depends on magnet shape! )

Now back to the project announced in OP, for which I have not yet bought any material.
Here is a 75 mm classroom electromagnet, just under the nominal size (3 inch) that I'm aiming for.
svs_magnet.JPG
svs_magnet.JPG (16.72 KiB) Viewed 19528 times
http://www.svslabs.com/Products/Product ... tegoryID=1
The EMU-75 spec claims 11 kG in a 10 mm air-gap between flat pole pieces, using 270 watts of electricity. The designers kept the power down by using a ton of copper, figuratively speaking, in two great big air-cooled coils. A longish coil aspect ratio contributes to efficiency, so the pole pieces and connecting yoke are correspondingly long and heavy. I haven't tried to reverse-engineer the circuit and coils.

The same power should develop 10 kG (one tesla) in an 11mm air gap. To increase the 1-tesla gap to a full inch, current would have to increase by the length factor, 2.3. The power would go up by the square of that factor, to 1440 watts. Inertial cooling ought to allow at least a minute of operation at that power. Wire temperature could be monitored by the increasing voltage needed to keep the current constant.

- - -
My design was inspired by a couple of rectangular plates of cold rolled steel, found on a remnants shelf. They're 3/4 by 5-3/4 inches, and almost 19 inches long. Just thick enough to serve as the ends of an H-frame, spreading the flux from 3-inch round pole pieces without bottlenecking. Here's a scale drawing, with some annotation of current and flux paths for discussion next time. Grid and dimensions are in inches (as is most metal stock the USA).
3in_magnet.JPG
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Re: lab electromagnet from scratch

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You now see the issues in getting a Tesla in a large gap. Doable with a lot of iron and copper, a KW or two and maybe a bit of cooling or short duration operational periods. It's all about ampere-turns, circuit permeability and cross section.... Good luck on your efforts.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: lab electromagnet from scratch

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Richard Hull wrote:You now see the issues in getting a Tesla in a large gap. Doable with a lot of iron and copper, a KW or two and maybe a bit of cooling or short duration operational periods. It's all about ampere-turns, circuit permeability and cross section.... Good luck on your efforts. Richard Hull
Thanks.
I knew the job was dangerous when I took it. (wink to Super Chicken, for those of a certain age).
http://www.digital-sledgehammer.com/sup ... gerous.wav
http://www.televisiontunes.com/Super_Chicken.html

Don't want to cut metal for pole pieces, until the deal for 55 lbs of 0.012 x 4 inch aluminum coil is sealed.

This is about exploring ways to minimize cost and fabrication effort.

George Schemermund expressed the same spirit in discussion about another yoke:
viewtopic.php?f=15&t=7236&start=20#p51771

So how can we adjust the pole gap? There's no provision for that in previous drawing.
It would not be easy for me to make a close-fitting hole in an end plate, in which a pole piece could slide.
[*]Plan 1 was to depend on interchangeable pole caps, a method which also allows different tip areas (as Chris alluded to).
[*]Plan 2 uses interchangeable spacers in series with the flux bars parallel to coil axis. Spacers can be made from bar or plate stock, which comes with two surfaces flat and parallel enough. The flux bars can still be bundles of scrounged bedframe angle iron, or rebar. :-)
[*]Plan 3 is infinitely adjustable, but flux bar parts need 2 flat & smooth surfaces at right angles.
[*]Plan 4 is also infinitely adjustable. Flux bars need one flat smooth surface, but interfere with access to the gap.
pole_adjustment.JPG
By the way, the last two need adjuster screws that can resist the magnetic attraction between the pole pieces.
At 1 tesla it's tolerably close to 60 psi or 400 kPa. That value, and its dimensional unit of pressure,
are identical to the air gap energy per unit volume --
J/m^3. The formula is BH/2, or as Chris said, B^2/2u.

Any better ideas, or important caveats from the experts?
Someday I'd like to try using rectangular plates that are stacked in weight machines at the gym. "Cast iron" apparently saturates at only around 1 tesla, but that can be compensated by using more of it.
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Re: lab electromagnet from scratch

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For Continous, Non-alternating current, many highly permeable carbon and silicon steels are available. Cast iron is OK, but as you are shooting for max flux, it is a weaker choice.

The pros almost always set up variable gap sections in max flux situations with interchangable pole pieces or pole inserts. Even though the gap appears to be zero with additions, there is still fringing and added reluctance losses unless mirror like near quater-wavelength flat finishes are maintained at joints. This is the cheapest high flux gap adjustment solution.

Sliding and continuously adjusting gaps, to be effective, demand special custom castings and milled components, more power and continuous cooling to maintain high flux in the gap.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: lab electromagnet from scratch

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The yoke pieces look a bit thin to my eye. It depends on the material as to whether they saturate. Your original diagrams look more like what I would expect to see.

(An aside; for my 'inverted yoke' I deliberately used thin mild steel so the plates did saturate near the magnets, which caused a more uniform field in the working space, rather than have a concentrated field around the magnets. (I tried some pieces of borrowed mu metal, and the field uniformity was very poor.) That won't apply in a conventional yoke like this.)

Your adjustable pole pieces will likely not need any significant mechanical fixing. Once the induction field is applied, they'll stay well in place and not jump off the yoke. So you could simply have removable coils and pole pieces of variable length, then just have a hole through the yoke and tap a thread into the back of the pole pieces to hold it in place with a small bolt.

Maybe fix the upper permanently, with a given pole piece length, and vary the lower one. You'd slide the coil and the pole piece out, together.

I'm still unclear what you want this for ... it might help with suggestions. Is it for a 'micro-tron' of some description?
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Re: lab electromagnet from scratch

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>> I'm still unclear what you want this for ... it might help with suggestions. Is it for a 'micro-tron' of some description?

What do people want fusors for? This magnet project is to:
* get my hands dirty
* test my purported knowledge of E & M, engineering, and practical scrounging
* explore the low-cost low-power corner of magnets producing 1 tesla in 1 inch air gap.

Have previously claimed that for fixed ampere-turns, average coil diameter, and material: the product of conductor mass and electrical power is invariant.
Just ordered my conductor: about 70 lbs of aluminum at a bit more than $1.50 per pound. Here's half of it: http://www.ebay.com/itm/321162065864
coil_small.JPG
coil_small.JPG (28.52 KiB) Viewed 14405 times
It's 1100 composition, about 5.25 inches wide, 0.007 thick,
with a separable layer of 0.0025 plastic film which -might- serve as inter-turn insulation as received !

Here are the preliminary design numbers.
Magnet will have 2 coils very close to the size of the spool in picture, wired in series.
22,000 ampere-turns
760 turns x 29 amperes
Then at 20 degrees C:
0.6 ohms
17 volts
496 watts
Given the large exposed area on annular surfaces of each coil, this might be able to run continuously with forced air cooling.
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Re: lab electromagnet from scratch

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Nice shot at making your magnet! I can't wait to see this thing perform. Smart move on the thin Al conductor. You might watch out for inductive kick back faulting your insulation. The insulation should extend a bit beyond the foil's edges, ideally. A very slow bring up with a variac and an ultra slow wind down on the variac at shut down will save the insulation. A 100 amp 1kv reversing diode across the coil would also help.

All the best.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: lab electromagnet from scratch

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My aluminum conductors arrived last week in coil form, almost perfectly filling two USPS flat rate boxes.
One is bigger and heavier than the other, and in much better shape (literally). Maybe that one can be used as a 400 turn electromagnet coil without being rewound. A 3" diameter steel pole piece, not yet procured, might be persuaded to slide through that core with no layers of cardboard peeled off.
DSCN6759.JPG
I immediately cut off a sample from the larger coil, on which to measure the material's thickness, weight, and electrical resistance.

The aluminum has a clear anodized coating on both sides, which I scraped away in a few places for electrical connections.
Questions up front. Can anyone cite experience to complement my Internet research on:
How to strip anodized coatings without rapidly etching the aluminum?
What fluxes and filler metals can solder aluminum at temperatures below 600F or 300C?

Here the sample is conducting 5 amperes from a benchtop power supply. Voltage drop is measured with a fancy DMM that can resolve microvolts. Keeping one meter probe at a fixed location, I used the other (with sharp pointy tip) to penetrate the anodized coating and map the potential.
DSCN6766.JPG
These contours are 1/2 millivolt apart, so the resistance between lines is 0.1 milliohms. The sample is barely long enough to demonstrate a rule of thumb for current spreading: Current density is practically uniform at places more than about 1 strip width away from a point source.
The sheet resistance worked out to be 0.24 milliohms per square, an unexpectedly high value. (Another rule of thumb: 1 ounce copper foil is 1/2 milliohm per square.) If my Al were 7 mils thick then its resistivity would be 427e-8 ohm-cm, a plausible value for 3004 alloy. But this is supposed to be 1100, practically pure Al, at around 300e-8 ohm-cm.
unimet_comp.jpeg
unimet_comp.jpeg (30.51 KiB) Viewed 14154 times
By the way, it's easy to remember the value for 100% IACS, a popular and practical reference value for stating the conductivity of metals. The International Annealed Copper Standard, which is 100 years old in 2013, adopted a standard resistance at 20 degrees C of a copper wire 1 meter long and 1 mm^2 in area: 1/58 ohm. Today we'd say 58 megasiemens per meter. It works out to 172.4e-8 ohm-cm. Modern copper wire routinely exceeds 101% IACS.

The discrepancy was resolved by careful thickness measurements, and some investigation of the maker's label inside the cores. Overall thickness is about 9.5 mils (0.24 mm), including the 2.0 mil (0.05 mm) clear plastic film. Originally I, like the used metal vendor, had peeled back the film and measured 7.0 mils. But that includes an adhesive layer that takes up 2.5 mils (0.06 mm). When that's cleaned off, the metal thickness is only about 5.0 mils (0.13 mm), consistent with resistivity of 305e-8 ohm-cm.
The labels inside the core say Adhesive Research, which is still in business. There's a special part number, but I think what I got is closely related to ARclad 5795 "EMI shielding foil". http://www.adhesivesresearch.com/Docume ... 0Sheet.pdf

So I got only about 28 lbs of Al in a 38 lb coil. The magnet power estimate must be revised upward, unless I want to strip that adhesive and use a thinner insulating film. On the other hand, original power estimate based on 7 mils used a very conservative value for resistivity.

Let's close with the one indirect measurement that's probably accurate to within 1%.
As wound, the thickness per layer is 9.73 mils (0.247 mm). I measured the metal ID and OD, and counted the 401 layers like rings on a tree.
DSCN0291_count2.JPG
The same measurements tell us that the strip is 803 feet long (1676 squares, so about 0.40 ohms). The AR label says the coil originally held 1200 lineal feet.

Thank y'all for reading this far. Both of you! :-)
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Re: lab electromagnet from scratch

Post by Richard Hull »

I'd leave the anodization on the foil. It is not electrically important. For contact, you can scrap a bit. There are fluoride based fluxes and low melting point solders made especially for aluminum, search around. You could always use thin Al or Cu strips bolted to the ends of your coil using 2-56 hardware if soldering doesn't work out.

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Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
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Re: lab electromagnet from scratch

Post by Rich Feldman »

OK, time for a one-month update.

On August 14, I made the first steel purchase for this project.
Some rusty old 3 inch diameter HRS, in two pieces 7 inches long.
That's to allow some working room on both ends of the 5.75-inch-long coils.
It fits through the core of the "good" coil, now that I have removed the innermost ply of cardboard (and the attached A.R. label).
DSCN6892.JPG
Saw cuts at the shop cost $5 each, and took about 1 minute each.
I was pleased that they produced surfaces within about 1/16" of being square (to the rod axis). Perpendicular would be a better word, because the cut surfaces are round.
The available tool for rough machining to flat-and-parallelness was a Bridgeport.
IMG_0738.JPG
Next step is to contrive a closed flux path and a temporary drive coil.
Will find out how many ampere-turns it takes to magnetically saturate these parts.
I think that will have to be done with slowly changing current (not 60 Hz) because of eddy currents in the steel.
On the bright side, those eddy currents might usefully reduce the flux ripple when coils are driven with rectified 60 Hz AC.
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Re: lab electromagnet from scratch

Post by Rich Feldman »

Another 23rd day of month, another update.

This will be a tale of two new coils, but first: an updated drawing of the proposed 3 inch pilot project.
1 inch per grid square. Aspect ratio is unusually skinny because there's enough conductor for a 6 inch magnet
but much less steel. Whole system should weigh less than me, and not need much heavy machining.
big3inch2.JPG
Design evolution is explained in earlier posts. Starting point was steel end plates 0.75" thick.
That drove choice of pole diameter = 3". Coil length and diameters are driven by the dimensions of adhesive-coated aluminum strip that I found at a surplus dealer by email inquiry. These particular coils, it turns out, are about 40% insulation. That increases the average turn length and reduces the number of turns, for a double penalty in electrical power per ampere-turn squared. But it only cost about $2 per pound of Al, like getting Cu at $1/lb.

The "plain old steel" will be characterized in a closed path test, with the pole pieces side by side.
DSCN7079.JPG
We expect the endplates to saturate before the poles, because their cross-section now has to carry the entire flux instead of just half.
My temporary drive coil is a 100 foot 3-conductor extension cord, on a bobbin made from nominal 3" ABS pipe and some pressboard annuli.
DSCN7053.JPG
DSCN7068.JPG
Got 5 layers of 14 turns, as planned, plus 2 extra turns. That's 216 turns of AWG14 wire (packed even less densely than the aluminum strip). Expected total resistance is about 3/4 ohm. 12 volts should be much more than enough to drive the steel to saturation at the bottleneck.

More later. Do any readers care about this much detail? Want to know how much the steel rod is oversize and non-round, and what that meant for spoolmaking? Would it make sense to start a blog for this stuff, instead of having to write it up for 3 forums? Any hints about simple blogging for beginners? Thanks!
All models are wrong; some models are useful. -- George Box
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Re: lab electromagnet from scratch

Post by Ross Moffett »

Rich, I imagine you have enough content to make your own website. They're inexpensive and available with lots of free templates, even if you go ad-free. If you don't go ad-free, you might even have some financial assistance for your projects by posting them to sites like hackaday.com and makezine.com after they're finished. The traffic is substantial. Nearly four years ago in my final semester of college I hacked my Rigol oscilloscope to change its bandwidth from 50 MHz to 100 MHz (a software / hardware restriction used to sell the same hardware as two models). To this day I still run into total strangers and if oscilloscopes come up, they'll say something like, "You should get a Rigol, you can hack it." I posted it to the eevblog forum, may have missed out on some bank!

I find your thread interesting, and I'm sure a lot of others do as well.
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Re: lab electromagnet from scratch

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Very interesting. This could all be in the construction forum in a single thread. You are making something here.

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Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: lab electromagnet from scratch

Post by Ross Moffett »

The forum title is "Other Forms of Fusion - Theory, Construction, Discussion, URLs." So as to not derail Rich's thread, I started a new one here.
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Re: lab electromagnet from scratch

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This forum is fine, especially if it is to be part of a cyclotron or related to some planned fusion effort.

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Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
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Re: lab electromagnet from scratch

Post by Rich Feldman »

First Pull. Seems like a milestone for any electromagnet.
I got there last night, quick and dirty, after weeks of not having time to do it scientifically enough. Measuring BH curves can wait.

In the meantime, I had measured the orange coil's DC resistance: a disappointing 0.99 ohms. While measuring that with no iron core, I got some tiny sparks and, later, made a neon lamp flash. Still want to compute (and measure) the air-core inductance.
DSCN7496.JPG
In the pictured setup, with a current of 1.28 amps and power input of 1.62 watts,
I was able (with care) to lift the whole rig while holding only the top plate.
So lifting force was at least 63 pounds, over 14.5 square inches, amounting to around 4.3 psi.
For that pull, the required B is about 1/4 tesla in the round parts (and slightly more in the end plates).
Figuring the magnetic path length to be about 0.7 meters, and knowing there are 216 turns of wire, the average relative permeability (including air gaps) is around 600. That's plausible.

Next step is to trace a BH curve well into saturation, with this flux path and this coil. For that, I want a bipolar adjustable supply that can do 12 V and 12 A. Am planning to build my first H-bridge inverter, unless someone has a better idea. Anyone want to talk about high-side gate drive methods, not involving GDT's?

[edit] Uh, it'll be simpler to try an adjustable unipolar benchtop supply, with V and I meters, that can do 10 amps. Just need to configure a reversing switch (and will wire in some clamp diodes).
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Re: lab electromagnet from scratch

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At 275 amp-turns that is hard to fathom! It is tough to believe you got a flux of 2500 gauss. Any flux meter on hand to check that? You have a lot of surface area though. That meant that you only put around 1 volt into the system! I would have thought it would have taken a few thousand amp-turns?!

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Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
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Re: lab electromagnet from scratch

Post by Rich Feldman »

Well let's see. 276 amp-turns along 0.7 meters (perimeter of 7" square) is 395 amps/meter = 5 oersteds.
Permeability of 500 would give us 2500 gauss.

Remember the pull strength, and the electric power, are proportional to the SQUARE of the flux density. And the only air gaps in this case are those from crudely finished mating surfaces. (Too bad nobody has invented a ferrofluid or ferroputty with Bsat much above 0.2 T.)

My announced target of 1 tesla through 1 inch of air WILL need more than 20,000 ampere turns. I hope to get that with 25 amps in 800 turns, using different coils whose total resistance is less than that of this orange one. The more metal, the less resistance.
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Re: lab electromagnet from scratch

Post by Richard Hull »

Yeah, I forgot this is a completely closed path with decent permeance and just a watt of two in the right coil will give a tremendous flux level within the core and mechanical separation of a core path element would be a bitch.

It's that nasty permeability in an air gap that cuts the flux to nothing in the gap, requiring a lot more amp turns.
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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