lab electromagnet from scratch

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Chris Bradley
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Re: lab electromagnet from scratch

Post by Chris Bradley » Tue Jun 18, 2013 7:10 pm

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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Rich Feldman
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Re: lab electromagnet from scratch

Post by Rich Feldman » Thu Jul 11, 2013 8:25 pm

>> 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
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.
Mike echo oscar whisky! I repeat! Mike echo oscar whisky, how do you copy? Over.

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Richard Hull
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Re: lab electromagnet from scratch

Post by Richard Hull » Fri Jul 12, 2013 2:57 pm

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Rich Feldman
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Re: lab electromagnet from scratch

Post by Rich Feldman » Tue Jul 23, 2013 4:59 am

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 5137 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! :-)
Mike echo oscar whisky! I repeat! Mike echo oscar whisky, how do you copy? Over.

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Richard Hull
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Re: lab electromagnet from scratch

Post by Richard Hull » Tue Jul 23, 2013 1:42 pm

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.

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Rich Feldman
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Re: lab electromagnet from scratch

Post by Rich Feldman » Fri Aug 23, 2013 11:52 pm

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.
Mike echo oscar whisky! I repeat! Mike echo oscar whisky, how do you copy? Over.

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Rich Feldman
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Re: lab electromagnet from scratch

Post by Rich Feldman » Mon Sep 23, 2013 8:50 am

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!
Mike echo oscar whisky! I repeat! Mike echo oscar whisky, how do you copy? Over.

Ross Moffett
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Re: lab electromagnet from scratch

Post by Ross Moffett » Mon Sep 23, 2013 3:29 pm

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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Richard Hull
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Re: lab electromagnet from scratch

Post by Richard Hull » Mon Sep 23, 2013 5:44 pm

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

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

Ross Moffett
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Re: lab electromagnet from scratch

Post by Ross Moffett » Mon Sep 23, 2013 6:14 pm

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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