lab electromagnet from scratch

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Rich Feldman
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Re: lab electromagnet from scratch

Post by Rich Feldman » Sun Jul 05, 2015 12:13 am

For a few weeks I was afraid I'd blown something up. Indicated current got squirrely after hitting 7 A. While troubleshooting, soon even the little motor wouldn't go in either direction. Had too-rapid voltage reduction kicked magnet energy back into DC PS? Had to wait before there was time for fresh, systematic attention. Seems I had hit an unexpectedly low current limit setting in amplifier as received (it has fast and slow limits). Also I was using an intermittent control knob.

With a new self-powered and filtered control knob, I got power satisfaction on the 4th of July. For loads of around 1 ohm, the knob goes from -15 A to +15 A. Here's the new, simplified configuration in schematic form:
magnet_sch1.PNG
and for real:
DSCN9902cr.jpg
Forgot to show in schem. that amp switches are set for Voltage Mode. The servo amp is shown converting DC 24.2 V 2.5 A to 7.19 V 7.5 A. A wooden clothespin secures a temporary 15 amp connection without the hazard of a conductive clip lead (whose far end would be energized). Magnet top plate has been replaced with a stiffly-supported crescent wrench.

Electromagnets are naturally more power-efficient as they get bigger, and it's great to see that even with a 3 inch pole diameter. The wrench demo was able to keep it up at 1 ampere.
DSCN9904.JPG
As the drive was turned down little by little after that, we read 313 mA before the still-magnetized wrench fell off.

Next step (same as stated a month ago) is to start measuring magnetic flux. Can skip the DC current sensor with ground-referenced output, if I manually tabulate data read from the analog meter.
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Got me an electric tesla

Post by Rich Feldman » Fri Jul 10, 2015 11:20 pm

Finally got to measure some magnetic properties of my "plain old steel".
First, restored the magnetic circuit to a simple gapless configuration. Here the top and bottom plates have much less cross-sectional area than the round pole pieces (27.8 vs 45.6 cm^2), so they will saturate first. My quick-and-dirty sense coil is four turns of wire, conservatively placed about as far as possible from the forcing coil (whose decoration is left over from Halloween 2013).
flux1.PNG
Fluxmeters depend on an electrical conductor going around the magnetic flux of interest. It's analogous to measuring electric current by putting a ferromagnetic core around the place of interest. Both are noninvasive, and can take readings on solid bars or empty space. Fluxmeters measure changes of flux; a change of 1 weber (at any speed) generates 1 volt-second per turn in a sense coil. This unit has an analog voltage integrator with a reset button, a very sensitive offset-adjustment knob, and a digital display.
flux2.PNG
To get flux change from a gapped magnet, one can reset the integrator while sense coil is at the place of interest, then rapidly move the sense coil away from the strong field. With my electromagnet, I could have run demagnetizing cycles and then set the fluxmeter zero. This time, started at 10 amperes and set fluxmeter output to half of known peak-to-peak value, using a slow motion control kluge. (6 volts through 10 kΩ, applied in parallel with sense coil.)
flux3.PNG
One effect stood out which I had never seen while fluxmetering transformers. After each current step that caused a large flux change, but not similar current steps with small flux change, I had to wait for the least significant digit to stop changing. This has got to be an eddy current / skin effect thing. Coil current lags voltage a little. Average flux (esp. the last percent) lags current a lot. It will be fun to see that in a quantitative way.
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Re: lab electromagnet from scratch

Post by John Fenley » Wed Aug 05, 2015 5:47 am

All the work I see here is discouraging me from attempting to build the magnet I need for my proposed fusion reactor... I'm going to need an extremely uniform .5T field over an area of about 18", and the thicker the better.

Fortunately, I realized that MRI machines have fields that meet my requirements. I'm going to be trying to get a permanent magnet MRI machine to act as a basis for my reactor.

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Richard Hull
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Re: lab electromagnet from scratch

Post by Richard Hull » Wed Aug 05, 2015 4:47 pm

Magnetics can be fun and exciting. I actually put out a two hour long VHS tape on an introduction to magnetism back in 1993 titled "Minimal Magnetics" as part of my Tesla coil educational series of VHS tapes. I think there were 7 or 8 of these 2 hour tapes back then and about 60 of my 2 hour Tesla Coil report tapes.

Magnetics can be frustrating and very limiting due to the limitations in the permiability of metals which can concentrate and focus large field flux. Air, of course, is infinitely permiable, but then there are those pesky amp-turn limitations leading to meltdown with no core only conquered by either pulsed operation or Liqiud nitrogen or both.

As noted many times way back, there is only charged mass and gravity that manufacture the secondary and tertiary forces of all magnetisim and all light. (note at the very far end of light it seems only collapsing nuclear forces can make extremely hard gammas, but again, these nuclear forces are only there to constrain and contain the electrostatic charge forces of charged mass within the nucleus. The intense charge field relaxation of the nuclear force braking down might be he source of those hard gammas.

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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
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 » Sat Aug 08, 2015 5:53 am

Good luck, John, with getting a permanent magnet MRI setup. Fusor.net has some discussions of permanent magnet systems shaped for small cyclotrons.
My electromagnet project is just slow to be executed, not really complicated. Magnetic flux path model is 1-dimensional, with a few discrete segments. Each has a length and area, permeability, saturation, and hysteresis. Pole diameter is only 3 inches to keep the steel weight manageable and the machining requirements easily accessible.

In other news, I got a bit Andrew Robinsoney on my pole pieces. Removed the mill scale by pickling in dilute hydrochloric acid, then gave them a nice coat of paint. Drilled carefully centered holes for alignment pins, to be followed by threaded holes for machine screws.
DSCN0213.JPG
Here is the "carrying case" for a 29 pound set of pole pieces.
DSCN0216.JPG
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Re: lab electromagnet from scratch

Post by Richard Hull » Sat Aug 08, 2015 6:54 am

Nice pole pieces!
Do you know the permiabilty of that steel? It is probably pretty good.

I am sure the field produced by the future magnetics will appreciate the paint job and keep that steel that is just itchin' to rust and rot, protected against future humid magnetic events. No one likes something that is rotten to the core.

Couldn't resist that one.

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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Re: lab electromagnet from scratch

Post by Tom McCarthy » Sun Aug 09, 2015 4:22 pm

Ain't talking about magnetics Richard? I know you've said it before...

Anyone? :wink:

Tom

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Re: lab electromagnet from scratch

Post by Andrew Robinson » Sun Aug 09, 2015 6:05 pm

Rich Feldman wrote:I got a bit Andrew Robinsoney on my pole pieces.
HAHAHA LOVE IT! Looks great Rich!
I can wire anything directly into anything! I'm the professor!

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Re: lab electromagnet from scratch

Post by Rich Feldman » Sat Apr 28, 2018 11:03 pm

Chris Mullins's project inspired me to get back to work on the electromagnet learning experience.
Two pieces of steel for the yoke had gone missing around the garage. They're still missing.
Last October, while passing a metals store in San Jose, I decided to buy replacements.
bars1.JPG
Two 12 inch lengths of HRS, rectangular bar, 1" x 4". Should have asked for them to be 12 1/4 inches each, instead of "please cut them long enough to be 12" when squared up". Measured back home, one was dismayingly too short (by about 1/4 inch) and the other might barely make the grade. A week later I took the short one back & asked for it to be replaced. The cutting man chose a mill-finished bar end and prepared to guide his torch with a well-worn T square. Then changed his mind and carted the work over to a big abrasive-wheel saw, which took about a minute to make the cut. (The pieces I'd lost were from Alan Steel, cut with a bandsaw, and had no shortness or roughness problems.)

A few weeks later I had time to square up the ends on the Bridgeport at work. Or at least get pretty close, after adjusting the vise angle but not the spindle angles. Rarely are users obligated to leave machines as square and parallel as some next user might need.
I stopped at about 12.05 inches, when the first bar still had some flame-cut kerf areas on both ends.
bars2.JPG
The other bar in picture has interesting patterns on three sides, after metal raised slightly by the steel mill's shear was cut back to the original surface planes.

Could have lived with the voids, which are a tiny fraction of the contact area carrying magnetic flux. Or changed the yoke design to use 11.95" bars. Or replaced the deficient bar, without quibbling, for less than $15 including tax. But no, Rich wanted to try filling in the low spots with a TIG torch.
Then nothing happened for about four months, except I took off the black scale by pickling in diluted HCl, then WD-40'd the bars for storage.

Filling-the-pits opportunity came last night, at the home of a friend I hadn't seen since he retired. My previous TIG experience was a few minutes with the same machine, about 10 years ago, cosmetically closing some cracks in a home-made brass casting. Long story short: after 2 hours, the bar looks like this on both ends.
bars3.JPG
Not without some porosity, I bet, after puddles got bubbly in a couple of places.
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Re: lab electromagnet from scratch

Post by Chris Mullins » Sun Apr 29, 2018 1:09 pm

Rich,

Ironic, since this thread was very helpful and a source of inspiration for my magnet construction originally! E.g. I stole your idea of using an extension cord for a quickie coil as shown here: https://mullinscyclotron.weebly.com/upl ... 5_orig.jpg

I experimented with copper and aluminum tape, and came to the conclusion that if all materials are bought new (and assuming copper), ACR copper tubing was the cheapest way to go. You got a great score with that wide Al foil tape! I was thinking of posting more on this if it would be helpful for others, but this is a rough dollars per pound for copper in various forms:

1" wide by 55 yard copper foil tape https://www.amazon.com/gp/product/B01HA31M4M: $54/pound of copper
16 AWG magnet wire, 50,000 foot quantity from http://mwswire.com/: $9.45/pound
1/4" ACR copper tubing, in 100 foot lengths http://coppertubingsales.com/copper-coi ... 1-8-7-8-od: $5.12/pound
bulk copper commodity price (absolute floor for pricing) https://www.scrapmonster.com/comex/copper-price/353/9: $3.15/pound

I had trouble finding bulk pricing on large rolls of foil tape, so that 1" wide was a low-cost representative example.

I was surprised that ACR tubing was cheaper than magnet wire, but that's what I found. I went through another analysis of tubing size (1/8" up through 5/16"), and 1/4" was the cheapest/pound of copper among ACR tubing sizes. Of course the big problem with copper tubing is that it's not insulated - we had to slide some sleeving over the entire length, which was extremely tedious and added to the overall coil diameter. On the other hand, cooling is much more feasible with the tubing.

1/8" ACR tubing is about $8.38/pound. Even though it's more expensive than 1/4" (per pound), it would have been a little cheaper in total, since the coil size was smaller for the same number of turns. I was worried about cooling with that small a diameter, and also that put the coil resistance higher than I wanted for the power supply. For a smaller diameter pole piece, I'd probably use 1/8" tubing.

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