lab electromagnet from scratch

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Re: lab electromagnet from scratch

Post by Richard Hull » Mon Sep 23, 2013 7:17 pm

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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Re: lab electromagnet from scratch

Post by Rich Feldman » Mon Oct 14, 2013 7:24 pm

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

Post by Richard Hull » Mon Oct 14, 2013 7:44 pm

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 » Mon Oct 14, 2013 7:53 pm

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 » Mon Oct 14, 2013 8:49 pm

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
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 Rich Feldman » Thu Oct 17, 2013 2:19 pm

For the first time in decades, I have purchased a stamp for philatelic purposes.
An Amp stamp.
amp_stamp.jpg
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History of Science is a fascinating and valuable subject, IMHO.
What was André-Marie Ampère up to in his lab 200 years ago?
Most images show his hair much more curly than the image in this stamp.
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Re: lab electromagnet from scratch

Post by Richard Hull » Thu Oct 17, 2013 3:33 pm

You may have seen an image of Ampere in a wig! They were still in vogue into his early adulthood. By the time he died, they were a thing of the past. The stamp image may be of a bust of him in later life. The droopy eyes seem to indicate this.

I have been a stamp collector all my life. Stamps can and do honor science history. One of my favorites is shown below. I have many mint, post office fresh, sheets of 50 each of these and use 'em for "makeup" postage sometimes. Can you believe they ever managed to deliver mail, twice a day, in the city, on foot, to you door for this 3 cent price?! They delivered around 10AM and then a second delivery, about 3:00PM. (in large cities only -rural only once a day)

This stamp was issued after Ike's famous Atoms for Peace speech at the U.N. His words adorn the framed edge of the stamp. This launched the Atoms for Peace Program in the U.S. and made isotopes available to the general public, but mainly for kids in school to help them train for careers in nuclear physics. "Project Plow Share" was a major component of this effort to find peaceful uses for atomic energy. In "Plow Share" they used nuclear bombs for large area excavation and natural gas rock fracturing tests ("Gas Buggy" test series).

I bought a bunch o' isotopes under Atoms for Peace from Oak Ridge, Abbott Labs. I attach a nice little price list I used back when you could get the rad stuff loose in bottles. Mailed in small lead pigs and delivered by postman in his big brown leather mail bag. I mowed more than a few lawns to get the fabulous "9 pack" (click the image to enlarge and read......)

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A list of what you could order sent to you home in 1963. I ordered many times 1960-1966
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Issued in 1955 when I was 10 and already collecting A bomb fallout from my rain gutters and bird bath.
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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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Re: lab electromagnet from scratch

Post by Rich Feldman » Tue Nov 19, 2013 3:31 am

OK, back to the electromagnet. My scrounging of NST's from doomed movie theater is over, except for telling the story about six more 15kV-ers etc. :-)

Plan is to measure, more properly than by pull strength, the magnetic flux vs. magnetizing current in closed-path stackup. The main unknowns are the permeability and hysteresis of the plain old steel materials in these parts.

Flux will be measured by the fluxmeter method. We integrate the voltage induced in a sense coil, as the current is changed. We can measure the flux inside a solid piece of steel, analogous to measuring total current in a solid copper wire using clip-on ammeter. Calibration amounts to knowing the sense coil's number of turns and area per turn. The latter can often be taken to be the area of steel around which coil is placed. To use a fluxmeter on a permanent magnet circuit, we move the sense coil between the place under test and a place where B is negligible.

To get the magnetic properties of my 3" HRS rod material by itself, without airgaps or complicated geometries, the plan has been to fabricate toriod-shaped cores from a surplus section. Then wind force and sense coils on them. That's how specimens are prepared for hysteresisgraphs in real industrial labs. Originally low priority, it became high when my brother said he'll need his hole saws back soon.

So I did the first steps of ring preparation:
1. Face the bad end of the surplus rod stock, which had been cut obliquely with a torch. Had to machine 0.300" off of the high spot.
2. Trepan some annular grooves more than 3/4 inch deep, using bi-metal hole saws from hardware store. It actually worked! See picture. Spindle speed for the 2 5/8" saw was 120 RPM. Backed the blade out every 0.030" of progress, to clear the chips & squirt in more cutting oil. The whole workpiece quickly became hot, in a way reminiscent of Count Rumford, so I set up a fan for continuous cooling (even during breaks from cutting).
3. Not yet done: separate the rings by sawing or parting on a lathe (not my toy lathe).

Note: the work is sufficiently secure in vise. Its non-roundness turned out to be two low areas on opposite sides. The diameter across 90% of circumference is 3.04 inches, but in the short direction it's only 3.02 inches.
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Re: lab electromagnet from scratch

Post by Richard Hull » Tue Nov 19, 2013 4:02 pm

Fabulous fields can be attained with toroidial strutures. Most often seen when tremendous lifting work is needed. Nice looking work there.

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 Rich Feldman » Sat May 30, 2015 5:36 pm

Yay! Rich has own work to report, after brushing 18 months of dust from sort-of-big magnet.

While looking into audio amps as coil drivers, I stumbled upon "servo amplifiers" made to drive brushed DC motors. Immediately procured a 30A8T on ebay for about $35. http://www.a-m-c.com/download/datasheet/30a8.pdf Two big terminals receive DC power at 20 to 80 volts. The other two big terminals supply motor with pulse-width modulated rail voltage in either direction. 15 A continuous current, 30 A peak. Four-quadrant operation, as much as the primary supply can sink current.

A motor from a scrapped printer was my first trial load, while measuring the transfer function in voltage mode. The command is for voltage, not duty cycle.
servo_amp_transfer.PNG
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Running magnet coil up to 6 amps (a limit set by windshield-wiper-blade-stiffener current shunt) had few surprises. Average current from primary supply is much lower than in coil, because coil inductance makes a buck converter. At 6 A the upper yoke plate could not be pulled off by hand, in spite of corrugated cardboard between vertical bars and lower yoke plate.
DSCN9837.JPG
DSCN9835.JPG
Since then I made a +/- 15 A shunt out of coat hanger wire. First measured some galvanized straps from lawn furniture, and painted steel bands that secured heavy loads: 4 mΩ is impractically long. Stainless-steel bands from lawn furniture and a rotted wooden flowerpot: 4 mΩ is impractically short. Stainless-steel hose clamp including worm gear for continuous adjustment: nearly ideal resistance, but I'm not equipped to solder or spot-weld stainless, and didn't want to drill holes for screws.
DSCN9838.JPG
Next steps are to properly stack the magnetic circuit, apply the fluxmeter, and see how much current is needed for saturation. The coil circuit has no common ground, so I need to find an isolated current sensor before a BH curve can be displayed on oscilloscope. 60 Hz is of no use here because of eddy currents, otherwise the BH curve would have been presented 18 months ago.
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