lab electromagnet from scratch

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Rich Feldman
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Re: lab electromagnet from scratch

Post by Rich Feldman »

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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Rich Feldman
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Got me an electric tesla

Post by Rich Feldman »

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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John Fenley
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Re: lab electromagnet from scratch

Post by John Fenley »

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 »

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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Rich Feldman
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Re: lab electromagnet from scratch

Post by Rich Feldman »

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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Richard Hull
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Re: lab electromagnet from scratch

Post by Richard Hull »

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
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 Tom McCarthy »

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

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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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Rich Feldman
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Re: lab electromagnet from scratch

Post by Rich Feldman »

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 »

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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Re: lab electromagnet from scratch

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Chris, thanks for the electrical conductor price-per-pound data. Its importance is one of the main learnings and teachings of our projects.

Now an update on the three-inch magnet. Filling voids using TIG torch, in my inexperienced hands, was not worth the trouble. Minor "pits" remain after both bars were finished to the target length, with ends flat and square, within about 0.001". (As we pretend that 1" x 4" shapes can be called square.) Then it was only a matter of drilling a few holes, and tapping four of them, before a declaration that yoke assembly has begun.
DSCN0367.JPG

Ever wonder about teenage amateur scientists driven by birthday deadlines? I'm facing the horrible spectre of five years elapsed since original post about this project. Here's one statement from early on:
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.


Here's the post which presents the starter plates. Sort of like the stone, or nail, with which to begin making a tasty and wholesome soup. Also the first scale drawing. viewtopic.php?f=15&t=8600#p59448

As mentioned more than once before, and as the Mullins family well knows, electric power requirement is not very sensitive to pole area. But it goes up as the square of air gap length, and as the square of flux density B. And it goes down in direct proportion to conductor mass going up, if the average turn length doesn't change.

The scale drawings below illustrate that principle, and the 2015 evolution of my design. Grid is 1", just like in that early drawing. In all cases the cross-sectional area of flux paths in the yoke is slightly more than that in the cylindrical pole pieces. All coils have the same winding area and current density, and generate the same flux density in air gap. All pole pieces are 7" long, a consequence of my coil conductor choice.
em_evol2.PNG
The 6" magnet is 20" x 20" on the outside, and to my eye has an ordinary aspect ratio.
The 3" magnet in the middle uses 22% as much steel but 71% as much conductor and power (same as ratio of average turn length).
The 3" magnet on the right is what's being assembled. The original flat-finished plates go on the sides instead of the ends. That makes the air gap length infinitely adjustable, without any precision turning or boring. Predicted and measured steel weight is 56 lbs for the four central parts, and forecast to be 102 lbs when side plates are included.
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Re: lab electromagnet from scratch

Post by Chris Mullins »

Rich,

Having an adjustable pole gap would be really useful. That feature was way beyond my construction abilities. You mentioned another temporary winding, at 100V and 100A to get 1 Tesla with a 1" gap. Even with much less current on a temporary winding, if you can adjust the gap continuously you could experiment with saturation limits. Are you still planning to use that aluminum foil tape for the final winding?
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Re: lab electromagnet from scratch

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Yes, Chris, I want to use the aluminum strip coils. The material will need some work to make connections to the inside, and to avoid any shorted turns on the exposed edges. Any rewinding onto different spools will need a fixture, I think, and a way to clean up damaged edges on the way through. Facing a sort of deadline, and not much spare time, I might shoot for 1 tesla-inch using lesser coil(s) and lots more power. Duty cycle could be 2 seconds on and 2 hours off. Must avoid throw-away work just to meet the demo date. :-)

Here's the first re-assembly after degreasing and painting the yoke parts.
Joints between horizontal and vertical bars are clamped with 1/4-20 threaded rods. Just as tight as they would be with two 1/4-20 machine screws at the same torque, like the pole piece connections, but without thick-steel drilling and blind-hole tapping.
DSCN0385r.JPG
At the bottom we see a low-profile caster cart, presently on a flat concrete block while glue dries.

The upper pole piece and top plate are secured with an identical clamp set. Presently the gap is set to be 1 inch.
Without further ado, it's my pleasure to present:
DSCN0388r.JPG
Next step is to do some magnetizing. Another high priority is making some accessory parts for aligning the upper pole, and placing the upper clamp at the right height. Clamp rod tensions are matched by sound, like bicycle wheel spokes. If the upper clamp had a quick release feature, this contraption might make an effective mousetrap. :-)
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Rich Feldman
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Re: lab electromagnet from scratch

Post by Rich Feldman »

My confidence was shaken to the core the other day, during casual magnetization experiments.
DSCN0390r.JPG
An unexpected phenomenon, which I think is from flux leakage, immediately reared its ugly head. The pole farther from the coil is _much_ weaker than the nearer pole. I bet the factor is worse in this slender design than in cores with stubby poles and stubbier gaps, like the Mullins'. Adding a coil around the upper pole won't improve the "loss" factor, it will just superimpose a strong pole on top and weak pole on the bottom.

Not explainable by the one-dimensional magnetic circuit model, indicated by blue lines in the scale drawings above, and in this signed one from a 2013 post:
3in_magnet.JPG
3in_magnet.JPG (35.56 KiB) Viewed 15941 times
Never previously considered by me.

No time for discussion now. Anyone else want to chime in, or point to a magnet design book? We can measure flux any place we can encircle with a wire. We can simulate using FEMM, but its restriction to axisymmetric geometries limits the realism in this case. Who has access to a real 3D FEA simulator for magnetics, or even for static heat flow or electric conduction?

If the problem is what I fear, then three remedies come to mind. Use more ampere turns, change the thick steel design, or magnetically insulate the flux leakage paths with sheets of superconductor. :-)
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Richard Hull
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Re: lab electromagnet from scratch

Post by Richard Hull »

Simple variable gap systems of your design are fine, but all systems of your type design have coils over each round pole section and much thicker flux path pieces equal to or of greater cross sectional area than the round poles of carefully chosen high permeability steels. It all depends on the final use for the magnets. Hysteresis loses in such special steel is not an issue in DC magnets, of course.

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Re: lab electromagnet from scratch

Post by John Futter »

Rich
I agree with Richard your flux return path steel is way too thin. I meant to photogragh several of our big magnets @ work but ran out of time.
These are made here in NZ by http://www.buckleysystems.com/what-we-make/ who make 95% of the magnets used in the semiconductor industry world wide and a great deal of the research type magnets as well. Click on the video at the top of the page to see one being assembled (a bit quicker than you are doing). note the massive flux return paths they use to get max field in the gap
I've seen their factory in Auckland and some syncnotron magnets they were making at the time 20 tons each and dozens of them. I also saw some magnets just out of the factory in test that had Danfysik labels on them
They use special steel that is made for them using Buckley systems steel recipe by a steel mill in Australia it starts to saturate at just under 2 tesla. They say that most normal steels start saturating around 1 Tesla.
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Re: lab electromagnet from scratch

Post by Chris Mullins »

Rich,

Can you power the coil with an adjustable, constant current source, and measure the field strength at the face of each pole? When I only had one coil, I had around a 3 to 5% difference at each pole face, across a 1.42" gap (e.g. I'd read 144.9 mT on the bottom pole face (the one with the coil), and 137.7 mT on the top pole face). That spread also varied with measurement position. In the center of the pole, there was almost no variation, while I'd have up to 5% near the pole edge. The spread also varied a bit with overall field strength. I attributed the variation from center to edge of the pole to the asymmetry of only having one coil (one big motivator for finishing the second coil, apart from doubling the max strength).

I used 3" thick steel due to two design requirements:
1. The total cross-sectional area of the frame should be at least equal to the pole piece area, like Richard said above. Since the flux divides into two directions (into left and right sides of the frame), the area of each section can be half the pole piece area. I had 8" pole pieces (from the design requirements for the cyclotron), or 50 sq. inches. So the frame has to be roughly 25 sq. inches, otherwise it will saturate before the pole.

2. The frame width should be roughly the same as the pole diameter. This was more a rule of thumb I picked up, and although there are several examples where it's not the case, many working cyclotron magnets roughly my size were shaped like that (e.g. this one: http://heidib.me/cyclotron/?page_id=284 from the Rutgers cyclotron), so it's an aspect ratio that works. In mine, that puts the frame at 8" wide, and the thickness at 3.125". I used 8"x3" = 24 square inches, close enough for me.

Even with that, my magnet efficiency is around 90% at 450 mT, and 80% at 820 mT across the 1.42" gap, so it's already starting to drop off.

If you're close to saturating that steel in the thin section, you might find it works closer to expectations if you lower the current. If you can make some measurements at low currents, and see how the performance varies as you crank it up, that might help in understanding what's happening.

I do plan on modelling my magnet, and in another thread Scott Moroch mentioned some tools to do that. The Rutgers and Houghton cyclotron folks have used those (some of which are free) and posted the info online, so we could start from their models and modify them to fit our magnets (which are pretty similar).
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Re: lab electromagnet from scratch

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Chris, Your measurement differential is a total non-issue. You seem to think it signifcant. So very much has to do with the critical placement of the sensor used to measure the strength. Equally important is the absolute flawlessness of the steel pole itself. Are there voids or imperfect alloying regions in the pole piece? Are there small imperfections in the face or numerous tiny bubble voids near the pole face measurement area? Most likely, however, the increased flux path to the other pole piece and any joints between the coil and the other pole piece would be the causative agent.

You are talking the difference of 1377 Gauss versus 1449 Gauss or a mere 72 Gauss differential. This is a pitiable differential which can be caused by you or the flux path very easily. Detailed magnetic theory and the issues involved with permanent or DC electromagnets are one of the most understudied areas by technical people forcing them to make general assumptions related to magnetic measurement and magnetic circuitry. That's right, a magnetic circuit! Look at the ohms laws for electrical circuitry versus the magnetic circuit laws.....They are, in effect, identical. (An exercise left to the student)

There is a slight difference in that in electricity we look at resistance in ohms and bypass the term conductance, its reciprocal. In magnetics, we look at permeability,(magnetic conductance), and not its reciprocal, magnetic resistance, reluctance, stated in "Rels". The Rel is virtually unknown, some consider it archaic, however electrical conductance in "mhos" is more commonly known and is still used.

Measuring magnetic field strength is like measuring neutrons. Both are less commonly followed paths for most technical people and just because you a have a neutron counter or a magnetic field strength meter, does not confer on you the ability to understand what you are measuring or how to apply and interpret information given by the instrument. A much deeper understanding of what you are dealing with at the material and theoretical level is needed.

I equate this with a person with a GM counter seeing a radioactive source read a 58 mrem/hr rate and another person with a different GM counter finding only a 20 mrem/hr rate from the exact, same source! Assuming each counter to have been recently calibrated by a national standards referenced facility, why the differential? Are they both right or both wrong or is only one reading really correct?? (Another exercise left to the student)

When I use the "exercise left to the student" phrase, it is a clue that we assume you to be inquisitive enough to be come a student and seek out references as self-directed learning would force a naturally inquisitive person to do in order to find out causative agents and , thereby, "learn".

In the end, I more or less blame the manner in which magnetics is dealt with, If the Rel and magnetic resistance were emphasized in magnetic circuits we would see more wire,(electrical), in the metal magnetic circuit and more joints,(electrical), in the magnetic circuit which would cause a diminution of current(magnetic flux,Gauss), at the pole face. We are more trained to see and recognize resistance in circuits, where magnetic engineers are more on the look out for keeping the most permeability, (conductance), in their circuits.

Reluctance is discussed here and its relationship to electrical ohm's law.

https://en.wikipedia.org/wiki/Magnetic_circuit

Richard Hull
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Fusion is the energy of the future....and it always will be
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Re: lab electromagnet from scratch

Post by Peter Schmelcher »

I believe you should split the electromagnetic, half around the top and half around the bottom. This should change the magnetic circuit potential in the support arms to zero opposite the air gap, the reduced coil diameter requires less wire so you will get more amp turns from your spool.

My fading memory also recalls an advantage to actually covering the air gap with the windings making the flux lines more constrained within the air gap.

Just my 2 cents
-Peter
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Re: lab electromagnet from scratch

Post by Chris Mullins »

Richard,

No doubt I still have much to learn!

I was thinking it could be significant since my difference was much smaller (0.5% or less) once I had both coils in place, compared to a single coil on one pole.
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Re: lab electromagnet from scratch

Post by Rich Feldman »

In case the pictures are misleading: Yoke parts in BTI have areas that are 113% and 122% of the pole area, counting both legs. So yoke metal is always farther from saturation than pole metal. I think the corresponding area ratio in the Mullins magnet (MCM?) is 95% for both part types.

The exceptionally slender aspect ratio comes from 1) need to reach around two coils with >10K ampere turns each. That's the minimum to get 1 tesla in 1 inch of air, for any pole diameter. Plus roughly 6% to magnetize about 1 lineal meter of steel to similar B value, same as in MCM. And 2) keeping the pole area down to facilitate DIY steel fabrication and handling, since no minimum diameter was required. I still believe this would be hunky dory if not for the leakage flux challenge.

Thanks for all the hints. After some reading, thinking, and just a little computing, my grasp of leakage flux is much better than it was a few days ago. The Internet teaches that all electromagnets and transformers have leakage flux percentages. It helps to keep air gaps narrow (duh!). And if wide, to place the coils as close as possible to the gap.

Some simulations, and multi-point flux measurements (with various gap lengths), are in the planning stage.

I bet they will support the view that in this application, aspect ratios are what matter most. Permeability is second, and nonlinearity (saturation) is last.

Let's assume the core's top half is a mirror image of the bottom. Take the magnet's nominal size (pole diameter) as the unit of length. I claim that the dimensions which matter most are air gap length, pole length, and radial distance between pole and "side bar". The last two are driven by coil length and diameter. The numbers for MCM appear to be in the mainstream -- about (0.24, 1, 1). For BTI they are (0.33, 2.33, 1.5).

For simplicity, keep B enough below saturation that the steel B-H curve is still sort of linear. Then we can look at the B/2 field generated by bottom coil only. Later superimpose a mirror image to get the total field.

Consider the flux impelled upward by the lower coil, when it reaches the bottom pole surface. If there's no air gap, the least reluctant return path is to carry on into the upper pole piece, and come back around the yoke. Nothing but steel! Total reluctance is low, and so is the total MMF for a design amount of flux. The "magnetic potential" is distributed around the whole circuit, so a small fraction of the flux will take a short cut through air from the pole pieces to the yoke.

It doesn't take much of an air gap to greatly raise the magnetic potential difference between the two pole surfaces, and demand lots more ampere turns. Now as the flux emerges from lower pole, the upper pole surface isn't so inviting. The yoke bars are still on the far side of some air, but their broadside view presents plenty of area.

I think simulations (and measurements) will show that when the air gap reaches 1 inch, between 3 inch diameter poles, the total reluctance of sideways leakage paths is less than that of the air gap itself. Both are much larger than the reluctance of the intended flux path through steel. Yoke bars could all be twice as thick, or twice as permeable, without greatly reducing the leak percentage. If 3D simulation were available, we might see if it helps to place my side plates with narrow edges instead of broad faces oriented toward the pole axis.

And that's one man's novice opinion.
All models are wrong; some models are useful. -- George Box
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Rich Feldman
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Re: lab electromagnet from scratch

Post by Rich Feldman »

Been talking with Chris, but haven't yet updated the MCM coil dimensions from guesses in this picture.
Discovered a cool way to name the nodes in one-dimensional magnetic path model.
bti_mcm1.PNG
In the round parts, blue lines are drawn where they bisect the semicircle area. :-) Chris, should the NS order in MCM figure be flipped?

Sketch at the top left shows axisymmetric model of BTI for the first FEMM simulations. Left edge is the axis of rotation. Yoke horizontal and vertical bars are represented by disks and a thick tube with the same cross-sectional areas (still 113% and 122% of pole area). So the model is configured like a gapped ferrite pot core. I bet it will overestimate leakage flux.

Between the poles, FEMM model has a few very short cylinders that can be steel or air.
bti1a.PNG
Here there's a 0.1 inch air gap, which gives a nice qualitative view of leakage flux. Only the bottom coil (with practice coil dimensions and turns count) is energized. That makes it easier to understand the leakage flux behavior. If we were to add an identical top coil, it would superimpose a mirror-image field distribution, restoring top/bottom symmetry at the gap.
bti1c.PNG
Since BTI's design target doesn't specify the strong field diameter, I'm thinking about tapered pole ends. Time to stop pinching pennies so hard, and shop out some of the fabrication work. Cost to date is under $100 for all parts in the picture, including extension cord and four casters. I've probably spent more time talking about it than working on it.
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Rich Feldman
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Re: lab electromagnet from scratch

Post by Rich Feldman »

Couldn't resist posting more axisymmetric simulation results, before trying a planar model of the same real object.
When there's no air gap, there's practically no leakage flux. 1-D model would be quite accurate. The new chart is |B| along a contour from bottom to top, at half of the pole radius. Current is 1 ampere in the extension cord coil.
bti2b.PNG
With a half-inch air gap, the peak B is vastly smaller (as predicted by 1-D model), and gap B is even smaller by another factor of three.
bti2a.PNG
Let's see what happens when we increase the vertical yoke bar area from 1.22 to 2.35 times the pole area.
bti3a.PNG
Not much, eh? Tends to confirm that this is a pole aspect ratio thing. Of course we need to repeat the experiment on a traditional, stubby geometry like MCM. Maybe Chris will beat me to it.
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Rich Feldman
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Re: lab electromagnet from scratch

Post by Rich Feldman »

Tried insulating the sideways leakage path with a sheet of superconductor. Set u = 0.001; maybe zero would work. Pattern changes, but the flux still finds ways to leak around the desired gap.
bti3i.PNG
A simulation with tapered pole tips also revealed little improvement, a finding that initially came as a surprise. Needs more review & thought, but much easier than having to turn a few pounds of steel into swarf.

Then came a planar-geometry model of BTI. Yoke part widths are fudged differently, to keep the cross-sectional area ratios right. All flux is parallel to the plane of the paper. (As in the axisymmetric model, where the plane is any that includes the axis.) I think the planar model underestimates leakage and fringing flux; the axi. model would be good about fringing but very pessimistic about sideways leakage.
btip1.PNG
The planar problem size could be halved, with the right boundary condition applied at the line (plane) of bilateral symmetry. The round shells were set up by a FEMM wizard, as a boundary condition to emulate unbounded space.

This calls for lab measurements. I've wound a round fluxmeter sense coil to fit around pole pieces. Got the bobbin made for a rectangular one, to fit yoke side bars. Sensitivity calibration is easy & very accurate. Rectangular coil will have 10.00 +/- 0.01 turns. Round coil also has 10 turns, with a tap at 5 turns.
The tap will allow direct comparison of flux in one sidebar with half the flux in a pole piece, without changing the instrument range or doing arithmetic. Come to think of it, the sense coils could be connected in series to read the difference between pole and sidebar fluxes. Then slid vertically to find null places (height pairs where pole and side fluxes are equal). No fancy voltage integrator needed for that!
All models are wrong; some models are useful. -- George Box
Mark Kimball
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Re: lab electromagnet from scratch

Post by Mark Kimball »

If you are not already using it, I have found FEMM's scripting capability (via LUA) to be useful for playing with different magnet configurations. It can speed things up quite a bit compared to manually setting dimensions of your parts (or coil currents etc.). It can take a little time to write functions to create components with rectangular and cylindrical profiles, but after that it is much less painful to experiment.

Mark
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