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Lab electromagnet from scratch

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klugesmith

Mon Sept 23 2013, 04:45PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

Another 4hv thread, begun two equinoxes ago,

, prompted me to actually make a cyclotron-style electromagnet.

One goal is to get my hands dirty again. To see the EM knowledge I spout so freely is correct.
Mostly, I hope to set a practical example for people who need "large" electromagnets and tight budgets.
There are used ones on the market, but not often close to one's home and for well below $1000.
The DIY approach lets us adapt the design to its end-use requirements and to material scrounging opportunities.

The magnetic field goal is to get 1 tesla continuously in a 1-inch gap, if possible without water cooling.
We should be able to crank it up to near saturation of the poles and yoke, for limited run times.

As we saw in the other thread, those B and L parameters set the coil ampere-turns requirement.
The conductor material, winding density, and I2R power density then dictate the cross-sectional area needed for coils.
I shopped mostly for aluminum, which at $2/lb (if the alloy is good) is like copper at $1/lb, but needs more room.
It didn't take long to find a deal on two rolls of adhesive-coated thick foil.

This pilot project has a relatively small pole diameter. That very much keeps the steel weight and cost down.
The whole thing will be transportable without help, and all machining will be on parts one could carry with one hand.
That & the bulky coils are the reason for a strikingly skinny aspect ratio, here juxtaposed with the 8 inch reference design from previous thread. Grid pitch is 1" (25.4 mm).

More to follow. Comments, questions, and suggestions are welcome.

First material purchased was the electrical conductor. It can be rewound later for a 6 or 8 inch magnet if the time comes to get heavy.

Sulaiman

Mon Sept 23 2013, 09:21PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

For beam deflection etc. wouldn't a square or rectangular gap/core be more useful?

Ideas;

. If you use the strip core material for/from "C" or toroid cores,
it is grain-oriented for maximum flux density ... >1T

. If you make the windings less height than the gap
you could fit/change windings without core dis-assembly ?

. I think that coil heating is proportional to (gap length.flux density)(squared)
so an adjustable gap length would allow long runs at high flux density.

. You could design pole-piece 'shims'

. some shims could be NdFeB magnets to establish a fixed field
and use the electromagnet to weaken the field
e.g. for a 1T +/- 0.1T field
make a 1.1T field and the electromagnet only has to provide equivalent 0 to 0.2 T

. consider forced air cooling during design?

Consideration at an early stage is easier than afterthoughts.

(I often add so many variables for consideration that I discourage myself from starting! )

klugesmith

Fri Sept 27 2013, 01:22AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

Good ideas there, Sulaiman.

One reason for round poles is to support things like cyclotrons. My design can be scaled up from 3" to 6" by rewinding the same actual conductor material, and scrounging 100 kg of additional steel.
Rare earth magnets for DC field are also a good idea, though it would take some pretty big ones to get a 1 tesla field in a 1 inch gap. The kinds that could crush a finger if they accidentally snapped together.

This is intended only for DC fields, and should be adjustable from 0 to +/- 2 teslas even without special pole caps.

We'll learn, quantitatively, how much eddy currents in the solid steel pole pieces and yoke
reduce the flux ripple, if the coils are driven with rectified mains-frequency AC.

Will the substantial magnetic hysteresis in "plain old steel" present a control problem, or cause the field in air to be unpredictably non-uniform?

We'll explore the limits of forced-air cooling on the outside of bulky aluminum coils.

Design evolution.

Kind of like the folk tale about stone soup (or nail soup), this project didn't start rolling until I found a couple of thick steel plates on a remnants shelf. They're 3/4 inch thick (19 mm) and almost 6 x 19" in extent.
They will be the end plates in configuration pictured above.

If any yoke parts have cross-sectional area smaller than the pole pieces, it will be hard to reach saturation at the poles. I think the wide endplate meets that requirement for round pole diameters up to D = 4 x thickness,
so chose 3 inches (76 mm) as the pole diameter. Then started shopping for coil materials, which is a story to be told later. After obtaining about 56 lbs of aluminum strip 5.75" wide, I chose to make the pole pieces 7 inches long.

First steel purchase

I found a suitable length of rusty old hot-rolled steel at Alan Steel.

Saw cuts cost $5 each, and took about 1 minute each. The sawed surfaces are almost perpendicular (to the rod axis). I won't say "square" because they are round. The first machining was to make the pole ends more nearly flat and parallel.

Material Magnetization Measurement

Before further work on the aluminum coils, I will measure the magnetization of the four steel parts on hand. Won't take so many ampere-turns if the flux path is closed:

The temporary drive coil is a 100 foot extension cord, wound on a spool made from nominal 3" ABS pipe and some pressboard annuli.
Yandersen knows that "nice" coil winding is easier if the bobbin rotates on a spindle, with a hand crank or motor to make it turn.

Now I'm trying to post a little video. Going to flickr 'cause I can't remember my youtube password. Try this:

klugesmith

Tue Oct 15 2013, 01:59AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

"First Pull". Seems like thatâ€™s a milestone to celebrate for any new electromagnet. We finally did that last night, quick and dirty, after weeks of not having time to be properly scientific about it. Measuring BH curves can wait.

In the meantime, I had measured the orange coil's DC resistance: a disappointing 0.99 ohms. Using 1.00 amperes for that, with no iron core, I got some tiny sparks & later made a neon lamp flash. Will have to compute (and measure) the air-core inductance.

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 (28 kg), from 14 square inches, amounting to 4.5 lbs/in^2.
For BH/2 to match that value, the required B is about 1/4 tesla in the round parts Would be higher than that in the endplates.
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 500. That's plausible, esp. since we could be dealing with substantial hysteresis in this "plain old steel". When I use alligator clips to switch between my three voltage sources (1.5, 2.5, and 3.3), must pay attention to whether the polarity is reversed or not.

To trace a BH curve well into saturation, without changing the flux path or coil, it would be good to have a bipolar adjustable power source that can deliver up to 10 or 12 volts and as many amps. Am thinking of build my first H-bridge inverter. Any hints about high-side gate drivers whose On or Off time can be up to 100%, as for DC motor controllers? I'll go read the recent 4hv thread about that. How 'bout optoisolators and 9 volt batteries? Switching frequency can be very low in this application.

I could get results sooner using an adjustable unipolar supply that can hit 10 volts & 10 amps. Just need to put together a reversing switch and, on principle, some clamping diodes.

Steve Conner

Tue Oct 15 2013, 10:08AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Sounds like an interesting project!

You could use gate drive optoisolators like the Toshiba TLP250 series, with 9V batteries for floating power. Or maybe you could hook up a Class-D audio amp:

Tetris

Tue Oct 15 2013, 09:56PM

Registered Member #4016 Joined: Thu Jul 21 2011, 01:52AM
Location: Gainesville, FL
Posts: 660

I remember seeing a documentary about these things made up of thousands of very carefully placed aluminum/copper/dielectric "disks" which were really intertwined 3/4th rings. Are you planning on setting it up the same way sans the copper?

hboy007

Tue Oct 15 2013, 11:36PM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 373

Looks like an awesome project. I saw a video that matches your description recently,

(fwd. to 04:20)
Keep up the good work!

Steve Conner

Wed Oct 16 2013, 09:01AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

ASHPoD wrote ...

I remember seeing a documentary about these things made up of thousands of very carefully placed aluminum/copper/dielectric "disks" which were really intertwined 3/4th rings. Are you planning on setting it up the same way sans the copper?

Sounds like a Bitter magnet:

klugesmith

Thu Oct 17 2013, 02:43PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

Yup, Francis Bitter invented that way to make solenoids with closely packed, low resistance, intimately water-cooled conductor turns.

There's an astronomical gap between my puny project and the 30+ tesla magnets in that video, whose coils dissipate more than 10 kilowatts per cubic centimeter. For fields up to 2 teslas, designers use lots of iron, vastly reducing the magnetizing force requirement. Electric power goes up as the square of magnetizing force and field strength. Without iron, my orange coil would need 14 amps (200 watts) to make 0.02 teslas.

For the first time in decades, I have purchased a stamp for philatelic purposes.
An Amp stamp.

History of Science is a fascinating and valuable subject, IMHO.
What was AndrÃ©-Marie AmpÃ¨re up to in his lab 200 years ago?
Most images show his hair much more curly than the image in this stamp.

[edit] I bet the lab station in the video, with slowly falling aluminum pipe, is the same as the one in this drawing.

The center feild [sic] strength is 35 T when the machine is heating water at the rate of 19.2 megawatts. Their posted calibration data shows that operating point to need 38.8 kA, implying 495 volts and 0.013 ohms in the coil set.

klugesmith

Mon Jun 01 2015, 09:45PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

Yay! Rich has own work to report, after brushing 18 months of dust from sort-of-big magnet. It's related to what artlav brought up on 4hv last week.

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.

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.

1433194759 2099 FT157436 Servo Amp Transfer

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.

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 mÎ© is impractically long. Stainless-steel bands from lawn furniture and a rotted wooden flowerpot: 4 mÎ© 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.

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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