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Laboratory electromagnet

Laboratory electromagnet

Laboratory electromagnet


I need to make a laboratory electromagnet that can produce an induction of ~1T over a 1"x1" area with an air gap of about 1" .

I have a small budget ~$1000. Which is why I think I'll have to make it. Can somebody give me a few pointers on what iron I should use? I know it should be of lowest carbon content as possible, but I'm afraid I won't be able to find low carbon iron at an affordable price. What commonly available steel could I use in it's place?

Also has anyone wound a coil on there own without a winder? How much should I expect to pay to have a coil wound?

Thanks for any ideas

RE: Laboratory electromagnet

Actually I thought of some more useful information.

Even though the gap between the poles will be around 1", it will ont be entirely air. Instead it will be a rubber filled with about 30% iron particles, so I'm guessing the relative permeability is around 20.

RE: Laboratory electromagnet

I did some calcs for the size of the coil and came up with this:  

.75"x1" pole face (only have access to trial 2D software)
.75" gap on a C-Frame
1"x1" bar stock for C frame 1010 steel

18gauge wire D=.0407" use .0515" for calcs
1" ID coil with 4" OD and height of 2.5" -> No.of turns 2830
Total length wire = 1870ft -> total R = ~12ohms
used 2amps for supply -> 12*2^2 = 50 watts

will use two of these coils, so FEA gave a flux around 2T at center of gap.

Do these numbers sound reasonable to you?

RE: Laboratory electromagnet

sorry, I meant 0.6T

RE: Laboratory electromagnet

I'm very sorry, for calculations above ID =1" OD=8"

RE: Laboratory electromagnet

I don't have the time right now to go further but I get between 140,000 and 193,000 ampturns required. (It depends on the answers to some questions)
This is for 10,000 gauss in a 1 inch airgap with a 1 inch by 1 inch pole face.

Copper wire coil material costs for this will probably exceed $1000. Any motor repair shop could wind this coil for you. But it would not be that hard to do yourself.

Maybe more tonight or tommorrow. I checked this against a similar magnet we've been building for about 50 years. I don't think there is any way you can get 2T with your design. Maybe 0.2T.

For steel 1020 will probably be just fine. But more later. Tell us more about how you pan to operate this device.


RE: Laboratory electromagnet

Thanks MJR2,

I'm sorry but I meant 0.6T not 2T. Could I wind the coil by hand? Also I plan on running the electromagnet for 20 min periods with say an hour break. I could calculate the heat transfer and all of that, but in your opinion will the coil I described above be able to handle say 50-100W without burning up? If so what do you think the max ball park power it could handle would be in the 20 min?  Is 18 gauge wire a good choice?

Thanks a lot, appreciate the help. Oh yeah coil ID = 1" , OD = 7", I really need to be more aware when I post things, but it was late.

RE: Laboratory electromagnet

Didn't see the last part of your reply.

I'm going to have a double lap shear fixture inbetween the poles. The fixture is attached to a high frequecy horizontal tensile tester. The specimen is going to be some soft rubber with iron particles, 30% iron 70% rubber (MRE magnetorheological elastomer, stiffness changes with applied field, if you want to google). The composite induction in the rubber needs to get around .8-1T.

So I plan on making the specimen and fixture as thin as possible to save moeny on the construction of the electromagnet. Also I don't have a lot of space for large coils. The material tester's linear motion is only 4.5" from it's base. So if the coils are mounted horizontally I have a 4" OD restriction roughly, which is why I'm thinking about mounting tehm vertically with a c-frame like approach.
              -----  ------
      coil 1 !             ! coil 2

With the mentioned restrictions, I'm thinkg the gap then will probably need to be around 20mm or .75inches.

RE: Laboratory electromagnet

For a C-frame circuit having a square 1x1 inch face and 0.75 inch airgap and 10000 gauss (1T) you need something like the following.

The square face tapers back for about 1 inch at a 45 degree angle finishing at 3.375 inch diameter. That diameter runs for 3.5 inches. The coil is located around this diameter. The C-frame circuit completes it's return in any cross section equal to the 3.5 inch diameter. Any reasonable length will be acceptable. This circuit is mirrored with a coil and like pole on each side of the airgap.

The coil assembly will be two coils wired in series. One on each side of the gap. The ID will be 3.5 inch, 13 inch OD and length of 3 inches. You could wind this on the core or make a bobbin. You will need 82 pounds of #14 AWG film coated magnet wire wound with 3093 turns for each coil. I suggest at least 180C class wire. The coil assembly will operate continuously at 115 VDC but will get very hot (165F) to the touch. Running less as you describe it will run at lower temps. About 2.5 amps DC will provide the field but you need the extra ampturns as the magnet gets hot. A constant current power supply is recommended.

For steel type 1020 was considered for the design. There will be considerable force working to collapse the C-frame. Expect and design for at least 2000 pounds.

You should expect to pay at least $1000 for coil materials alone maybe twice that. I have not looked up that fine of wire for a while. Commercially with control it's not likely you could get this for less than $10k.

The reason for the big difference in ampturns between this post and last is the I used a tapered pole. A straight pole goes into saturation and drives the ampturn requirement out of sight.

As to answering your design/build questions what I would build I have described above. I'm not sure of your requirement on the coil OD. Politely put your design does not appear to satisfy your requirements. However my design is only one of many ways to satisfy your request.

Merry Christmas.


RE: Laboratory electromagnet

Could you operate in pulsed mode (___-____-____-____) with
low duty cycle?

A coil 1 mm diam. wire generally can take 2 Amp-s without

Plesae read FAQ240-1032
My WEB: <http://geocities.com/nbucska/>

RE: Laboratory electromagnet

I've really not been clear, so please excuse me, I'm usually never this bad. What I need is a electrolmagnet capable of nearly saturating a rubber with 29% iron particles. I've seen journal articles where a air gap (nothing inbetween the poles) flux density of 0.45 T was sufficient to get to the level I want with the rubber in the air gap (then the composite flux is around 1.2T).

I have some space restrictions which make buying a off the shelf electromagnet not possible (also I would like to save money, especially if I had to have a custom unit made). The restrictions are primarily that the pole face center has to be 4.5 inches off the base, no higher, no lower. I was thinking of using 1.75in poles tapered to a .75inch diameter with a .75inch gap.

So with a 1.75inch ples

RE: Laboratory electromagnet

so with 1.75inch poles and 1.75 inch (diameter frame), I have a coil ID of 1.75in and an OD of 5.5in.  The coils can be as long a reasonable.

The electromagnet needs to be able to operate for at least 5 min at a time with say 20 min inbetween, pulse operation won't work.

RE: Laboratory electromagnet

Okay. For operating at 0.45T in an open air gap of 0.75 inches with a 0.75 inch diameter pole face and 1.75 diameter core the following steel and coil is required.
Pole tapers at 30 degree angle for about .625 inches to the core.

#20 copper magnet wire
18.4 pounds per each of two coils. One on each side of the airgap.
110 watts total
115 VDC
0.96 amps
1.75 inch ID
5.5 inch OD
4 inch length
6315 turns each coil
12081 total ampturns

I doubt that the field will increase to 1.2T with 29% iron filling.

This will not be suitable for continuous operation. Probably be okay for for 5/20 operation. When the amps can no longer be maintained at 0.67 or greater with 115VDC it has become to hot to continue. General comments apply here as well as my last post.

To do what I think you want I have been designing fields of 0.75 - 1.0T.

Merry Christmas


RE: Laboratory electromagnet

Thank you MJR2,

Are you working with something similar to MREs also?

RE: Laboratory electromagnet

I design and evaluate designs for devices both permanent and electromagnetic to produce magnetic fields as requested by others. Projects from around the world every day. From rather small to really big. I use a set of proprietary programs and a commercially available program called Magnet from Infolytica. I don't do any high frequency designs.

I did not use FEA for your question. I have a set of programs idealy suited to answer your question in a few minutes. I would suggest you use an FEA analysis with your final design.


RE: Laboratory electromagnet


Actually the trial version I have been using is the 2D version of Magnet. I like it a lot better than the other trial version by Maxwell, but it's still a trial version so I haven't been able to simulate a exact design, only an approximation.  Thanks for all the help, I'm going to talk with the machinist today and see what we can do. I'm thinking of something similar to www.gmw.com/electromagnets/dipole/3470/3470_short.html. It should be easy to fabricate, without water cooling.

RE: Laboratory electromagnet

That is a fine looking magnet. But I didn't make it. Physics applies the same for all of us. I mentioned about $10K in an earlier post. Looking at their pricing I see I'm not too far off. Remember as you progress and what I've found to be true. The people that do well at things for a living always make it look easy.

Magnet 2D can be used for this.... Back in the early days that was all we used. A 2D C-frame circuit is difficult to do properly. You'll probably have enough assumptions made to cloud th accuracy of your answer. When we used it we always had physical models to check on.

I used the same tools recently on several large 0.85T C-frame (poles were about 8x80 inches, 6 inch airgap). The results were VERY good. But everything was built right. Steel quality is important.


RE: Laboratory electromagnet

If you don't need to vary the magnetic field, consider a permanent magnet design.  Simple, no power worries, always "on."

RE: Laboratory electromagnet

For an update, I have nearly finished constructing the electromagnet. The frame is done. As I mentioned before I have a space restriction that limits the pole center to be 4.5" off the ground. What I ended up with was 1.5"x4" cross section steel for the base and two yokes (I think that's the right terminology) and poles with a diameter of 1.75" that taper at 60 degrees to a 1" diameter. the poles were made long enough to accommodate coils each of 6" in length and an OD of roughly 6", with a maximum air gap around 0.75".

The construction went well. I clamped the poles with a pinch bolt incorporated into the yokes. The only thing I have left is winding the coils. I'm going to use 14ga wire that is rated at 200C. I found a journal article ("A General Purpose Electromagnet", Journal of Scientific Instruments, 1956 Vol.33) describing how they wound their coils with a lathe and basically being careful to wind each turn without overlapping. Any ideas on how to make this experience go well the first time?

As a preliminary test I attached a 1" Dia by .5" length NdFeB magnet to both sides of the poles with a gap of .5" and the field in the gap was symmetric but not uniform ( reading was around .7T at center with .5T at edges), but I guess this is to be expected since the short stubby magnets are not uniform at the poles.

RE: Laboratory electromagnet

Get yourself some nomex sheet, kapton tape, and electrical grade glass tape.

The coil you describe with #14 wire will run at 40VDC for 110 watts power input. I didn't review why we had that power earlier. Ampturns at that will be 15669. You'll be putting about 2849 turns and 36 pounds per coil. This will give you a coil with 6 inch OD and 1.75 inch ID and 6 inch height. That is if you do a good job layer winding it.


RE: Laboratory electromagnet


I think you meant 18 pounds per coil and roughly 14000 turns per coil, right? Else I'll need to get some more wire.


RE: Laboratory electromagnet

For an indiviual coil weighing about 18 pounds or a total of 36 for two coils.

#14 wire
110 watts
28 VDC
3.93 amps DC
Series connection
1.75 ID
6 length
4.38 OD
1764 turns
6931 NI per coil
13862 NI total
If you fill the area you described it will take more wire.


RE: Laboratory electromagnet


Sorry, my fault. I checked my spreadsheet and realized I used 10% oover the gauge diameter (for insulation) and then an additional 15% for random wound so that's how I got 18lbs per coil. Oh well, if the magnet doesn't end up being strong enough I'll get some more wire.    

RE: Laboratory electromagnet

I'm done!

Everything went well. I ended up with 14.5gauge wire rated at 200C. The vendor wasn't going to have 14gauge in for a few weeks, so I settled. For a recap:

Structure (frame):
"As I mentioned before I have a space restriction that limits the pole center to be 4.5" off the ground. What I ended up with was 1.5"x4" cross section steel for the base and two yokes (I think that's the right terminology) and poles with a diameter of 1.75" that taper at 60 degrees to a 1" diameter. The poles were made long enough to accommodate coils each of 6" in length and an OD of roughly 6", with a maximum air gap (between poles)around 1.0."

Bobbins (2 of them) were ~6in in length, 1.75in ID, and about 6.0in OD. With 18lb (18lb per coil) of 14.5 gauge wire The OD of the wire wrapped on the coils turned out to be about 4.5in. So I have room to add wire if I ever need to. Also I used a lathe at 25rpm to wind the coil, and made a counter with a cheap calculator and a reed switch. The layers were packed well, except at the ends of the coil where I would stop every 2 layers and put some fiberglass tape down to even things out.

With an air gap between the poles of 0.9in :
   1amp 0.2T
   2amp 0.35T
   3amp 0.45T
   4amp 0.50T
   5amp 0.55T
   6amp 0.6 T
Field uniformity is better than 90%.

The coils in series have a Res of 9.1ohms cold. I'll probably be running the magnet around 3 or 4amps for my experiments with the rubber.

The total cost was $1200, $500 for the magnet wire, and $500 for a used LHP100-10 power supply (100V@10amps, 1000W), plus $200 for misc. hardware, tape, etc..  

What I would do different:
I think the steel I used wasn't a good choice. I used it because it was free.

For my situation I'm glad I built the magnet, if I had bought something off the shelf I would have had to modify it or make other fixtures to get everything to line up right. It took me around 40hrs to build.

RE: Laboratory electromagnet

Just out of curiosity, would a permanent rare earth magnet pair have met your needs?  It seems like you could achieve that 0.6T over that volum cheaper that way.

Eng-tips forums: The best place on the web for engineering discussions.

RE: Laboratory electromagnet

I don't think so. I have two 1in diameter by .5inch length NdFeB magnets and they would give around .4T over that gap, but the field is not uniform at all, a lot stronger in the middle and weaker as you head towards the edge (by about 50%). I probably could have designed the circuit to eliminate this, but that seems like a lot of work and a little mickey mouse since I want to be able to adjust the strength.  

RE: Laboratory electromagnet

You're getting quite a bit of saturation somewhere. But you got the field you want so it doesn't matter.

Looks like you accomplished quite a bit here. Well done.


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