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need help designing and building electromagnet

need help designing and building electromagnet

need help designing and building electromagnet

(OP)
I want to build a c frame electromagnet that will be powered with a variable dc source range of 3 to 24 volts. I need to create 200 gauss to 800 gauss in a 3/4" air gap. The c frame made of iron can be 2 inches on each side and maybe tapered down at the pole tops to help concentrate flux in air gap. Can somebody help me with this electromagnet?

RE: need help designing and building electromagnet

Looks pretty easy. How big an area do you want for the field? That depends on the application and how much fringing is tolerable. So, over what size and shape do you need the field to be reasonably uniform, and how uniform does it need to be?

RE: need help designing and building electromagnet

(OP)
A 3 inch by 3 inch area this electromagnet needs to fit. I need the field to be across the c core poles and mostly uniform

RE: need help designing and building electromagnet

(OP)
Is there some place I can have this built

RE: need help designing and building electromagnet

I'm guessing English isn't your first language. Fair enough.

The magnet needs to fit in a 3" by 3" cross section. Ok.
What about the other direction? How long can it be?

You still haven't said what the size of the "uniform" region needs to be. Is it 1/2" x1/2"? Bigger? Smaller?

RE: need help designing and building electromagnet

(OP)
The field needs to be uniform across an air gap of 3/4" by 1". It can be up to 5" long

RE: need help designing and building electromagnet

Air gap 5" long? Sounds pretty tough with those dimensions... And uniform, too.
That usually calls for a cylindical coil.

But, OTOH, 200 Gauss isn't extreme. Equals .02 Vs/m2, if I'm not mistaken. But you need to talk to someone that knows about these things. Not so cheap, I guess.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: need help designing and building electromagnet

Now this "design" is almost certainly no good for your application. You have really not said very much and all the "hidden" parameters will mean my design is "wrong". Of course I am pre-supposing that I haven't made an arithmetic error or other error.

The idea is to go through the steps with you so you get the idea how to start on the design.

You will note that I have just apparently plucked values out of the air. That's pretty much how it is done. They don't usually mention this at college because they can't justify their fees if you just have to guess!

RE: need help designing and building electromagnet

(OP)
I appreciate all your help. Do you know where I can have a core that shaped machined?

RE: need help designing and building electromagnet

(OP)
Also, what are the dimensions of the pole faces?

RE: need help designing and building electromagnet

"What about the other direction? How long can it be?" I took the 5 inches to be the maximum length of the magnet. I may be wrong.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: need help designing and building electromagnet

(OP)
Yes, 5 inches is maximum length of magnet

RE: need help designing and building electromagnet

Why you guys take the easy way and refer to 200 Gauss only? Gus2014 asks for 200 to 800 Gauss. 800 Gauss is not as easy as 200 Gauss. No one refers to iron saturation possibility at high Gauss values.

RE: need help designing and building electromagnet

OK, then it's doable. Do what Logbook says. PLS for him.

One question: is the taper needed? Or can't you just use right-angle pole pieces? The flux will find its way to the air gap all the same.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: need help designing and building electromagnet

Logbook does use 0.08 Vs/m2 in his design. I was thinking 5" air gap. So I kept the flux density as low as possible.

The iron area in the coil can be one tenth of the pole piece area, or even less, because saturation isn't usually a problem before 1.2 - 1.4 Vs/m2

The DC excitation makes things somewhat easier. You do not need laminations, which is an advantage.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: need help designing and building electromagnet

[skogs] One question: is the taper needed? Or can't you just use right-angle pole pieces? The flux will find its way to the air gap all the same.
The taper is only there to reduce the weight. It adds machining time so a trade off can be made. Making the whole iron 30mm x 30mm cross section is quite a lump!

RE: need help designing and building electromagnet

[gus2014]Do you know where I can have a core that shaped machined?
As it was, it was really difficult to machine.

I have drawn it to scale and improved the design so it is readily machinable using a cheap 2-axis mill. Any machine shop will be able to make it, including quick turn around places like this place

The design is tighter than I at first thought. The coil is a bit too close to the gap for comfort (due to the 3" constraint). This will affect the field uniformity, but without magnet simulation software the field uniformity is just a guess.

RE: need help designing and building electromagnet

(OP)
Is there a place where I can send them your sketch with figures and they can plug it into there electromagnetic field simulation software to test it.

RE: need help designing and building electromagnet

(OP)
How much bigger would I have to make electromagnet core, so that the coil would so close to the gap?

RE: need help designing and building electromagnet

[gus2014] Is there a place where I can send them your sketch with figures and they can plug it into there electromagnetic field simulation software to test it?
Yes, it is called a design consultancy. I don’t personally know any but I bet google does.

[gus2014]How much bigger would I have to make electromagnet core, so that the coil would so close to the gap?
The 62 dimension could usefully increase to 67 and still just stay within your requirement.

I tried to buy some simulation software recently. MagNet
Of course none of these software guys have price lists so you already know it is expensive. By the time I got all the options so I could actually do 3D modelling with transients it got up to £20,000. Naturally I just pointed out that I could make a whole lot of cut-and-try models for that so no thanks.

The real problem for you going to a design consultant at the moment is that you haven’t actually stated the problem you are trying to solve clearly enough. You have stated a uniform area of ¾” by 1” but you never answered the question of what you mean by “uniform”.

Let’s look at the field at the middle of the gap. The field will be strongest there. Now move outwards (at right angles to the field lines) and the field strength will drop. But also the field lines will be angled outwards. Is it the total field strength you are interested in or is it only the vertical component? You need a criterion to give to the designer.

For example, “I want a uniform area of ¾” by 1” such that the vertical component of the field at the edges is not less than X% lower than the field at the middle”

You obviously need to replace X with a number such as 10. If you want X=0.1 then forget it. You cannot achieve that spec in the space given. The point is you need a starting point and the designer can then come back and say if it is not possible and give you options.

Also you have defined an electromagnet for a DC application. 8W seemed like a lot to me but I don’t know your application. Maybe a permanent magnet solution would be better (if possible).

The designer is also going to want to know how many you want to make. If you want to make 1 and you have a budget of £30 for the designer then forget it!

RE: need help designing and building electromagnet

(OP)
Can I use a 15 volt battery with this application? Also, x can be 10.

RE: need help designing and building electromagnet

I use a small quite decent B probe to measure magnetic fields. If you get yourself a few pieces of mild steel and of suitable size, then you can easily mock up something to start testing with. The probe I use is the i-prober 520. It works from DC up to several MHz and can be used either as a current probe or a field probe.

It works up to 2.5 mT - so you will not be able to use it all the way up to full field strength, but you can reduce the excitation current and check uniformity, it will be very much the same at 2.5 mT as it is for 20 - 80 mT. I see no risk that saturation cuts in at these levels.

Tip for improved uniformity: Use shims made from mild steel plate and arrange at the perimeter of the pole shoe faces. It makes it a lot easier to "mould" the field you want.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: need help designing and building electromagnet

Quote (Gus2014)

Can I use a 15 volt battery with this application?
The idea of the forum is to give help and advice, not to do it all for you.

I have given you a worked example with 0.5mm wire requiring 8.2V to operate at 1A.

Try working it out for more turns of thinner wire running at a lower current. Post your result and it can be checked.

Alternatively use a switched mode regulator so you can adjust the field without incurring excessive power loss.

RE: need help designing and building electromagnet

(OP)
I appreciate all the help, I will post my results

RE: need help designing and building electromagnet

On a DC magnet you will find that a greater number of turns will lower power consumption but will not make much difference to the strength. For greater magnetic field strength you must use either higher voltage or larger diameter wire.

Example:
The field strength depends on the number of Amp Turns. That's Amps time turns.
"I have given you a worked example with 0.5mm wire requiring 8.2V to operate at 1A."
Now double the number of turns of this wire. The resistance will be substantially doubled and the current will be halved.
Half the current through twice as many turns results in the same Amp Turns.
You may add a second coil with the same number of turns and run both coils in parallel for greater strength.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: need help designing and building electromagnet

(OP)
The more options I have to get the desired results the better chance I have of achieving them. So, thanks for all this info.

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