need help designing and building electromagnet 1

[Gus2014]

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?

 

[logbook]

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?

 

[Gus2014]

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

 

[Gus2014]

Is there some place I can have this built

 

[logbook]

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?

 

[Gus2014]

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

 

[Skogsgurra]

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

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[logbook]

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!

 

[Gus2014]

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

 

[Gus2014]

Also, what are the dimensions of the pole faces?

 

[waross]

"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
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"Why not the best?"
Jimmy Carter

 

[Gus2014]

Yes, 5 inches is maximum length of magnet

 

[israelkk]

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.

 

[Skogsgurra]

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

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Skogsgurra]

Logbook does use 0.08 Vs/m[sup]2[/sup] 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/m[sup]2[/sup]

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

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[logbook]

[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!

 

[logbook]

[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.

 

[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.

 

[Gus2014]

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

 

[logbook]

[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!