Help with electromagnet flux/field calcs

Todd619 · Oct 17, 2005

I'm designing an electromagnet and found the thread "Designing a simple elctromagnet and need help" very helpful, especially UKpete's formula tutorial. I've been playing around with those formulas, but I would like to try to refine the calculations and try to get the calculations as accurate as possible. I want to figure in the reluctance of the yoke and pole pieces and also hopefully calculate losses due to fringing and leakage which was not part of the calculations shown below. Can anyone describe how to do that using the same type of formulas provided by UKpete below. I would prefer to avoid calculus as I only had one year and that knowledge is rapidly rusting away (no line integrals please)

*********************************************************
reluctance S = g/(?0*A)

where g is the airgap length (m), A is the airgap cross-sectional area (m²),
?0 is permeability of free space 4*pi*10-7

flux ? = NI/S where N = no. of turns, I = current (A)

flux density B = ?/A - in Tesla (1T = 10000G), A is airgap csa as above.
******************************************************
I combined and rearranged these so that I can estimate my amp turns needed; NI = Bg/?0

My electromagnet will differ in that it will have two coils in what I believe is called an "H" configuration (square yolk w/ 2 pole pieces on the inside each with a coil and a single gap) instead of a "C" configuration and will be larger and more powerful than the one in the thread I mentioned before. It will be about 6" or 15.24 cm dia. at the pole faces and generate a field of about 1.5 T across a 1.5 cm gap and I plan to try to design it so that it can be air cooled, I'm not sure what type of steel or iron the yoke and pole pieces will be made of yet.

Thanks for any help
-Todd619

Todd619 · Oct 17, 2005

Well the symbols for BH (?) value and flux (?) didn't read in my post before in the calcultions so let me try and paste it again

***********************************************************
reluctance S = g/(?0*A)

where g is the airgap length (m), A is the airgap cross-sectional area (m²),
?0 is permeability of free space 4*pi*10-7

flux ? = NI/S where N = no. of turns, I = current (A)

flux density B = ?/A - in Tesla (1T = 10000G), A is airgap csa as above.
***********************************************************
I combined and rearranged these so that I can estimate my amp turns needed; NI = Bg/?0

Todd619 · Oct 17, 2005

Oh I give up on the symbols. It previews fine, but then they don't show, I'm pasting them from another thread, is there a toolbar I can download for those or something?

-Todd

Lakey · Oct 17, 2005

Hi,

Thread 237-84957 may be of some assistance to you.

Happy hunting.
Regards,

UKpete · Oct 17, 2005

Todd - re the special characters, I've been up that particular learning curve!

If you look at the bottom of the box you type your post into, click on the bit which says "Process TGML". This will open a small window, scroll down about 1/3 to the section headed "Special Characters" and click on the underlined bit "TGML Character Entity Reference". This brings up a table of characters including the Greek ones (if thats what you are looking for) - I cut and paste the bit in square brackets in the first column into my post and it seems to work. If you want to add a subscript look in the first window (after clicking "Process TGML" as above), it tells you how. Good luck!

Re the original question, I'm still looking in Lakey's thread, there is a lot of good info in there.

MJR2 · Oct 17, 2005

Well I have not yet left for my hunt and things have slowed down a bit here. So I took a closer look.

I think what you are desiring to build is something like a figure 8 with the air gap where the intersection of the 8 is. What follows is a rough design for a square pole of 5.3 inches and 15000 gauss in an airgap of 0.6" and two coils providing the ampturns. Cold this coil provides just under 60,000 NI.

The face of the pole is 5.3 inches square with a 30 degree taper, 2.5 inches long to a cross section of 8.1868 inches. This pole face has the same area of your 6 inch diameter. My utilites are set up for square. But the answer would be considered similair.

Two coils built of 39 lbs #11 and 146 lbs #10 each. The coil height is 4.75 inches with inside dimensions of 8.25 inches square. Power is 4000 watts at 230VDC total both coils. Moving the coils to be immmediately adjacent to the poles or air gap would allow for 1000 watts at 115VDC for about the same result. This coil will require force cooling or intermitent use. A hot NI value of 70 percent of cold was used for the FEA work.

All steel is type 1020. The core through the coil will run at about 13500 gauss. That cross section carried through to the pole runs at only an average of about 9000 gauss so it could be smaller. Say about 2/3's without suffering much loss. Across the center line of the air gap we have 16200 CL to 14600 at the edge and 13600 at an outside corner. A round pole of course would not have a corner. It was intentionally about 10 percent higher than 15,000 gauss. Getting iron that is as good as it is supposed to be has become challenging. So that is our SF.

Have fun.

Todd619 · Oct 18, 2005

Thanks UKPete, Lakey and MJR2. I'm looking over all the good info.

-Todd619

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