Small electromagnet design guidelines.

[salukikev]

I've recently built a very small electromagnet the size of a pencil eraser. It works, but I would like to improve it. It is not a high precision application- right now I'm just shooting for "better". I already understand that I can use more turns of a thinner wire to yield a coil which operates at higher voltage and lower current, or reverse that and have a lower voltage higher current setup- maybe the overall watts remains the same and the resultant flux is the same, or maybe one would make the case that more turns can fit onto this coil when they are a smaller diameter wire and so that is the way to maximize gauss in something like this. (random, intermittent 1 second pulses).
Secondly, I took a guess at the core profile ratio and diameter- I think right now my diameter is around .1" and the overall diameter at the flanges is around .24" What dimension(s) will most effect my overall magnetic pull? Are the steel (I don't have any pure iron handy) flanges helping, hurting, or not effecting my magnetic attraction? I could press on some plastic or other material flanges if needed. Their purpose now is just to contain the wire. I also have an opportunity to make the design a little bit longer if necessary, and I may order a soft iron core if the performance improvement would be significant. Thanks for any general suggestions!

 

[EdStainless]

What is the current NxA (amps and turns)?
We can estimate the field, and size the core.
The flanges should not be steel.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[salukikev]

It is a LOT of turns, I don't know exactly as it would take me all day to count (I wound it with a power drill). I can estimate based on wire diameter and core dimensions. If I destroy one then maybe I could manage to unwind and measure length but the wire is so thin it will take a lot of care. The current is easy though, the 4 solenoids I made vary from around 14mA to 22mA. I actually cracked open a typical (larger) relay to extract the wire as I didn't have any that small available. The way they manufactured that relay was to have a core with a flange on one end and the other end was stamped onto a flat steel mounting bracket- thereby making essentially two steel flanges. I was partly motivated by that in thinking that steel flanges should be ok, but I will update based on your suggestion. What material should they be? I have a lot of choices (brass? plastic? composite?)
Thanks!

 

[dgallup]

What are your design goals? Do you want speed, force, low power????? Without goals, there can be no optimization. If you really want to dig in and understand, download FEMM and learn how to use it.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[EdStainless]

People use a turn counter when they wind coils so that they can get the correct number of turns.
Using a steel flange will produce a lower fields strength, but it will be over a larger area. So you need to know what strength and shape of field you are after. There are lots of tricks in this.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[salukikev]

I have FEMM now but only just getting familiar. Its a handy tool but I won't learn it overnight.
EdStainless- Thanks! That type of general tip is just what I'm looking for. Dgallup: I can vary the power quite a bit, at least on the prototype. Anywhere from 2-30v dc is pretty easy to manage for this version so long as we don't burn out the coils. Even your lowest spec speed would be fine for this application I'm sure. Force/range would be the area of highest concern, and per ES's comment it sounds like I need to get rid of my flange(s?). I'd like a focused, strong, field. I will try to get more information to better define the existing info, but I don't have those #'s just yet. The flanges do serve a valuable purpose of containing those coils though, so if you have a suggestion for what material they can/should be (instead of steel)- I'd love to hear it.

 

[dgallup]

We always wind coils on molded nylon bobbins, but that's for volume production. We have occasionally machined bobbins from nylon rod for prototypes.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[salukikev]

dgallup:
Thanks! So plastic- that's easy enough, although for your production I'd guess cost is a bigger motivator. My motivation on the prototype would be to have very small and thin flanges, and that works a bit counter toward using plastic as opposed to something that will stay in place better on a metal core with only a thin line in contact. I may just machine a full bobbin with as thin an interior wall as I can manage, but that will subtract from the amount of copper I can fit (or core diameter).

 

[dgallup]

We have powder coated metal parts to wind directly on them when space is really tight.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[salukikev]

I've used that trick (with paint though) when I need a super thin insulator, but in this case the motivation is just to have something structural to help secure the plastic flanges.

 

[EdStainless]

Machine plastic flanges that will be a tight press fit onto the ends of your core.
Can you estimate turns or the field strength? If it is below the saturation of the iron core then you could make core smaller and add more turns.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube