Optimum electromagnet dimensions

Number of turns is a key factor.

ons would be optimum for maximum force against a permanent magnet in close proximity? Should the electromagnet be wound with a large diameter and shor t length, or a small diameter with a longer length?

Hmm maximum force makes me think*. But if you want the maximum B-field the n I learned here (maybe a year or so ago.) That it's a stubby coil. (I don 't know exact dimensions) But short, with lots of turns.

*The force on a magnet depends on the field gradient. So to make a big gra dient you might wind two coils with their fields opposing each other. Thin k something like Helmholtz coils with one coil flipped.

George H.

Not really. Ampere-turns is a key factor, but if you replace ten feet of wire of a given area with twenty feet of wire with 1/2 the area, you'll have something that fits in exactly* the same space, takes exactly twice the voltage to achieve exactly half the current, and burns the same amount of power to generate exactly the same number of ampere-turns.

• Exactly if you ignore how well the space is filled with actual copper, and not either enamel or air. But you get the drift, I hope.

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Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com

I do get the drift but the OP stated "Using an air core electromagnet with a given length of wire".

You don't need to increase the voltage. A coil with twice as many turns usi ng a smaller gauge wire in the same area will be more efficient since it dr aws half the current for the same total amp-turns. Question is how to arran ge the turns. If the wire is just one giant single turn, then there is only one amp-turn spread out over a wide area. If the core is skinny, there wil l be many more amp-turns, but only a few small turns will be near the perma nent magnet. Maybe the winding area should be matched to the physical size of the permanent magnet? I thought there might be some magic ratio for the problem.

-Bill

I missed the "given length" part. Sorry -- I just solved a different problem.

Bill:

Specifying the length of the wire isn't very helpful -- in general, the volume that you fill the wire with (assuming good packing) determines the performance/watt; the exact impedance is just a detail that can be changed by using the same weight of a different gauge of wire.

This is why, when you start looking at families of motors, you used to see a motor figure of merit that boils down to torque/unit power -- because that figure of merit is roughly constant for any winding that you put on that motor's armature.

(They don't do that much anymore, to my knowledge. You saw it on data sheets for American servo motors. American companies don't make servo motors any more, and Swiss engineers think that English-speaking engineers are stupid because we get the job done with just a bachelor's or master's degree, where they need doctorates to do the same thing. Go figure.)

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Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com