Optimum electromagnet dimensions

[Bill Bowden]

Using an air core electromagnet with a given length of wire, what dimensions would be optimum for maximum force against a permanent magnet in close proximity? Should the electromagnet be wound with a large diameter and short length, or a small diameter with a longer length?

-Bill

 

[Tom Biasi]

On 2/12/2014 11:17 PM, Bill Bowden wrote:

Using an air core electromagnet with a given length of wire, what dimensions would be optimum for maximum force against a permanent magnet in close proximity? Should the electromagnet be wound with a large diameter and short length, or a small diameter with a longer length?

-Bill

Number of turns is a key factor.

 

[George Herold]

On Wednesday, February 12, 2014 11:17:59 PM UTC-5, Bill Bowden wrote:

Using an air core electromagnet with a given length of wire, what dimensions would be optimum for maximum force against a permanent magnet in close proximity? Should the electromagnet be wound with a large diameter and short length, or a small diameter with a longer length?



-Bill
Hmm maximum force makes me think*. But if you want the maximum B-field then 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 gradient you might wind two coils with their fields opposing each other. Think something like Helmholtz coils with one coil flipped.

George H.

 

[Tim Wescott]

On Thu, 13 Feb 2014 01:05:08 -0500, Tom Biasi wrote:

On 2/12/2014 11:17 PM, Bill Bowden wrote:
Using an air core electromagnet with a given length of wire, what
dimensions would be optimum for maximum force against a permanent
magnet in close proximity? Should the electromagnet be wound with a
large diameter and short length, or a small diameter with a longer
length?

-Bill

Number of turns is a key factor.

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.

--

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

 

[Bill Bowden]

On Thursday, February 13, 2014 12:08:33 PM UTC-8, Tim Wescott wrote:

On Thu, 13 Feb 2014 01:05:08 -0500, Tom Biasi wrote:

On 2/12/2014 11:17 PM, Bill Bowden wrote:
Using an air core electromagnet with a given length of wire, what
dimensions would be optimum for maximum force against a permanent
magnet in close proximity? Should the electromagnet be wound with a
large diameter and short length, or a small diameter with a longer
length?

-Bill

Number of turns is a key factor.

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.

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

You don't need to increase the voltage. A coil with twice as many turns using a smaller gauge wire in the same area will be more efficient since it draws half the current for the same total amp-turns. Question is how to arrange 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 will be many more amp-turns, but only a few small turns will be near the permanent 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

 

[Tom Biasi]

On 2/13/2014 3:08 PM, Tim Wescott wrote:

On Thu, 13 Feb 2014 01:05:08 -0500, Tom Biasi wrote:

On 2/12/2014 11:17 PM, Bill Bowden wrote:
Using an air core electromagnet with a given length of wire, what
dimensions would be optimum for maximum force against a permanent
magnet in close proximity? Should the electromagnet be wound with a
large diameter and short length, or a small diameter with a longer
length?

-Bill

Number of turns is a key factor.

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.

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

 

[Tim Wescott]

On Thu, 13 Feb 2014 21:05:56 -0500, Tom Biasi wrote:

On 2/13/2014 3:08 PM, Tim Wescott wrote:
On Thu, 13 Feb 2014 01:05:08 -0500, Tom Biasi wrote:

On 2/12/2014 11:17 PM, Bill Bowden wrote:
Using an air core electromagnet with a given length of wire, what
dimensions would be optimum for maximum force against a permanent
magnet in close proximity? Should the electromagnet be wound with a
large diameter and short length, or a small diameter with a longer
length?

-Bill

Number of turns is a key factor.

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.

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

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.)

--

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