# Opposing electrical magnets

Discussion in 'General Electronics Chat' started by Gdrumm, Aug 2, 2014.

1. ### Gdrumm Thread Starter Distinguished Member

Aug 29, 2008
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Can electrical magnets be made to oppose each other the way earth magnets do?

Thanks

Gary

2. ### tom_s Member

Jun 27, 2014
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i would assume the answer is yes, my understanding - reversing the current flow would change the polarity.

3. ### Alberto Active Member

Nov 7, 2008
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When your coil is powered it will produce a magnetic field with north in front and south on the back ( depending on polarity if DC if AC you will have N and S changing at the AC frequency)

Alberto

4. ### studiot AAC Fanatic!

Nov 9, 2007
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An electromagnet has north and south poles in just the same way as a permanent magnet.

A linear solenoid generates the same field configuration as a bar magnet.

The direction is given by the right hand screw rule, with the North at the point of the screw as shown in the second part of the sketch on the right.

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5. ### ErnieM AAC Fanatic!

Apr 24, 2011
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Of course they can. A magnet is a magnet is a magnet.

6. ### shortbus AAC Fanatic!

Sep 30, 2009
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That's how a "wound field" electric motor works.

7. ### wayneh Expert

Sep 9, 2010
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An electromagnet in a black box can not be differentiated from a permanent magnet in a black box. As Ernie noted, a magnet is a magnet.

8. ### Gdrumm Thread Starter Distinguished Member

Aug 29, 2008
684
36
What would be the determining factors regarding how much of a gap I could achieve between the magnets, when they are opposing; more voltage, more core wire?

I'm thinking of an application where a machine, when in set up mode, would have two electrical magnets opposing each other, creating a gap (of about 1/8") through which I could thread a substrate.

Then, when switched to the run mode, the magnets would come together, grabbing the substrate, then by mechanical assist, feed the substrate into a die, to be cut.

The unit already exists, and has an electromaget that feeds the material into the die.
The bottom portion is the electrical magnet, the top portion is simply a steel plate.

But I'm looking for a way to thread the material into the magnet area during set up, that does not involve touching the magnet carriage mechanism with human hands.

If I can aviod touching the carriage mechanism during set-up, I can avoid having to lock out the machine, for OSHA concerns.

I've tried a simple spring attached to the top portion, and it does create some seperation, when the magnet is off, but not enough to do what I need to do.

If I increase the spring pressure, then when the unit is turned back on, it is not powerful enough to overcome the spring, and re-engage the substrate.

Thanks,
Gary

9. ### wayneh Expert

Sep 9, 2010
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I don't see why that would not work.

Is it conceivable to replace the steel plate with a permanent magnet? Then all you'd have to do is reverse the current in the existing electromagnet and it would go from attraction to repulsion. A negative, from a safety point of view, is that there would still be attraction with all power off.

Another approach would be to simply replicate the existing electromagnet and replace the steel plate, but you probably don't need as much field strength as this would enable. The 2nd magnet can probably be a much lower field strength.

Field strength is proportional to amp-turns. The usual challenge is to choose which wire gauge to wind so that, in the volume you have to place the coil, you get a reasonable compromise of DC resistance and turns; both go up with wire gauge and length. Of course current - and thus field strength - goes down with DC resistance. Heat dissipation is another concern.

10. ### studiot AAC Fanatic!

Nov 9, 2007
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To a first approximation you can adapt the formula, which is used to calculate the lifting capacity of lifting electromagnets.

$P = \frac{{{B^2}A}}{2{\mu}}$

Where B is the field induced by a single electromagnet by current I.
A is the cross sectional area of the facing steel areas.
P is the force developed between the faces.

Last edited: Aug 5, 2014
11. ### Gdrumm Thread Starter Distinguished Member

Aug 29, 2008
684
36
Replacing the steel plate with a permenant magnet in this case would create a problem. There must be a release between the two during the return stroke, otherwise it would pull the paper (substrate) back out of the die.

I will sleep on it.

It might be best to try to make one, and see how much seperation I get, using parts I have in stock.

It might be a Rube Goldburg too!

Thanks,
Gary