Electromagnetic force 5

[lemwells]

I know this is a basic question, but I cant find an answer in literature that match the units I have measured. I have a magnet whos strength is measured in Gauss. I have a set of electromagnets with known number of turns, size of wire, AC voltage and current. The electromagnets face each other so that the field lines flowing out of one flow into the other. If the magnet is placed in the feild of the electromagnet, it will want to move perpendicular to the feild lines. The question is, how can I calculate the force that makes it want to move.

 

[stardelta]

Is there a good reason why you need to know this, or is it just because its exam time??

 

[lemwells]

The magnet's oscillation is used to power a diaphragm pump. I need to improve that pump and the motor is one place where that's possible. I just cant find my reference and thought maybe an online forum of likeminded people who would be willing to help. Sorry, no test.

 

[spargher]

If i am remembering well... you should calculate the magnetic energy of the system for each position of the mobile magnet.
The force is then the derivative:

F = dE/dx (or -dE/dx)

Where E is the magnactic energy function of the displacement x

 

[electricpete]

"If the magnet is placed in the feild of the electromagnet, it will want to move perpendicular to the feild lines"

I'm have a hard time imagining your geometry. You have two permanent magnets with a gap and flux flowing between them. If the permanent magnet is on a pivot directly between two electromagnets, then it will want to align with the lines of flux - N toward one and S toward the other. Then when polarity reverses it will want to align the other way.

I am just guessing at your geometry. Pls explain more.

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[fsmyth]

IIRC, I ran across a good treatise on exactly this at one of
the web sites for a vendor of BLDC motors. Been too long,
but I think it was a white paper on rotor design. Surf around.
<als>

 

[lemwells]

Thanks fsmyth, I've been surfing and continue to do so. I guess I'll do a search on rotors, too.

electricpete, there are no rotating parts in this assembly. There are two e-core electromagnets facing each other. In the gap between the two faces are a pair of permanent magnets suspended by two rubber diaphragms so that they can only move translationally along an axis perpendicular to the electromagnetic feild. The N and S poles of the permanent magnet are parallel to the electromagnetic feild lines and are not allowed to rotate as the field changes direction. This is not an issue as the dominant force is attempting to move the permanent magnets perdendicular to the electromagnetic flux direction and it only reverses, never changes angles.

spargher, I agree a derivative will be more accurate and I didn't think about the energy relationship. I've been looking for a Force equation. I really only need an average and a peak value but if I have to integrate, so be it. It will be nice to have a good plot of whats going on in there. Thanks.

 

[electricpete]

If the permanent magnet poles are aligned with the electromagnet magnetic field lines and sitting squarely in the center of the gap between pole pieces, I don't see where there is any perpendicular force. Unless the permanent magnet is offset from the center of the gap such that it wants to pull toward the center or repel from the center?

In any case, it's going to be a tough road to come up with a force. I don't know the units used to express strength of permanent magnets.

This might be a good post for the Magnetic Engineering forum.

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[lemwells]

The permanent magnets are indeed positioned so that their "at rest" positions are in the gaps of the e-cores. I'll try this ( E:3 ) , if the E and the 3 are the facing, e-core electromagnets, the two dots of the colon are the two permanent magnets.

I have found the following vector equation: F= q*v X B where q is a charge on a particle in Coulombs, v is the charge velocity vector in m/s, and B is the electromagnetic flux vector in Gauss. Because this is a vector cross product, the force will be at a maximum when the charged particle (permanent magnet) is moving along a vector that is perpendicular to the lines of electromagnetic force. The problem with all this is that the permanent magnet's strength measured in Gauss, not Coulombs. Is it possible I have lost my mind and that B is the MAGNETIC flux and that qv (which is coulomb*meter/sec or Amp*meters) is the ELECTROMAGNETIC force? Could the A*m be simply the Amps flowing through the coil multiplied by the length of wire in the coil? That would make it easy, too easy. It also doesn't match what little literature I've found which says that q*v is the charge and velocity of the moving permanent magnet and B is the electromagnetic flux. There are reasons I am not an electrical engineer, this is one of them. I thank you all for all of your help.

 

[lemwells]

I just noticed a typo, in the 6th line from the bottom, I wrote "force", I should have written "charge velocity"

 

[IRstuff]

Your equation has zero bearing on this problem. It is for a moving CHARGE in a magnetic field.

TTFN

 

[lemwells]

Ok, I thought a magnet was a volume of charged particles. Am I stuggling with a gross missinterpretation of something I learned in physics? My kingdom for an electrical engineer!!!!

 

[Skogsgurra]

A moving charge IS a current.

Gunnar Englund

http://www.gke.org

 

[IRstuff]

Magnets attract magnets. There's no EE involved.

TTFN

 

[lemwells]

Right, a charge as measured passing a point is current in Amps or Coulombs per second. An A*m is a Coulomb*meter/sec or the velocity of a charge multipied by its magnitude.

 

[lemwells]

You're half-right IR, if you reverse the poles they attract each other.

 

[lemwells]

electricpete, the magnet poles are in opposite directions and they are indeed being attracted and repeled from the center of the flux feild. As one is repeled from one feild, the other is attracted to another feild so that they both move in the same direction. Once you gave me the idea, I drew it out and checked to see if the poles were opposed. Thanks

 

[lemwells]

I finally found a reference, complete with formulas and fundamentals on the Knovel site. For those who may be interested, enter Knovel, go to Electrical and Power Engineering, then Electrical Devices and Equipment, then Handbook of Small Electric Motors by Yeadon, W. H. and Yeadon, A. W. As spargher pointed out in the beginning of this thread, it is in fact the change in energy over the change in position. Mr's Yeadon and Yeadon already took care of the calculus and even went into designing the electromagnets for specific flux.

Again thank you all for all your help and any other thoughts you may have.

Lem