Author Topic: Electromagnetism Question  (Read 3126 times)

Taurus

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Electromagnetism Question
« on: March 16, 2005, 08:58:01 PM »
If I have an electromagnet and I want to boost the flux density and its magnetic strength, do I increase the voltage or current through the conductor(s)?

Also ... which makes a more efficient and just overall better electromagnet? AC or DC?

Thanks
J. D. White

BillBlank

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Electromagnetism Question
« Reply #1 on: March 16, 2005, 11:22:53 PM »
Hmmm, what do you need a powerful electromagnet for? Nosy sods who enjoyed their physics lessons would love to know. Digging out old textbooks as we speak btw Smiley
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DustinD

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Electromagnetism Question
« Reply #2 on: March 16, 2005, 11:34:46 PM »
Keep in mind I am not an expert, just a lay person.

I am not sure about flux density. I think a higher voltage will push the fields out further, but that is just a guess. Maybe rewind the coil to a different shape? Also keep in mind that magnetic strength is equal to distance squared.(twice the distance has only a fourth the magnetic field)

Your limiting factors for current through the conductor will be IR^2 losses(twice the current means four times the losses). Too much current and you will over heat your magnet. For voltage your limiting factor will be insulation(I am guessing enamel?) breaking down due to high voltage combined with high heat, and electric arcing.

As for AC vs DC efficiency. I think DC is slightly more efficient for wattage through a given wire size and type. By that I mean more power can flow through a thinner wire, not that you will loose less power to heat. If the frequency is high, such as 60Hz or more, and you have three or more phases than it does not really matter, as both AC and DC will carry about the same energy.  If you meant energy losses that depends on current, components that you use, and the quality of your electromagnet.

Another thing to keep in mind is that AC is much more gentle on electronics because it slowly builds voltage and repeatedly crosses the zero potential mark. Many switches and breakers can handle a lot more AC power than DC because DC will try to arc weld the contacts when they open. AC does not have that much of a problem because when the voltage reaches zero the electric arc shuts down and will not try to jump back across when it builds back up.

As for flux and electromagnets. I think depending on what you are trying to do there may be a big difference between an AC magnet and a DC magnet. The AC electromagnet will have it's fields constantly growing and collapsing with whatever frequency (Hz) it is running at. That can make things more complex than a DC magnet with fixed magnetic fields.

I hope something I said was useful. I will try to attach a freeware magnetic flux program that I have. Edit: I Found it and uploaded it to my site. http://www.dustin.o-f.com/storage/femm34bin.exe

Try www.otherpower.com There are many experts there that can help you, or search their archive. Also www.eng-tips.com can help you.
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Taurus

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Electromagnetism Question
« Reply #3 on: March 16, 2005, 11:38:47 PM »
Bill, it's funny how you assume this needs to be a powerful magnet. I never stated it needed to be powerful, however, your assumption this time is correct. I want to produce piece by piece and tinker with a miniature rail gun from which to launch light grain projectiles. BTW: My conception on launch differs and strays somewhat from current physics.
J. D. White

BillBlank

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Electromagnetism Question
« Reply #4 on: March 16, 2005, 11:46:10 PM »
Fair point lol, I never said I was smart did I? Assumptions are a bad habit of mine.

On the other hand mini rail guns could make a fun project. Combined with a REALLY big carbon arc beam weapon, hmm.
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yesterdaysyouth

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Electromagnetism Question
« Reply #5 on: March 17, 2005, 04:16:04 AM »
faraday's law: the amount of voltage induced in a coil is directly proprotional to the rate of change of the magnetic field with respect to the coil.

lenz's law: when the current through a coil changes, an induced voltage is created as a result of the changing electromagnetic field, and the direction of the induced voltage is such that is always opposes the change in current.

so in theroy you could do it either way.... it's just a matter of application, and power available....

Nathaniel Firethorn

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Electromagnetism Question
« Reply #6 on: March 17, 2005, 04:17:38 AM »
(a) Current.

(b) "Better" how?


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jamz

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Electromagnetism Question
« Reply #7 on: March 17, 2005, 08:43:27 AM »
Well, you could start making it really cold.  Our magnets here use liquid helium to stay cold enough to have good field strength.  :p



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Werewolf

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Electromagnetism Question
« Reply #8 on: March 17, 2005, 11:05:32 AM »
I don't remember the formula for calculating magnetic field strength (measured in Faradays IIRC) but for a stronger magnetic effect use more turns.

I.E.
Using the same size wire @10 Turns wrapped around the core

|   |   |   |   |   |   |   |   |   |

and @30 Turns in the same length

||||||||||||||||||||||||||||||

You will get a stronger field out of the 30 turn magnet.

Upping the voltage will result in a stronger field too but since current is a function of voltage and resistance if you up the voltage too high you'll melt the wire.

You can use a smaller wire to get more turns but smaller wire=higher resistance.

This assumes you use DC. If using AC you now have to consider the inductive resistance that exists in the coil and that is a function of turns and frequency IIRC.

Fun little experiment to do though.
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P95Carry

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Electromagnetism Question
« Reply #9 on: March 17, 2005, 05:57:13 PM »
As an engineer I am afraid to say I have forgotten much - books would have to come out.

IIRC tho - large dia wire windings and very high current is way to go.  The volts are just the ''push'' (pressure)  - the current is quantity (volume flow).

I am thinking Coulombs and ''Q'' factors but cerebral rust is most inhibiting!  Then we have flux density - the strength and numbers or frequency of lines of field - which would be A/C.  I seem to think that DC will or can produce the strongest force, power factor for power factor.  It is also therefore a polarized field ... north is north - south is south.  A/C being alternating is changing field poles at frequency of supply - tho something tells me the big magnetic ''picker-uppers'' in scrap yards are A/C driven.

Oh my - have to go back to books for this game Cheesy
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