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Science Forum Index » Physics - Electromagnetic Forum » attraction force on electromagnets...
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| Camilo... |
 Posted: Sat Jul 05, 2008 8:26 pm |
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Hi everybody! Speaking about industrial electromagnets, like this one:
http://www.coolmagnetman.com/images/em500a.jpg
¿Is an electromagnet like this, strong for attract nearby
ferromagnetic objects? ¿How closer they need to be to get attracted
strongly? or ¿are they strong just for holding ferromagnetic objects?
(particulary, at the webpage where shows this electromagnet, they say
that it can hold 500 lb just using a 1,5 v "D" size cell, so it´s
obvious that this class of electromagnets are too good for holding
ferromagnetic objects; what i need to know is how good are the to
attract and how strong they attract nearby objects) |
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| Camilo... |
| Posted: Sun Jul 06, 2008 8:42 pm |
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Ok, but in the practice, leaving aside formulas; on an industrial
electromagnet which have all the design conditions to be strong, ¿is
it only strong to hold, or is strong to attract too? |
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| Don Kelly... |
| Posted: Sun Jul 06, 2008 9:20 pm |
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----------------------------
"Camilo" <camilogil at (no spam) 100cia.zzn.com> wrote in message
news:4037f2ec-b634-4257-a330-007f3e002add at (no spam) e53g2000hsa.googlegroups.com...
Hi everybody! Speaking about industrial electromagnets, like this one:
http://www.coolmagnetman.com/images/em500a.jpg
¿Is an electromagnet like this, strong for attract nearby
ferromagnetic objects? ¿How closer they need to be to get attracted
strongly? or ¿are they strong just for holding ferromagnetic objects?
(particulary, at the webpage where shows this electromagnet, they say
that it can hold 500 lb just using a 1,5 v "D" size cell, so it´s
obvious that this class of electromagnets are too good for holding
ferromagnetic objects; what i need to know is how good are the to
attract and how strong they attract nearby objects)
The attractive force depends on the ampere turns in the winding, the
cross-section and length of the air gap and the geometry of the magnet and
gap. Ignoring the iron, for a total air gap length g and area A, with N
turns and current I, the force is (mu0/2 )N^2I^2A/g^2. In the magnet shown,
there are two air gaps, preferrably of the same area. (use 2g instead of g)
muo is the permeability of free space and fringing of flux is ignored.
This is not valid at a gap very near 0 as the iron will limit the flux.
You can then use force =1/2 BHA where B is the maximum flux density with no
gap. Then B is determined by H=NI/L where L is the iron path length and B
is generally found from a B-H curve for the material rather than using B=muH
because u is so variable.
--
Don Kelly dhky at (no spam) shawcross.ca
remove the X to answer |
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| Benj... |
| Posted: Mon Jul 07, 2008 6:04 am |
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On Jul 7, 2:42 am, Camilo <camilo... at (no spam) 100cia.zzn.com> wrote:
Quote: Ok, but in the practice, leaving aside formulas; on an industrial
electromagnet which have all the design conditions to be strong, ¿is
it only strong to hold, or is strong to attract too?
In short, a magnet attracts iron based upon the rate of change with
distance of the magnetic flux flowing through the magnet and attracted
piece. As you move a piece of iron away from a magnet, the air gap
that appears acts like a "resistance" to the magnetic "circuit". This
means that as the air gap is increased, the magnetic flux flowing
through the magnet and lifting piece "magnetic circuit" is greatly
reduced. The greater the distance the greater the reduction. Therefore
Electromagnets tend to hold items with a LOT of force when the gap is
small or nearly zero, but do not "attract" items from a distance very
well. It is possible to adjust these things but as a general rule that
is true. Just go find any magnet and stick a piece of iron to it.
Clomped on to the magnet it will stick with lots of force. But at some
distance the force of attraction will be much less. |
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| Camilo... |
| Posted: Mon Jul 07, 2008 11:04 am |
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"Electromagnets tend to hold items with a LOT of force when the gap is
small or nearly zero, but do not "attract" items from a distance very
well. "
ahhh... that´s what I was thinking about, just I wasn´t too sure, so
thanks my friend for your answer!! |
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| Don Kelly... |
| Posted: Tue Jul 08, 2008 5:20 pm |
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----------------------------
"Camilo" <camilogil at (no spam) 100cia.zzn.com> wrote in message
news:3162a128-61c9-4a68-ac43-c411bed0b28e at (no spam) 79g2000hsk.googlegroups.com...
Ok, but in the practice, leaving aside formulas; on an industrial
electromagnet which have all the design conditions to be strong, ¿is
it only strong to hold, or is strong to attract too?
The holding strength is the important issue. It is the maximum attractive
force possible. The objective is to bring the magnet into contact with the
load and then lift it mechanically. A magnet of the design that you showed
is designed to hold at zero gap. --
P.S. I have just read Benj's comments- and they are excellent.
Don Kelly dhky at (no spam) shawcross.ca
remove the X to answer |
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| Autymn D. C.... |
| Posted: Fri Jul 11, 2008 3:36 pm |
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| No, a battery is /not/ a complex 1,5 voltage. |
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