Hi everyone,
>
>I took part in a competition of my college's electronics club to build
>an electromagnet under £100, with the strongest and cheapest one
>winning.
>
>Approach #1:
>I bought a 500W ATX power supply and used the 12V 35A lead to energise
>an electromagnet. I used a full 100m length of 0.8mm thick enameled
>copper wire - that length had a resistance of about 5 ohms. The wire
>reel in the as-bought state conveniently had both leads exposed.
>
>I put a 10mm steel hex bolt through the reel's centre hole as the core
>material. The electromagnet was feeble and was no where even close
>enough to carrying its own weight :(
>
>Approach #2:
>I also tried attaching a 6kV Cockroft-Walton voltage multiplier (as in
>instructables.com) to the same 100m 0.8mm thick enameled copper wire
>reel, but although the current was presumably very high it was of such
>short duration that some 5mm chrome steel bearings I had nearby didn't
>even budge :(
>
>So I'm back to square 1 and am open to suggestions on how to create
>the strongest possible electromagnet under a tight budget.
>
>Incidentally, am I correct in concluding that the length of the wire
>is not relevant to the magnet's strength as the resistance is
>proportional to the length but the strength is proportional to the
>number of turns (hence length) so both terms cancel out?
>
>So presumably the only electrical considerations as far as a resistive
>electromagnet is concerned is how many amps you can put through it
>before you burn it out. Does this imply that thick copper wire (a few
>mm at least) with a large thermal mass and a very high voltage, high
>capacitance power supply is the way to go?
>
>Thanks,
>Jessie xx
The geometry of the thing that you're trying to pull is critical. As is the motion that will be associated with the pull.
At any rate, you need a nearly-closed magnetic path that is reduced as the load is pulled. Any large air gap will be lethal, and your setup sounds like mostly air gap.
Think about an iron kitchen pot with an added iron center post. The interior, around the post, is wound with wire. The lid of the pot is held a bit above, on a spring scale. When you run current through the coil, the cover is pulled down. The air gap was small and gets smaller as the cover moves down. That setup makes a lot of force, and the force increases as the gap gets smaller, max when the cover is just about to close the gap.
What you want is maximum field strength, so you want the closed system to have minimum reluctance. So the "pot" and lid must be made of thick, high-permeability material.
A more practical approximation would be to use an E-core pulling against an I-core, or against the test plate if there is one. You could make that from a big old transformer.
You've never defined the geometry of the contest, so it's hard to give specific advice. Are the rules in writing? Do the people who designed the contest know what they're doing?
John