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.
--- For a solenoid, its internal magnetic field is described by:
µ N I B = ------- l where µ is the permeability of the core, N is the number of turns surrounding the core, I is the current in the coil, and l is the lenghth of the core
so, you can see that B will increase if the permeability of the core increases, the number of turns increases, the current increases, or the length of the core decreases.
To get the maximum strength, then, you want to wind a short solenoid with a lot of low resistance wire because, as the resistance increases the current will decrease for a given supply voltage.
Also, you'd like the external field to add to the internal field, so you'd want the solenoid to be encased by a high-permeabilty core.
To do that you'd want the core to be shaped like a bundt cake pan, with the coil nested inside of it, like this side view: (View in Courier)
. +---+ +---+ +---+ . | |ooooooooo| |ooooooooo| | . | |ooooooooo| |ooooooooo| | . | |ooooooooo| |ooooooooo| | . | |ooooooooo| |ooooooooo| | . | +---------+ +---------+ | . | | . +-------------------------------+
That way, the part of the magnetic field which would be lost will be captured in the walls of the "core" and will add to the pull of the center leg.
That's how junkyard electromagnets are made, BTW.
---
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?
--- Both lengths _don't_ cancel out since one is the length of the wire used to wind the coil and the other is the length of the wound solenoid.
---
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.
--- In a sense, yes, but you also need to be concerned with how many turn of wire you can get on the thing because that, and the current in the coil (ampere - turns) will determine the strength of the magnet.
---
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?
--- Not necessarily.
If you want to get a feel for the numbers, here ya go:
Let's say we have a core that looks like this:
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