Electromagnet - source of power

[hubihubi2]

Dear All,

I have constructed opposite poles electromagnet. The windings are made out of the enameled copper wire (~100m, ~300 loops, diam. 0.9mm). The core is made out of the recycled transformer plates (covered with translube varnish). The side plates are made out of the mild steel plates.
I have isolated the electromagnet core from the wires with the insulating board (to avoid enameled layer peel off during the wire winding). But I did not isolate enameled wire from the side plates (north and south pole). I am going to try this electromagnet with AC230V 50-60HZ as well as with DC24V 20A.
I wonder whether this electromagnet is safe to try (assuming that there is no short circuit between the wires- they were recycled from old transformer, and no short circle between wires and side plates. I have checked that with the multimiter- 'beep' option and no short circuit was indentified between side plates and the wire. But I do not know whether e.g. 20A will not cause some problems here).
I am going to put the electromagnet on the piece of wood or ceramic material during its testing.
P.S. The resistance checked for the coiled wire is 3.2[Ohm].
If the power of the electromagnet will be insufficient then I am going to increase number of copper wire loops.
I kindly ask for your opinions.
I send the picture of the electromagnet attached to this message.

 

[waross]

Counter-intuitive though it may seem, increasing the number of turns may have no effect or may reduce the strength of the coil.
The strength of the magnet depends on the number of Amp:turns.
DC
The Amps depends on the resistance of the magnet.
Doubling the number of turns generally doubles the resistance which cuts the current in half. So you have half as much current through twice as much resistance. This results in the same number of Amp:turns and the same strength.
But, double the resistance is a quick estimate. Generally the added turns will be on top of the existing turns and so will be longer. That means greater average resistance per turn. The end effect of this is that the magnet is weaker.
AC
The current is the vector sum of the resistive current and the reactive current.
The effect on the resistive current is similar to the DC effect.
The inductive reactance increases with the square of the number of turns (with some second order effects due to skin effect).
Again the magnet will be weakened.
So why not use just one turn???
I²R Heating.
As the number of turns increases, the R increases linearly (neglecting increases in the average resistance per turn) and the I² decreases as an inverse square function.
To increase the strength of the magnet, increase the voltage. If it gets too hot, then increase the number of turns.
You may also reduce the frequency of an AC magnet supply.
Please read an elementary text book on electro-magnets.
As for safety I make no suggestions and no assurances. I do not have enough information to make an informed suggestion. I don't know what safety codes are in effect in your part of the world.
I comment only on the technical effects.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[IRstuff]

I don't think the ampacity of your wire is anywhere close to what you are attempting to pass through it:

http://en.wikipedia.org/wiki/American_wire_gauge

TTFN
faq731-376

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[OperaHouse]

Testing with a lamp in series with the 120V AC is always a good first step to see if there is a potential short to the core. That will prevent any wire blow out. It can also give you a sense of the current draw. Something high current like a toaster or a large number of lamps for actual load testing. If a variac is handy plot current with voltage or view V/I with a scope. Re3member, once the core saturates its just a resistor..

 

[waross]

Let's do some numbers;
3.2 Ohms on 24 Volts = 7.5 Amps
Watts = I²R = 7.5 Amps x 7.5 Amps x 3.2 Ohms = 180 Watts.
12 Volts may be safer. About 45 Watts.

Bill
--------------------
"Why not the best?"
Jimmy Carter