Electro magnet

[Gregory]

I have a transformer that I am making into a electromagnet using a 12 v battery . The wirer size is 3 mm dia and there is 2 wirers joined.
I have provided photos of the battery and trans former.
I connected the wires to the battery but the insulation started to melt but the magnetic field is very strong . What can I reduce voltage or current to prevent the wire I connected to the battery from melting but to maintain the magnetic force.

 

[ronsimpson]

You have this:

Change to this:

Do not do this:

 

[Gregory]

When I change the connections do I have to reduce the Voltage , or current . I should be able to cut 2 wires to arrive at what you have posted and join 2 together .

 

[ronsimpson]

My change will reduce the current. Give it a try. It may get hot and them we can find a way to reduce the voltage and current.

 

[Gregory]

I have done what you requested and the wires melt the insulation and now the transformer windings get hot and started to smoke.
Have not damaged the transformer.

 

[dr pepper]

The original windings are probably not suitable for using on dc as an electromagnet, to get it to work well you'd really need to recalculate the number of turns, and wire size then rewind the core.
There is a balance between magnetic field, current flow and number of turns, you'll probably get nearly as good a magnetic field with a lot less power designed correctly.

 

[Gregory]

Do I require a smaller wirer more turns with a higher resistance would It be better to use a resistance of 132.2 ohms or a winding with a resistance of 1.8 ohm which one would be the best winding.

 

[Tony Stewart]

Depending on duty cycle of operation needs, you can judge what resistance of wire can fit the form, if you can't find a form design tool.

The thermal resistance of the wire will limit how much power it can handle with a a corresponding temperature rise and rate of rise thus determines maximum on time.

The power dissipation will be 12V^2/R and the thermal resistance will be much poorer than a heatsink but it has some mass, so it may be on the order of 30 deg'C per wat but have a 2 minute time constant. So if just using for seconds, that may be acceptable. A PTC polyfuse can protect it with the desired holding current.
Let's say you wanted 50 Watts for 10 seconds with 10% duty or 5Watts average. Then assuming my guestimates are reasonable, it might rise 150'C in a few minutes but ok for 10 seconds with a 30'C rise.

Thus for 50 Watts or 4 A@12Vdc you need to choose the AWG of wire and length to replace the existing volume of magnet wire with 3 Ohms worth of wire.

There are ratios often used to estimate the change in AWG to resistance per meter but I wont elaborate on that, you can use the AWG tables to convert your 3mm (AWG pairs to something like 0.5mm. You can use an Ohm meter or lookup in tables.

Basically to find ohms/meter, divide Volume Resistivity by area in circular mm^2.

 

[Gregory]

I am looking at a 35% duty cycle

 

[RODALCO]

That thick gauge wiring is unsuitable for 12 Volts DC and will overheat. Better to rewind or find an other transformer with a thinner gauge of wire.

 

[Gregory]

The 2 resistances I have given you are 2 different transformers which would be the best one to use

 

[Tony Stewart]

Do I require a smaller wirer more turns with a higher resistance would It be better to use a resistance of 132.2 ohms or a winding with a resistance of 1.8 ohm which one would be the best winding.

somewhere in between

1.5Ω = 80W @ 12V
132.2Ω=1.1W @12V
15W at 35% d.f.= 5.25W which might rise to 60C if used in short durations is my guess using

R=V^2/W=144/5.25W = 27Ω of magnet wire
AWG? depends on volume avail.

 

[audioguru]

An electromagnet has two exposed poles, a north pole and a south pole with high magnetism between them.
But a transformer does not have the poles exposed so its external magnetic field will be weak.

A DC electromagnet uses the resistance of its windings to limit the current. But a transformer uses its AC reactance to limit the current.

The iron core saturates at a certain current then increasing the current increases the heating but the magnetic field does not increase.

 

[Tony Stewart]

yes you would do better with a high permeability iron core with open ends as in a solenoid design?

why are you reinventing the wheel? or trying...

 

[Les Jones]

I think Gregory is just using the "E" laminations so the open ends if the "E" should provide a reasonable magnetic path.
I do not think he will achieve the same strength of magnetic field with a sensible winding. From the picture I suspect the original winding is well under one ohm. So the current would be greater than 12 amps.
Gregory, the magnetic field is proportional to the current times the number of turns.

Les.

 

[audioguru]

He is measuring the resistance of the leads of his multimeter.

 

[Tony Stewart]

He could get a nice fusion arc from making/breaking contact repeatedly, also good EMI generator.

 

[dr pepper]

The higher resistance winding 132 ohms will pull much less current, 100ma or 0.1a, however you will have much less physical pull force, a rough guess would be it'd lift about 500g.
12 ohms would be a ballpark resistance for that size of magnet.
Also current draw will be affected if there is something across the end of the e plates, if the magnetic circuit is completed that is.

 

[Les Jones]

I do not agree that closing the magnetic path will change the current. (Other than a fluctuation in the current at the instant the magnetic path is changed.) If it was being powered with AC then the current taken would be reduced when the magnetic circuit is closed.

Les.

 

[Tony Stewart]

Steady state DC Current is control by only the wire resistance but flux density is controlled by turns ratio, current core cross section, permeability leakage flux and few more variables.

AC Current would be controlled by the core effective inductive impedance (f).

Magnetic force would depend on the orientation at right angles to the winding and magnetic (mu) coupling of the core and the degree of saturation current x N windings. CRGOS laminate can normally handle 1 to 2 Tesla depending on quality.