dc electromagnet question?

I have constructed a DC electromagnet for a special use at my shop. Being a electrical contractor I thought this would be simple. (NOT) Made the electromagnet, connected it to a 12vdc power supply at 5 amps and it works great with one problem, it starts to heat up quickly. My desired results was for it to run on a 80% duty cycle. Will resistors keep this heating problem from happening? DC voltage is not something I have dealt with indepth. Can anyone help me with this problem? snipped-for-privacy@cox.net Vick

Reply to
vick
Loading thread data ...

A resistor in series will decrease the current, decrease the dissipation in the coil and decrease the strength of the electromagnet. If that is ok, then fine, stick a resistor in series or lower the voltage.

Otherwise, you need a way of getting rid of the 60W that you are dumping into the coil - or redesign the electromagnet. A fan or two (or four) might be an idea.

Another idea, that might be appropriate, is a resistor that can be switched out. So, the full strength of the electromagnet will be available when needed, but some form of automatic or manual switch reduces the current when the full strength isn't.

Reply to
Palindrome

Is there a way to reduce the wattage from the power supply or should I just get another power supply with less wattage? Thanks for your help!!!!!!!!!!!!!!!!! Vick

Reply to
vick

You can get a power supply with less *voltage*. That will reduce the current and hence the power dissipated - and the strength of the electromagnet.

Reply to
Palindrome

If you're an electrical contractor remind me never to employ you to do anything electrical. You clearly haven't a clue.

Reply to
Stuart

Being "positive" this time.

Rewind your coil with a finer gauge of wire but use more turns.

The higher resistance will reduce the current and hence the heating but as magnetic flux is the product of the number of turns and the current you can maintain the required strength of field/

For example, if you reduce the wire size from 1mm square to 0.5mm square and double the number of turns. The current, and hence the power dissipation, will be halved but you magnetic flux will remain virtually the same.

Reply to
Stuart

Reply to
vick

Reply to
vick

----- If you use resistors, you will reduce the heating-but you will also reduce the force produced by the magnet in proportion. Using a lower voltage source will also do the same.- In that case increase the number of turns of wire to compensate -this will also add some resistance.

For example, twice the number of turns (same wire) doubles the resistance and halves the current resulting in half the heating for the same force(proportional to ampere-turns)- but a bigger magnet. Life consists of tradeoffs.

Reply to
Don Kelly

----- You indicate a doubling of the wire length and half the cross-section--resulting in 4 times the resistance. From this the current and losses will be 1/4 the original value but the force will also be approximately 1/4 (proportional to {NI}^2). (unless the original setup was driving the iron into saturation ).

Reply to
Don Kelly

Reply to
vick

Glad to be of service - I trust you read my other post?

Reply to
Stuart

Hmm! I must remember not to post at 20 mins past midnight, when I'm already half asleep, in future :-)

Your other post is of course the correct answer.

Reply to
Stuart

That was why, at a slightly earlier time, I settled for writing, "Or redesign the electromagnet" - without explaining how. :)

I thought, if he really wants to know, he can ask and I can answer - when the old brain cell isn't still swimming in cherry brandy..

Reply to
Palindrome

Nice!

Personally I prefer a single malt but cherry brandy's nice too.

Reply to
Stuart

You are not the only one to boob due to lack of sleep, surfeit of wine, or simple haste. I have done it more times than I want to count.

Reply to
Don Kelly

I hate to be the wet blanket here, but winding a coil with smaller diameter wire will NOT change the heating if the field strength is kept the same! It will change the voltage required to give that heating.

As a hint: A coil wound with 950 feet of 20 gauge wire will weight about 3 pounds, run on 12 volts dissipating about 15 watts in the coil (think of a soldering pencil in there!) and produces a field of about

250 Gauss (without any iron present). The coil will draw about 1.2347 amps at 12 volts and have a resistance of roughly 9.7 Ohms. If you change the wire gauge keeping the coil size identical everything will remain the same except the voltage needed to get that field. Note that coil inductance will be greatly changed but that is not of concern here. (Inductance of the 20 ga version about .042 H.)

(Calculation of the above values left as an exercise for the interested student!)

You can only lower the heating by lowering the voltage drive which of course lowers the field strength of the magnet. Winding with finer wire but keeping the voltage the same will reduce heating but also the field. If you can get away with a lower field that is fine. Otherwise, the usual solution is to use lower voltage and heavier wire! Namely tiny copper tube with insulation on it. You run water through the tube to take the heat out of the coil. Except for dealing with water it makes a killer electromagnet. Takes a heavy current low voltage power supply though.

Reply to
Benj

Why (NI)^2?

MMF is just NI!!, as in ampere-turns!!

Reply to
BFoelsch

-----------

mmf is NI but the mechanical force is dependent on (mmf)^2 . Check it out.

Reply to
Don Kelly

Sorry about that.

A "senior moment," I guess.

Thanks again.

Reply to
BFoelsch

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.