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Max current in wire

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cduma

Fri Feb 10 2012, 04:01AM

Registered Member #1822 Joined: Fri Nov 21 2008, 08:04PM
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Ive found several charts that show the maximum current for power transmission. Im trying to find what the max peak current would be for a certain amount of time such as 5-30s for use in electromagnets.

m4ge123

Fri Feb 10 2012, 04:21AM

Registered Member #4118 Joined: Mon Oct 03 2011, 04:50PM
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For around 5-30s I'd guess about 100A/mm^2 for a copper coil at room temperature. At liquid nitrogen temperature copper has about 1/40 its resistance at room temperature IIRC, so you might be able to push 4000A/mm^2. You really just have to try it out to know for sure.

Bjørn

Fri Feb 10 2012, 04:56AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
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Calculate the conductor mass, decide what temperature rise you are comfortable with (take into account any previous heating) and look up the heat capacity of copper. Then you know how much energy it will take to reach your limit. Use Ohms Law to figure out the current that will give you that energy in the time available. This will be a pretty good approximation for the maximum current of coils for periods in the second range.

cduma

Fri Feb 10 2012, 12:54PM

Registered Member #1822 Joined: Fri Nov 21 2008, 08:04PM
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Is there a type of wire with a higher resistance that can be used for electromagnets?

Dago

Fri Feb 10 2012, 01:32PM

Registered Member #538 Joined: Sun Feb 18 2007, 08:33PM
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cduma wrote ...

Is there a type of wire with a higher resistance that can be used for electromagnets?

Why would you want your electromagnet to heat up more (=waste power)? You'd want lowest possible resistance for the electromagnet. That's why MRI machines and such use superconductive magnets.

Pinkamena

Tue Feb 14 2012, 02:46PM

Registered Member #4237 Joined: Tue Nov 29 2011, 02:49PM
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cduma wrote ...

Is there a type of wire with a higher resistance that can be used for electromagnets?

You want low resistance, not high. You also gotta take into account that many materials gets higher resistivity when it heats up. If you have had some basic university level differential equations, it shouldn't be a problem to set up the differential equation for the temperature.

Dr. Brownout

Wed Mar 21 2012, 03:28AM

Registered Member #2405 Joined: Fri Oct 02 2009, 12:59AM
Location: Wisconsin
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I like to base the current handling of any wire based off the circular mills of the wire gauge. for example if you figure 14 gauge wire has 4107 circular mills and it is rated at 15 amps then that wire is rated for 274 circular mills per amp. That rating is a continuous rating in a pulsing condition you can run them much higher. However it really depends on the temperature rise you willing to tolerate.

But anyway the example I gave can be used as a starting point from there you can play with the numbers and come up with a number for you application. You can find charts all over the internet that has the wire gauges and circular mills and so forth.

Dr. Brownout

Wed Mar 21 2012, 04:11AM

Registered Member #2405 Joined: Fri Oct 02 2009, 12:59AM
Location: Wisconsin
Posts: 140

cduma wrote ...

Is there a type of wire with a higher resistance that can be used for electromagnets?

Basically Electromagnets, Solenoids, inductors, and anything like that is really computed using Amp turns. And what I mean by that is the force generated is generated by current, knowing that tid bit lets say you have a one turn coil with 100 amps flowing into it in that case you would have 100 amp turns, now lets say you have a 100 turn coil with 1 amp flowing though it, now guess what... you still have 100 amp turns. of course there is more to it than just that, such as core material, shape of the core, length of the winding, diameter of the winding...etc but in any case the amp turns remains the same.

So just keep in mind if you use a higher resistance then you will need more voltage to increase the current back to the amp turns you are targeting for the amount of force you want.

Calculating the temperature rise is not as difficult as it sounds and there really is no tricky math to it its kind of time consuming if you do it by hand especially if you have multiple layers with layer insulation and multiple windings each time you make a change you have to do the whole process over again. So if you need more information on how design such a critter let me know and I will help you along.

radiotech

Wed Mar 21 2012, 09:53PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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You might start with the fusing current table. #22 AWG Cu 41 Amps.

3/2
I = Kd^ d inch k Cu 10250

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