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Registered Member #8032
Joined: Tue Nov 13 2012, 01:22AM
Location: Chicagoland USA
Posts: 33
Can anyone tell me what would happen if I make a coil from a 30 gauge wire, about 40 turns, only about 4ft of wire. 4 ft of wire should have about .4 ohm resistance. I need to run pulses of 80amp 40v through it. Can the coil survive that much amps if I keep the coil at the rated temp, say below 120c?
Seems to me that 80 amps through 30aw wire is a lot but 40v / .4ohm seems to allow up to 100amps? Whats wrong with my calculations?
I need a coil that is 1 layer, 1cm wide, with a power supply of 40v 80a max and need to figure out the best geometry for the coil.
Registered Member #90
Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301
supak1 wrote ...
What would happen if I make a coil from a 30 gauge wire, about 40 turns, only about 4ft of wire. 4 ft of wire should have about .4 ohm resistance. I need to run pulses of 80amp 40v through it. Can the coil survive that much amps if I keep the coil at the rated temp?
This is relatively large current for a very small wire. So first look at the wire's fuse limit: at high current density the wire will melt like a fuse.
This manufacturer's datasheet at www.hmwire.com says that 30 gauge wire will melt at 7.5 amps.
Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600
How long do you need it to carry the current for? It can do it, for a short time.
You can get the heat capacity for the metal of the wire, and plug in the energy, and calculate the heat rise.
Make sure you allow for the rise of resistance with temperature though. Also I think that hotspots can sometimes form, and then you can get thermal runaway, so it's a good idea to allow for some leeway.
Registered Member #8032
Joined: Tue Nov 13 2012, 01:22AM
Location: Chicagoland USA
Posts: 33
Fuses are exactly what I was thinking, didn't seem right that 80a would go through 30a wire.
Its not for long, coil would rise to 80a in 1 - .01ms. Problem I see is that I may need lots of HZ so. My coil will be very low inductance, goal is to make a very fast coil.
Metal is just pure copper with 200c shielding, I wish I know how to figure out the heat, is there a calculator? What do I search for?
BTW I am planing on coiling the coil with water, this experimental setup where I am going to have my coil sit in water, the actual coil rings/wire will be sitting in water for maximum surface area.
OMG, it will handle it without a problem if it is a single spike. You are building a reluctance coilgun? The thicker the wire, the longer the current will oscillate in it. Just use 2mm copper stick if you are able to bend it.
Registered Member #8032
Joined: Tue Nov 13 2012, 01:22AM
Location: Chicagoland USA
Posts: 33
Yes building a reluctance coilgun. My projectile is 1cm so I want to make the length of the coil 1cm. I also want a really really fast coil, so only about 10-20uH and one layer. To get 10-20uH I need to have about 40 turns and to fit 40 turns in 1cm 1layer I need to use a 30aw wire.
Yandersen are you sure I can do quick 1ms or less pulses of 80a through 30aw wire? How many HZ could I run through such coil at 40V* 80a?
2mm wire is 12aw, I'll only get 5 turns with that, that is way too low of inductance.
Registered Member #8497
Joined: Tue Dec 04 2012, 06:24PM
Location:
Posts: 74
I'm with Yandersen on this one. A 1ms pulse of 80amps in 30AWG is no big deal. Although I have a feeling it'll be far less than 80 amps, and much more than 1ms.
Supak, didn't i told you about the spiral flat coils and flat projectiles for reluctance coilgun? Wanna try the most inefficient way to build the most inefficient type of coilguns (at such energies)? OK, go ahead. Maybe your projectile will move a little proving that your thing may "work"... Your wire will sustain single pulse, not continuing series of those. Trying to plug your coil to the outlet, touching the wires for a little time, huh? It will burn. But be more aware about the soldering points - they may blow before the wire. Put drops of solder all around wires.
Registered Member #8032
Joined: Tue Nov 13 2012, 01:22AM
Location: Chicagoland USA
Posts: 33
I didn't understand what you meant by "Just use 2mm copper stick"? Please explain. Thanks for the heads up on the soldering points, something I hadn't though about. I should be able to keep the pulse below 1ms. I'll be using IGBTs with a opto-switch for fast trigger times. Thinking about using compressed air to speed up the projectile to X speed and drop the amount of time the projectile is the the opto-switch to less then 1ms, and then the multi coils will speed it up more.
Registered Member #8497
Joined: Tue Dec 04 2012, 06:24PM
Location:
Posts: 74
That's cheating ;)
Hybrid coils have been discussed before - electo-chemical, exploding wire, compressed air, spring-start, etc....
If you're looking down that road, you realistically should switch out from HV completely. The major benefit of these CG's is that they are damn close to being silent, relying purely on electro-magnetism.
Some people have done 'injector' coils which is essentially a small coil to accelerate the armature into the main coil.
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Coil construciton: 80amp through 30aw wire?
