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Patrick

Mon Dec 12 2011, 07:29PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

I think the Onderonk 32mS case is the closest standard thats useful in your application, as i dont believe the time is variable in Onderonk's original work.

Lrt me look again at his work.

Anyway, youll have to be careful, since very fast rise times squish the current from the center towards the skin surface of the conductor, so pulsing current will behave differently than its average AC or DC equivelent. Skin effect and proximtiy effect will both become significant, me thinks. (assuming a multilayer coil on the latter.)

Ill offer this advice: "One experiment is worth a thousand expert opinions." Perhaps its time for you to make some copper incandescent.

EDIT: upon further research, i found that time can be solvable. ->

HVGuyAFSL

Mon Dec 12 2011, 08:14PM

Registered Member #4252 Joined: Fri Dec 09 2011, 10:43AM
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I saw the equation here ->

So you're saying time is a constant??

Patrick

Mon Dec 12 2011, 08:47PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

HVGuyAFSL wrote ...

I saw the equation here ->

So you're saying time is a constant??

I was looking at the same PDF ! lol.

No, you can solve time as a variable. In a given circumstance, it will vary based on your inputs. you can rearrange, by means of algebra, to find the conditions needed for a desired time interval.

short of real testing i think this method gives you good, but not great results.

HVGuyAFSL

Mon Dec 12 2011, 08:56PM

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I also looked at this table

The last columns apply the Onderdonk equation for 2 different time intervals.

Patrick

Mon Dec 12 2011, 10:47PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

HVGuyAFSL wrote ...

I also looked at this table

The last columns apply the Onderdonk equation for 2 different time intervals.

Ah yes, Preece and Onderdonk, also there is the 32mS comlun, i see that one quoted often by others, though i wonder if they know anything about fusing formulas.

HVGuyAFSL

Wed Dec 14 2011, 01:57PM

Registered Member #4252 Joined: Fri Dec 09 2011, 10:43AM
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Hey Patrick. Then what can i use to know the conductor section? I just know the discharge will have a peak current of 35kA at 17us.

Patrick

Wed Dec 14 2011, 02:51PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

HVGuyAFSL wrote ...

Hey Patrick. Then what can i use to know the conductor section? I just know the discharge will have a peak current of 35kA at 17us.

Im tempted to say 1/t or 1/0.000017s = 59kHz, but that may not be right. then use that freq (if its right) to find the skin depth , then calculate the solid wire's resistance as if it were a hollow pipe with the wall thickness equaal to one skin depth.

I might be wrong about all this though, but thats where id start if i were you.

klugesmith

Wed Dec 14 2011, 04:12PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

HVGuyAFSL wrote ...
Hey Patrick. Then what can i use to know the conductor section? I just know the discharge will have a peak current of 35kA at 17us.

Welcome!

Back to what Bjorn and Steve have said:

If you intend for your coil to survive, then it's an adiabatic heating problem. (Mechanical strength is another matter). What temperature rise can you tolerate? Tables and formulas relating wire size to impulse current rating often have stated assumptions like "75 C before pulse, and no damage to thermoplastic insulation".

Path 1: Estimate I^2 t (technically, the integral of I^2 dt) for your current pulse. You don't need to be too exact. Then refer to table/formula. Or multiply by the resistance of some length of wire, to get the joules of heat absorbed. Divide by the wire's mass and specific heat to get its temperature rise.

Path 2. Figure the mass of copper in your whole coil (including connecting wires if they are the same wire gauge). How hot will that get if it absorbs all the energy from your capacitor? This method has you covered even if the wire's resistance increases significantly from skin effect, or from temperature coefficient as it heats up in microseconds.

[edit] I shudder to think of doing all that arithmetic without a spreadsheet calculator program.
[edit edit] We will be glad to review your answer, if you provide the necessary background data.

Patrick

Thu Dec 15 2011, 01:12AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Ok the adiabatic situation does simplify things, perhaps thats the better path to pursue, also not having your coil fly apart is an issue. (IE explosive quarter shrinkers on you tube)

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