AVR-Chat

Done the exploding wire deal Dave, both intentionally and non.

1.  How are you measuring your shunt voltage and waveform?  A scope I 
presume.

2.  Your shunt will be somewhat inductive at these frequencies and currents.

3.  Your frequency components will appear because of the pulsed 
waveform, so you'll need to be aware of that.

4.  Have you tried to cancel fields by running the cable into and out of 
your shunt in opposite directions?  At these kinds of currents, physical 
placement of the wires will have an affect.  For example; we built a 
multi-station circuit breaker calibration system for distribution 
transformers, substation types up to 2500 amps, some years ago and the 
way we had to match station impedances was to change the routing of the 
350 MCM cables.  To achieve 0.25% system accuracy this was no minor 
accomplishment and the station impedances had to be matched even better 
than this.

5.  Could the difference be caused by an induced field in your probes?  
1KA at ~1mSec is a huge field to contend with.  200uSec rise time makes 
things even more interesting.  Physical routing of your wires might 
reduce any problems of this nature as well.

6.  Have you thought about "skin effect?"  That makes life even more 
interesting at high currents and itsy bitsy rise times.  If you 
calculate this for 60 HZ you'll be amazed at the difference it will make 
in the resistance of a wire with frequency.  If you can do an FFT 
analysis of your wave?  Then take the highest significant harmonic 
component and calculate the skin effect you might be in for a little 
surprise.

          http://en.wikipedia.org/wiki/Skin_effect

7.  16 AWG copper wire has a DC resistance of 4.02 milli-ohms per foot.  
Can you measure or estimate the length of your wire?

8.  Can you take a comparable length of copper wire, stretched out 
straight, and get the same or similar readings?

9.  You're right about the timing of your pulse not being great enough 
to cause serious heating.  How about your duty cycle?

10.  The TCR of copper is +0.39% per degree C at 20C.  Not significant, 
but still a factor.

11.  Don't forget about your magnetic pulse and it's affect on item 5 
above.  We built a small transformer impulse test system once that went 
on a pilot line in the lab.  They were supposed to build a steel 
shielded room, but it wound up being copper as any normal RF screen room 
would be.  Impulse testing is basically generating about a million volts 
then jumping an arc to the transformer to simulate a lightening strike.  
We struck that baby off the first time ... and blew up every instrument 
in the lab with the magnetic pulse.  Did I mention that we only did that 
one time?

I'm going to be gone for a couple of days but this ought to give you a 
couple of days worth of experimentation.  Let's see what results you get 
from the above and then look deeper if it's still a problem.


REB



David VanHorn wrote:
> On Thu, Feb 19, 2009 at 9:37 PM, Jim Wagner <wagnerj@proaxis.com> wrote:
>   
>> In this mode, the current capacity depends on a whole bunch of things.
>> Thermal mass, length, heat sinking capability of the end-terminals,
>> melting point of the alloy are just a few. You might be able to model
>> it with some sort of finite-element simulator but you would spend a
>> lot of hours doing that and what you got would be hard to validate.
>>     
>
> It's in the form of a 44mm ID two-layer coil, with 64 turns.
>
> I was hoping someone here had done "exploding wire" experiments and
> could offer guidance.
>
> My risetime looks great, and the flyback diode is damping the ringing
> fine, but I am nervous that at 1kA I'm pushing the wire too hard.
> Still, it hardly has time to get hot.
>


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