AVR-Chat

Roy makes a good point about skin effect. Forgot about that. Could be  
part of your excessive impedance.

Jim Wagner

On Feb 19, 2009, at 10:25 PM, Roy E. Burrage wrote:

> 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
>



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