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A yen for serious coin shrinking

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LAN_403

Tesladownunder

Wed Jun 07 2006, 01:18AM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

Bert wrote ...

.. The marked non symmetrical shrinkage was probably due to the coin being constrained only on one side. ..., I've never tried shrinking with only one dowel rod in place.

I didn't use any dowel at all. It was slipped in to the split gap of the PVC but wasn't a good fit. The ice (presumably) filled the gap. I tried to avoid an air pocket but who knows.

Bert wrote ...

...The estimated open core (no coin) Wheeler inductance of your work coil was perhaps ~1.6 uH or so. The low power scope shot indicates that you got four complete cycles over a period of ~475 usec, for a resonant frequency of about 8.4 kHz. Assuming a 215 uF bank capacitance, your overall initial loop inductance (including "loaded" work coil, caps, switch, and bus bars) was about 1.67 uH - in the ballpark.

The system rings at 25kHz with a 6 inch wire in place of the coil giving a system inductance of 0.2 uH.

Bert wrote ...

Assuming 5 kJ of bank energy (~6800 volts) and no change in work coil inductance, the predicted current peak would be about 77 kA. However, we know that the coil stretched and expanded as the current increased. In order for the measured current peak to only be ~50 kA, the "effective" work coil inductance must have increased to ~3.9 uH, implying that the work coil expanded to an average diameter of about 1.7" by the first current peak. ..

So my measurements might be right. It also means I can push a lot harder with the power and still be within the caps ratings. Using 10kJ is not out of the question with my set up.

Bert wrote ...

.. In any event, I have little doubt that arcing is the source of your excess system losses.

What I really want to know is if my losses are less than with the plain coil. Is it worth pursuing the ice containment?

Bert wrote ...

.. I'd recommend repeating the experiment, but this time using a simple close wound coil of heavy build polyamide-imide magnet wire, centering the coin with identical dowels on each side, and simultaneously monitoring coil voltage and current.

Sounds like that is next. I have avoided voltage measurements as it means a connection to the HV which makes me nervous.

Bert wrote ...

And obviously, you also need to develop a more robust containment method before repeating the experiment.... :^)

If I don't have ice to absorb the energy I most certainly will.

BjÃ¸rn BÃ¦verfjord wrote ...

What about using a photodiode to detect when the first arcs appear?

My camera (same as yours) goes to 1/8000sec so won't be too good for the 8kHz full cycle. It's all over by then.
Just the time for a Rapatronic camera...

Peter

Omicron

Thu Aug 03 2006, 08:05PM

Registered Member #131 Joined: Fri Feb 10 2006, 09:25PM
Location:
Posts: 185

Another good coin for shrinking is the canadian nickel. They are almost solid nickel so they are very conductove.

Steve Ward

Thu Aug 03 2006, 09:02PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Another good coin for shrinking is the canadian nickel. They are almost solid nickel so they are very conductove.

Have you actually tested this? I know (from first hand experience) that the american nickel is probably the most difficult coin to shrink (im not sure if its metal composition, though). Nickel is about 4X more resistive than copper. Aluminum (pure) is only 1.7X more resistive, and is much softer, so it should be by far the easiest coin to shrink.

Bjørn

Thu Aug 03 2006, 09:59PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

I forgot about this thread...

My camera (same as yours) goes to 1/8000sec so won't be too good for the 8kHz full cycle. It's all over by then.

What I meant was to use a photodiode to detect the arc and compare it to the current graph to make sure you interpret the current graph correctly. Try something new, learn something unexpected and all that.

Bert

Fri Aug 04 2006, 04:26AM

Registered Member #118 Joined: Fri Feb 10 2006, 05:35AM
Location: Woodridge, Illinois, USA
Posts: 72

I've shrunk both types of nickels. A Canadian nickel (~100% Ni) actually shrinks somewhat better than a US nickel (75% Cu, 25% Ni). At 6300 Joules, a US nickel shrinks to about 92% of its original diameter, while a Canadian nickel shrinks to about 86.7% of its original diameter. Strangely enough, pure nickel is a significantly better electrical conductor than the cupro-nickel alloy used in the US nickel, even though the latter alloy is 75% copper. The Cu-Ni alloy only has 8% of the conductivity of electrolytic copper (IACS - International Annealed Copper Standard), while nickel has 22% of the conductivity (almost 3X better). A bit of nickel in copper really messes up its electrical conductivity.

Because it is so soft and has good electrical conductivity (~61% of IACS), aluminum coins are probaby the easiest coins to shrink - like Japanese 1 Yen coins.

Bert

Fusion

Tue Sept 05 2006, 02:09PM

Registered Member #354 Joined: Thu Mar 30 2006, 10:12AM
Location:
Posts: 55

GREAT.
It would be very cool (also for me) if you test using a FLAT spiral coil. You can get lots of Teslas as it depends of coil volume that is very little.
(the coin must be placed inside the coil center)
I think that the coil must not suffer very much

Here it is an attached excel file used to design spiral coils
]1157465391_354_FT10774_flat_spiral.zip[/file]

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