Bert Hickman wrote...
Ignoramus26172 wrote:
Cydrome Leader wrote:
In rec.crafts.metalworking Ignoramus26172 wrote:
RoyJ wrote:
Why not keep them and start trying to shink coins?
http://205.243.100.155/frames/shrinkergallery.html
I want to keep one or two caps at most. I could build
a coke can crusher... I think that 2 uF at 18 kV could
be enough to deform the cans.
For can crushing you need a minimum of about 400 Joules, and
for coin crushing at least 2000 Joules. I happen to use a bank
rated at 140 uF at 12 kV. I also use Maxwell energy discharge
caps, but mine are Series C 100 kA high current type. Because
mine are rated for only 20% voltage reversal, I only take the
bank up to about 9500 volts (6300 Joules) for coin crushing.
I thought you used four 70uF 12kV caps in series parallel,
for a 70uF 24kV rating? The coin crushing is where you
count on the wire coil disintegrating within the first half
cycle, with the arc rapidly extinguishing to limit reversal
the voltage? Or are you charging to a smaller fraction of
the 24kV bank faceplate rating?
How much of the energy is taken up by the coin crushing and
coil stretching?
For can crushing, I only go to about 3500 Joules (mainly to
reduce wear and tear on the spark gap switch).
That's using 140uF? That would be two paralleled 70uF 12kV
caps from your bank, charged to 7kV, or about 60% of the cap's
faceplate rating? Implying only 35% voltage reversal while
staying under a 20% limit? Is that with a 3-turn coil, which
would be about 1uH? What's the Q of the 13kHz resonance?
Using all of your 15 caps in parallel would give you a capacitor
bank capable of delivering ~3 kJ, so you are in the right
ballpark. However, can crushing (especially) and coin crushing
can cause highly oscillatory discharges. Rapid voltage reversals
are very stressful on a HV capacitor's dielectric system, and
most of Maxwell's pulse caps are only rated for 10-20% voltage
reversal (at faceplate voltage), so you don't want to run these
caps anywhere near their full faceplate voltage if you are doing
can or coin crushing - they WILL prematurely fail. And, you
definitely don't want to be anywhere near the caps when the energy
from the other 14 capacitors dump everything they've got into a
single faulting cap... :^)
So, sticking to 60% of the faceplate rating, that'd be 13kV
allowed on the full 15 x 1uF = 15uF cap bank, which would be
only 1.3kJ available, where 3.5kJ is needed for can crushing?
Looking at the "Frankenstein" insulator style used on your caps,
they are likely not rated for more than 2 - 5 kA peak (the folks
at General Atomics can probably provide you with their actual
specs): http://www.gaep.com/capacitors.html).
For a 3-turn 1uH coil and 140uF caps at 7kV, that's 83kA peak
in your case, Bert? 83kA/15 = 5.5kA. But a higher voltage
would allow using more inductance and lower peak currents.
Running more caps in parallel will help to share the peak current
seen by each capacitor. If you plan to do any coin shrinking, treat
the coil like a small bomb, with copper shrapnel being ejected at
hyper velocities.
There's more information on my site:
http://205.243.100.155/photos/shrinker5.pdf (1 page summary)
http://205.243.100.155/frames/shrinker.html (more gory details)
And, always remember to be afraid - very afraid - of the energy
stored in these caps. They will not give you any second chances.
=:^[
Bert
--
Thanks,
- Win