Winfield Hill wrote:
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?
Although my capacitor bank is capable of being configured for 24 kV (+/-
12 kV with cases of all four caps grounded), I presently use only half
of the bank since it provides excellent results and is completely
compatible with previous charging/control/safety hardware from the
earlier bank that used three 54 uF 15 kV GE caps. [The previous GE caps
weren't up to the task and they began catastrophically failing. Ruptured
cases, gunky arc-blackenned Geconol dielectric fluid oozing onto the
floor.... it wasn't pretty.] The pair of Maxwell caps have delivered
well over 6000 trouble free shots over the last few years.
How much of the energy is taken up by the coin crushing and
coil stretching?
It's really hard to say... but I'd be surprised if even 50% of the
energy actually ends up going into shrinking the coin. Considerable
energy goes into explosively ejecting coil fragments. A fellow shrinker
in Texas has calculated fragment velocities of up to 5000 fps.
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?
Yes, with a maximum bank voltage of 7.1 kV (for can crushing). BTW,
that's the MAXIMUM energy I use - but can crushing can be done with
considerably less energy. Since the work coil remains intact during can
crushing, I assume a high Q load (100% worst case voltage reversal).
Under this scenario, the capacitor dielectric system would see a peak
voltage swing of about 14.2 kV, which is an ~18% voltage reversal based
on the 12 kV faceplate rating of the caps.
I haven't measured the actual circuit Q, but anticipate it's at least
15-20, with most of the losses coming from the spark gap. BTW, can
crushing is quite hard on spark gaps - lots more evaporated metal than
with coin crushing. I recently bought a Pearson Model 301 50 kA wideband
current transformer to allow for isolated current measurements, but
haven't had a chance to hook it into the system as yet.
Also, my current coin shrinker is really not very "efficient" for
crushing cans since its operating frequency is comparatively low. The
system actually oscillates at about 11 kHz (including loop inductances
from cabling, capacitors, and spark gap switch). The compressive force
on the can is a function of skin depth, which at 11kHz is about 0.024".
Since a typical aluminum beverage can only has a wall thickness of about
..0035" (about 1/6th of the skin depth), most of the work coil's magnetic
field passes through the can, leaving only a small portion to do
crushing. Using a lower capacitance, higher voltage bank would work
significantly better for can crushing.
However, the current 140 uF system is almost ideal for crushing coins
(from an esthetic and practical standpoint). Lower capacitance/higher
voltage systems begin encountering coil flashover problems once you go
beyond ~20-25 kV. The coins also begin to develop "toroiding" (i.e.,
having thicker edges versus the interior). For example, here's a Silver
Eagle 1 Oz coin shrunk with higher voltage lower capacitance shrinker in
Texas:
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?
Using 60% voltage derating should work assuming these caps are rated for
20% reversal. And, since it uses a higher operating frequency, can
crushing should be considerably more "efficient". 400 Joules should be
more than sufficient to demonstrate the effect.
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.
Yes. However, increasing the inductance lowers the operating frequency,
reducing can crushing "efficiency", so there's a trade-off. YMMV...
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
Bert
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