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Re: Cap break down voltages

Subject: 
        Re: Cap break down voltages
  Date: 
        Fri, 4 Apr 1997 12:19:56 -0500 (EST)
  From: 
        Esondrmn-at-aol-dot-com
    To: 
        tesla-at-pupman-dot-com


In a message dated 97-04-03 20:00:46 EST, you write:

<< 
 I am about to start capacitor building again and want to know a good
 working voltage to choose.   I currently have
 3 home made poly/foil caps 0.03uF -at-10KV.   These caps have
 40mils of good quality poly between the plates.
 
 Some time ago I remember Richard Hull posting about chemical breakdown
 of the dielectric where voltages exceed 10KV.   
 
 If this is the case then I see little point adding to the thickness of
 the poly for a higher voltage rating.   I will simply build more of my
 0.03uF 10KV caps and wire them in series.
 
 I would also like to ask about primary operating voltages.   Currently
I
 am using a 11KV pole transformer with no balast, just RF chokes in the
 primary circuit.   When I fire the coil up I make sure my rotary gap is
 running at full speed before applying the power via a variac.
 
 As far as I can tell the current drawn by the pole transformer is low
 (no AC amp meter fitted yet).  The coil runns very smoothly generating
 no more than 36" arcs.  Heating of the rotary electrodes is also low. 
 The coil has no toroid to speak of being a 12" on a 10" coil.  New
 toroid in under construction.
 
 To get more arc should I:
 1)  Add more primary capacitance and use fewer turns on the primary
 (currently tapped at 10 turns using 0.03uF)
 2)  Up the primary input voltage by adding a second pig. This will also
 mean wiring caps in series and using more turns on the primary.
 3)  Add balast and PFC
 
 Coil spec is 1000 turns on 10" x 36" secondary.  11 turns
 primary with 3/8" pipe.
 
 Comments welcome
 
 Julian Green
  >>
Julian,

What is the kva rating of your pole pig?  I am very surprised that you
can
run it with no inductive or resitive ballast.  My 5kva pig would
probably try
to draw 100 amps with no ballast.  With my welder set wide open and a
few
2000 watt oven elements in parallel with the welder, it draws 30 to 35
amps.
 I tried to fire it as a jacobs ladder once with no ballast and just
about
melted everything involved.

You have all the basics for a kick butt coil.  The .03 cap should be
just
about right.  If you have a Powerstat variac that will go to 130% of
line
voltage, I would use that option and boost the output up to 14,300
volts.
 Shouldn't hurt the pig a bit.  Your system will want a large toroid.
40" in
diameter won't be too large.  This will drop the resonant frequency and
require more primary inductance (can you add more turns?) or less tank
capacitance.  Large primaries, tapped out near the last turn or so do
seem to
perform well.  You should be running a static gap in series with the
rotary
for better quenching.  I have reached 80" discharges with my 6" coil and
a
14.4 kv  5 kva pig.  You should be able to get to 90" or 100" I would
think.

PFC is not necessary and won't improve performance.  I would certainly
want
to get some variable primary inductance with a system as large as yours.

Ed Sonderman