Just got back from a ten day holiday and just starting to get going on
my cap again. I decided to do a destructive test on a couple of plates. The
only oil i had to hand was SAE 10 motrcycle fork oil, a very thin clear oil
with almost no colouration. I dont know if it contains any additives but it
served its pourpose for the test.
For the first test a pair of plates were put together with two layers of the
polypropylene film in the oil. Each foil and film were layered in the oil
and then a small roller was used to expel any remaining air bubbles. A
single 10KV 25mA transformer was connected with the variac to the cap. The
output voltage was wound up to 1KV and then left for half an hour for the
electrostatic attraction to pull the plates in hard together. The voltage
was then increased in 500V steps and left for ten mimutes between each step.
After an hour 4KV was reached and the cap quite happily held this, although
even through the oil corrona was starting to appear at the edges of the
plates. From these corrona sites streams of what looked like discoloured oil
spread out to the edge of the plates. After removing the power and close
inspection with a magnifying glass the discolouration was found to be caused
by tiny gas bubbles. I assume the bubles were due some additive in the oil
starting to break down under the electric stress.
After rolling the plates again to expel the gas bubbles, the experiment was
run again but this time with a spark gap across the cap. The first imediate
effect was, as noted before, that the gap fired quite readily at around
2500V with the same gap that would be required for it to fire at 4KV. The
second effect was the corrona at the plate edges was about twice as long as
before and pulsated with the spark. The voltage was increased as before
untill the cap failed, not with the nerve wrenching bang as before but with
a rather pathetic "splut" and a single bubble of gas rising to the surface
of the oil. The cap held 3Kv pulsed for 10-12 minutes quite happily but
failed after about seven minutes at 3.5KV. The cap was then dismantled and
the failiure point was noted, as before when i did the same experiment with
dry plates, to be close in to the center of the plates and not at the edge
where the corrona was seen.
My main conclusions from these experiments are that the absolute max.
voltage for this cap. Is going to be 3Kv, possibly slightly higher with
proper transformer oil. Using a safty margin of X3 (Thanks for the quick
line Jim.) i'm allowing a working voltage of 1KV per cap. I know this has
been stated before on the group but i just wanted to prove it by practical
Based on the fact that at the moment i have 10Kv transformers, our standard
pole pig voltage is 11Kv and in the not too distant future i will be
decommisioning two 1KW Klystron amplifiers each containing a 6KV -at- 800mA
plate transformer (and hope to be able to save them from the sccrap heap),
i'm going to build my cap for 15KV. Im not quite shure how big to make the
cap. I was orrigionally planning on 0.01MFD but 15*0.15MFD caps will only
fill 2/3 Of the boxes that i bought, so i might up it 0.015 I intend to
build at least two of these caps giving me a total of .03MFD, a bit big for
the trans formers i have at the moment.
Anyway must dash, i seem to have a lot of plates to cut out.
Ian Hopley ----> i_hopley-at-wintermute.co.uk
Scotland Callsign M M 1 A B A