Black Poly Cap - RIP

From: 	Peter Electric[SMTP:elekessy-at-macquarie.matra-dot-com.au]
Sent: 	Tuesday, June 24, 1997 4:21 AM
To: 	Tesla List
Subject: 	Black Poly Cap - RIP

(This seems to have gotten lost last time so I have re-sent it)

Well folks, after about 3 months of faithfull service, my black poly -
engine oil rolled cap finally gave up the ghost. I did a post mortem on
the beastie today and discovered some interesting things.

Details were as follows - 55" by 14" black plastic sheets, supposedly
200 micron thick but measured at closer to 180 (around 7 Mil). 10 layers
of plastic and four alfoil plates giving me .015uF total. I was running
this with 15Kv 60Ma in my tank circuit. Dielectric constant was about 3
according to the TCH program.

The cap had arced over near the bottom feed and melted a big hole
through the plastic. The total thickness of around 70mil was a bit thin
for that voltage so maybe not surprising, but there were two
observations that surprised me. All along the edges of the alfoil
plates, there was a buildup of blackish "grunge". When wiped off, there
were no marks on the plastic so it must have come from the effect of
corona in engine oil. It may have been this buildup that caused the
arcover near the feed wire. I would say that the old 20/40 engine oil is
not good for rolled caps and I won't be using it again. The other thing
that intrigued me was that the inner turns of plastic in the cap had
buckled and wrinkled, almost as if the oil had caused the plastic to
swell slightly. This would seem to be a major limitation when using many
thin layers of Poly, bundled together. Maybe the thin layer concept is
only successful when used with flat plate caps (as the layers would then
expand uniformly)

The oil appeared to have penetrated all through the cap well and it did
work OK for 3 months so I don't think the thickness of the oil was the
main problem. Have any of you experienced coilers seen this "black
grunge" buildup before?

Cheers, Peter E.