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Re: [TCML] 942 destructive high voltage test
On 3/26/13 5:52 AM, Mark X2 wrote:
A little while ago I posted to find out if anyone did some recent testing
on the 942 caps.
There was no response, so I decided to sacrifice 5 of my 942s for the
cause.
(In the mean time I found that Terry Fritz tested one of these caps back in
2001
http://www.tb3.com/tesla/capacitors/GeekCaps.htm)
Each cap was hooked up to the output of a BAUR PGK 25 High Voltage Test Set
to measure the leakage current while increasing the (dc) voltage until they
finally started to fail internally or blew up.
Anyone who's interested in the results: 942 cap
test<https://www.dropbox.com/s/qjcuskwabhhrsat/942%20Cap%20test.pdf>
I'd like to invite you to draw your own conclusions and to discuss them
here.
So it looks like they're good to 4000-4500V before they really start to
fail (which I think is similar to Terry's test data back when). I don't
know that I'd push them that far in normal use. I seem to recall some
discussions about corona inception voltage, where the part doesn't
outright fail, but starts to degrade more rapidly (something that
wouldn't be caught in a short test like you did).
There's no question that increasing the voltage, particularly in an AC
application, increases the internal stresses. Maxwell has a empirical
equation for the life of their caps and they have a huge exponent of 7.5
on the voltage. (that is, doubling the voltage reduces the life by a
factor of 180!) .
http://home.earthlink.net/~jimlux/hv/caplife.htm
These are differently constructed with different materials, so the
voltage exponent will be different (I think Fruengel has tables in one
of his books). CDE might have life curves for that matter.
Of course, TCs have relatively short run times. A part designed for
24/7 operation as a snubber gets through 8000 hours in a year, and I'll
bet there's not a lot of TCs out there that have even 100 hours of total
run time on their capacitors.
the 942C has a rated life of 60,000 hours at rated DC voltage, 30,000
hours at rated AC voltage (which is 1/4 the DC voltage!). (and I note
they're tested at 150% of rated voltage for 60 seconds).
http://www.cde.com/catalogs/942C.pdf
SHows derating curves for AC, and it looks like the AC voltage rating is
down to 250V at 100kHz (from 500V).. That probably reflects the
decreased loss from the voltage reversal in AC use.
Tesla coiling is really a sort of unique load. Pulsed damped sinewaves.
So you'd really need to set up a special test rig to know for sure.
(This is what GA/Maxwell does...since they make pulse caps, they have a
test rig that does just this)
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