The corona is eating your caps up internally.
This is a very common problem incurred by experimenters who simply do not
use enough series caps in the MMC bank.
You need Erms x 2.5 for long term reliability. So, 12 kV x 2.5 = 30 kV DC
minimum
This equates to 30 kV / 2 kV/MMC = 15 series MMCs per leg of your cap
bank.
The peak potential from a 12 kV nst is 12 x 1.414 = 17 kV -- and this is
without any safety factor whatsoever. With your 8 caps string you are
just too close to this value for safety, and, as your are discovering, the
interior corona slowly eats away at your caps. As one fails even
partially, it puts too much strain on the remaining string, and they also
start failing one after another.
I know some experimenters on the list have recommended using less than 2.5
x Erms, but it is poor engineering practice to do so. I always use 18
series caps in my strings with nsts for a cap value per string of .0083 uF
at 36 kV DC. These strings have never failed. Just parallel these
strings for the cap value you require.
I have received my guidance from commercial mfgrs like PCI, CSI, and
Maxwell --- all use 3.25 as their safety factor. For experimenters use I
think 2.5 represents a very solid compromise --- less than this,
especially 8 or 10, is a sure path to cap death.
If you have no method of accurately measuring leakage current on each of
your caps to determine which is healthy and which are not, your only
correct choice is to replace all of your MMC caps using 18 per string.
You simply can not do less than this procedure as any remaining caps that
have even a 10% internal partial failure will keep failing and again put
all the remaining series caps at high stress causing them to also start to
fail. There is no "short cut" to repair the problem you are facing ---
total replacement is the only solution.
Using the proper amount of MMCs is a very robust solution to your
problems. In my commercial coils for small museum applications, the coils
are run 2-3 times per day, at least 6 days per week, year after year
without any failures whatsoever. Use 2.5 x Erms and it will work out
fine.
Sorry to be the bearer of bad news, but hopefully this "constructive
critisism" will help you develop a long term plan for a healthy Tesla
coil.
Happy holiday sparks,
Dr. Resonance
----- Original Message -----
From: "Vinnie" <teslatech@xxxxxxxxxxx>
To: <Tesla@xxxxxxxxxx>
Sent: Saturday, December 01, 2007 11:53 PM
Subject: [TCML] MMC failure / question or advice.
Hello people
Recently I have been running into some trouble with my coil and what I
believe to be an MMC failure
but not sure whats causing it.
I am running 8 series CDE 942C20P15-F .15MFD 2000VDC caps in my MMC. First
failure burned a hole
through two of these caps. I replaced the two bad caps with the same type
and did two more 20 second
runs which resulted in a loud snap. No visible damage this time but I
could still hear a loud snap or arc
even after the coil stopped functioning. One bleeder resistor on one of my
caps looks to have heated quite
a bit but it doesn't look burnt. Primary power input is 2 12/30 NST's and
a RQ style gap. This functioned
great for years but it seems somewhere something is breaking down now.
Does anyone have an ideas what could be happening. I am thinking maybe
it's time to build complete new MMC.
Would anyone know if there is a better more robust MMC solution for my
setup?
Thanks
Vinnie
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