Re: [TCML] MMC failure / question or advice.

["resonance" <resonance@xxxxxxxxxxxx>]

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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