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




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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