[TCML] MMC failure / question or advice.

[Vinnie]

Thanks for all the advice.

 Now just to find the caps. :)
I was also wondering if I need to beef up the RQSG?

Thanks

Vinnie

----- Original Message ----- 
From: "resonance" <resonance at wildblue.net>
To: "Tesla Coil Mailing List" <tesla at pupman.com>
Sent: Sunday, December 02, 2007 1:55 PM
Subject: 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 at verizon.net>
> To: <Tesla at pupman.com>
> 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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