Another MMC Failure

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Finn Hammer <f-h@xxxx>

All.

I just had a MMC fail on an OLTC coil that I built a year ago.
The coil resonates at 70kHz , primary cap is 16.2µF and the pri-L is 350µH
With a firing voltage of 1150V this translates 
into a primary current close to 8000A
The primary cap is made up of 18pcs. 942C12P9K so 
each of them sees 450 peak amps.
They are rated at 861Apeak, so it is no wonder 
that the cap failiure is consistent with what we 
have come used to see from an overvolted cap.

I am sort of surprised they gave in to over voltage......

The caps show multible punctures of the 
dielectric that leads to the evaporation 
deposited layer of the intermediate electrode, with signs of self healing.
In a couple of the caps, the intermediate 
electrode has almost completely lost all of it`s 
conductive material, so they started to turn 
resistive, and heated up, although not seriously so.

The coil is in adventure park service, and the 
duty cycle is low. every 4.5minutes, it puts out 
112 bursts of 124 discharges at 500BPS.

This killed the cap in a bit more than a month.

From
http://home.earthlink.net/~jimlux/hv/caplife.htm
I can gather that if I double the voltage rating 
of the cap, the expected lifetime will increase 
180fold and that suits me fine, because I can 
then expect the caps to last 15 years.

What I wonder is this, should I put 942C12P9K 
caps in 36 strings with 2 in each string, or 
should I rather use 942C20P15K and put 108 in parallel.

A Maxwell cap is another option, of course.

Cheers, Finn Hammer