Re: CD 942 failure - Found it!!

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Terry Fritz <teslalist-at-twfpowerelectronics-dot-com>

Hi Malcolm,

At 08:30 PM 3/8/2004, you wrote:
>On 8 Mar 2004, at 18:08, Tesla list wrote:
>
> > Original poster: Terry Fritz <teslalist-at-twfpowerelectronics-dot-com>
> >
> > Hi John and All,
> >
> > It's a peak current failure.
> >
> > Our new failure mode...
> >
> > Although the CD942 caps have solid metal plates that never blow
> > out.  Apparently, the floating metalized layer can.
> >
> > http://hot-streamer-dot-com/temp/JohnRcap.jpg
> >
> > http://hot-streamer-dot-com/temp/JohnRcap2.jpg
> >
> > The entire area between the electrodes has bad current stress failure
> > areas much like a fuse.  About 1/4 of it is blown clear.  There does
> > not seem to be any RMS heat melting, but the peak currents probably
> > are twice what the cap is rated at:
> >
> > http://www.cornell-dubilier-dot-com/film/9422000.htm
> >
> > maybe 860 amps per cap or a total of 2600Amps peak in the four
> > strings!!  John mentions that the primary and such get very hot!!
> > What I don't know is why the high currents?  People use one string
> > with 12/30, two with 12/60 without trouble.  So it would seem John's
> > coil is drawing unusual very high peak currents.  But I don't know
> > why.
> >
> > Cheers,
> >
> >  Terry
>
>Two possibilities spring to mind: the coil has a very low primary
>inductance, or there is a safety gap connected directly across the
>caps that is firing?
>
>Malcolm

If we look at the data sheet, we find they are 0.15uF, 32nH, 0.005 
ohms.  They are rated to 432 amps peak.  The resonant frequency is 2.3MHz 
and the reactance is 0.461 ohms.  Thus, a dead short with only 200 volts 
would hit their peak rating.

In John's case, assuming he got to 2000 volts, and it takes say 1000 amps 
to really break the internal floating layer, we would get a reactance of 2 
ohms.  Fo = 530kHz and the inductance is 601uH.  I am just thinking out 
loud here...

But, those numbers are a bit concerning since they are in the "range" of 
such a coil.  No doubt directly shorting the caps would be "bad".  But I 
wonder if the 4 x 9 MMC array is correct?  Unless John knows he sorted the 
caps, we must consider the possibility that the array size is wrong...

Much to ponder here...

Cheers,

         Terry