Re: CD 942 failure tonight

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

Original poster: Bart Anderson <classi6-at-classictesla-dot-com> 

Hi Gerry,

No clue if the primary is arcing somewhere, but I doubt it. It would be 
easy enough for John to observe with the lights off. My thoughts are still 
pointed towards current. Obviously, with cap strings of similar cap 
quantities at lower energy this isn't a problem (since there are many 
running in that fashion), but I don't know how many are running these 
higher energy levels with these few caps. That's the correlation I'm 
suspicious of, but when Terry diasects the blown caps, he will be able to 
identify if that is a probability or not.

Take care,
Bart


Tesla list wrote:

>Original poster: "Gerry Reynolds" <gerryreynolds-at-earthlink-dot-net>
>
>Hi Bart and John,
>
>I'm thinking that the fact that the wiring and .25 inch copper tubing is
>getting so hot is a BIG clue that is making me think that the problem is
>excessive rms current maybe more than should be.  If we are cooking CU pipe,
>we must be cooking the caps.  Im wondering is there is a arc occuring in the
>primary coil somewhere thus removing its current limiting effect.
>
>Gerry R
>
>
> > Original poster: Bart Anderson <classi6-at-classictesla-dot-com>
> >
> > Hi John,
> >
> > Ok, if the rsg total gap is 0.15" or less, then it's not the problem.
> > Phasing may be a possibility. You are correct, the safety gap should be
> > firing if the voltage is climbing up (unless it's set too wide). What are
> > you using for electrodes in the safety gap which is across the rsg?
> >
> > The caps rms current rating is excellent "if" circuit resistance is 3
>ohms.
> > If it's more like 1 ohm, then your looking at 22 amps or so. The tempeture
> > rise is then 6.7 per deg C and what you want to keep your eyes on. I like
> > to see this value down around 2 or less.
> >
> > The cap bank voltage rating (18kV) is set near the NST's Vp (17kV), so
> > there is basically no derating built in - it's on the edge (which many
>here
> > claim to get away with and obviously do). When I built my Geek MMC, I went
> > with 18 caps per string (36kV). For you to do this and keep the same
> > capacitance, you would end up with 144 caps (8 strings). If you look at
>the
> > cost, large capacitances and derating can get quite expensive, even for
> > MMC's. The only way to reduce the cost for larger capacitances is to have
> > individual MMC caps of larger value (maybe someday) or simply take your
> > chances with less derating built in.
> >
> > If you get the same heating with a static gap, then it may be worth
> > converting the 4 strings of 9 into 2 strings of 18 which drops the tank
> > capacitance to .033uF. This will allow more primary turns (higher surge
> > impedance),  a higher cap bank voltage rating (36 kV), and deg C is down
>to
> > 1.5 at 1 ohm. It also puts you that much closer to resonance rise problems
> > if an srsg is used. The .033uF is about the static gap LTR value and it
> > would be prudent to run it first with a static gap to check heating. If
>you
> > then run with an srsg, be aware you may risk the NST as it's twice as low
> > as the srsg LTR value.
> >
> > I'm not sure what the happy medium is, but the wiring and caps heating up
> > as you have mentioned tells me the rms current is  high and an indication
> > that circuit impedance is low. That coupled with the low voltage rating on
> > the cap bank sounds like a good probability of cap failure.
> >
> >
> > Note, a similarity to consider where the number of caps per string play a
> > major role:
> > 1) Cap voltage rating.
> > 2) Individual cap temp rise.
> >
> > Both need to be considered for MMC's.
> >
> > Take care,
> > Bart
> >
> >
> > Tesla list wrote:
> >
> > >Original poster: "John Richardson" <jprich-at-up-dot-net>
> > >Hi Bart,
> > >
> > >
> > >
> > > > Original poster: Bart Anderson <classi6-at-classictesla-dot-com>
msnip...