Re: CD 942 failure tonight

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

Original poster: "Dr. Resonance" <resonance-at-jvlnet-dot-com> 


Here's why:

If you are running 9 pcs at 2 kV/cap DC rating, 9 x 2 = 18 kV DC rating

AC rating should be 2.5 x Erms, so with a 12 kV NST you were really pushing
the caps hard.

Commercial mfgrs use 3.0 x Erms as their factor (Maxwell, PCI, CP, etc.)
They also have another advantage: special types of oil to retard corona.
These oils specifically retard corona formation at the edge of the foil ---
a major breakdown area in HV caps used in pulsed RF service at high rep
rates.  This gives a Maxwell or PCI an advantage over the MMC type cap
especially at high power levels and high rep rates.  The oil provides a
natural cooling whereas the solid dielectric can only slowly dissipate the
heat, and worse, a solid dielectric does not offer the best edge of foil
protection.


In your case,

12 kV Erms x 2.5 = 30 kV DC rating / 2 kV/cap = 15 pcs per string for use in
AC circuit

You can violate this rule, but you will experience failures from time to
time as you have just experienced.
Run at 2.5 x Erms and you will probably retire before you experience a
failure.  At $3 per cap it's a great bargain and give you piece of mind you
won't have to troubleshoot during an important demo with your coil --- like
at a local school.

Dr. Resonance

Resonance Research Corporation
E11870 Shadylane Rd.
Baraboo   WI   53913

 > Capacitor bank:  4 strings of 9, total of .0667, per MMC chart.
 >
 > After rough tuning and ramping up variac, I heard one cap blow.  Black
soot
 > and a hole about a quarter inch deep.  Soldered up a new one, and ramped
 > variac up to 120 volts.  Sparks about 4 feet, and gap won't fire until
 > variac is above 60%.  After that it will fire all the way down to about a
 > quarter turn on the variac.  Checked rotary, and there is no follow around
 > with the gap arc.  She fires right on electrode alignment, so I am
assuming
 > that phase is pretty close.  Started it up again, and tried to fine
 > tune.  A half turn either way from the tap point of turn 6 5/8 and
 > performance diminishes.  Put tap back at this point and fed the trans the
 > full 120 volts.  I stopped things again to check everything out.  First I
 > checked the variac, an ancient Ohmite, to see if it was getting warm.
It's
 > only rated for 10 amps, and I made a new brush out of a piece of brass due
 > to no access to new parts.  Everything looks good and runs cool.  The
lash
 > up wiring to the coil primary is 14 gauge, and it is hot to the touch, and
 > the primary is warm.  Will go to #8 for final set up.  Looked at cap bank,
 > and the new cap looked like a piece of popcorn that had just popped, black
 > and all.  The one on the next string over was bulging like you know what,
 > but no black stuff.  I can see the foil, however.
 >
 > Why is this happening?