Re: Final Capacitor Test Results

Tesla List wrote:
> >From Esondrmn-at-aol-dot-comWed Jun 12 21:01:50 1996
> Date: Wed, 12 Jun 1996 11:40:49 -0400
> From: Esondrmn-at-aol-dot-com
> To: tesla-at-pupman-dot-com
> Subject: Final Capacitor Test Results

> Now the big news - so it turns out that Condenser Products was right.  The
> capacitor was indeed good.  Not so any more.  I was running at 35 to 40 amps
> of primary currrent (about 9KVA) and I was slowing increasing the rotary gap
> speed to see its impact on performance when the capacitor exploded.  It blew
> one end off, breaking the plastic around the area where it was glued, the
> glue held.  I heard a muffled explosion, saw a flash of light (fire?) and
> everything went dead.  These things are made up of 5 or 6 (I haven't taken
> them all out yet) sections all in series.  Each one is just a little less in
> diameter than the inside of the case and about three inches long.  It looks
> like it is wound with very fine foil and thin dielectric.  What a hell of a
> mess, it puked oil all over everything.
> Why did it fail?  Too much voltage or just too much total power?  This cap
> was only rated at 15kv.  This is 15kv Tesla coil service.  They knew my
> supply was a 14,400 volt pole pig.  Do I need to order a 20 or 25kv capacitor
> next time?
> Sorry this was so long.
> Ed Sonderman


Sorry to hear about the death of your cap. However, it sounds like a good 
opportunity to learn exactly how these units are constructed (for 
education and plain curiosity, if nothing else). Please let us know 
more about how these look on the inside!

I'm sure that the folks at CP can perform an autopsy on the "pieces" and 
figure out exactly why the unit failed in the manner it did. Based upon 
the previous symptoms and the final explosive conclusion, I'd be willing 
to bet that an interconnect between capacitor sections became 
intermittent or marginal. 

A proposed scenario:
Earlier capacitance and pulse measurements looked OK unless you really 
"hammered" it in a TC tank circuit. CP's earlier tests may not have been 
brutal enough or long enough. When you stressed it, the marginal 
interconnect became the weakest link... As you increased the power level, 
the primary discharge current climbed, causing localized (I^2)*R heating 
in the defective area. If, while the tank circuit was under heavy stress, 
the marginal interconnect openned, you would get an internal high energy 
discharge of heavy primary current inside the capacitor IN THE OIL. At 9 
KVA, you had about 12 horsepower of energy stampeding in the primary 
circuit...the resulting shockwave from the initial arc discharge and 
vaporizing oil would propagate, through the incompressable oil, causing 
one end of the cap to be blown off. 

In hindsight, it appears that a somewhat larger voltage rating cap might 
be justified in the future, even though CP rates theirs conservatively. 
Additional safety margin NEVER hurts as long as the costs aren't  
prohibitive. It also could be that your cap may have simply had a 
manufacturing defect (weak interconnect weld??) that was there since the 
day it was made. 

Finally, if the CP autopsy indicates dielectric failure, increased 
voltage rating would definately be warranted in the future. 

Time to get that homemade cap out again??

Good luck and good coilin'

-- Bert --