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Re: Advice needed on capacitor tests: update: BIG resonance!
"Dear all,
I will add to this after another test....
I hooked up my cap across my NST again, but this time put my multimeter
on
the output of the NST. I turned up the voltage, and at only 1 or 2
percent
of line voltage I was getting about 600-700 volts on the meter!! I
tried
it without the cap and all I was getting was around 80-100 volts on the
same setting... All I can think of is that the NST windings are
resonating
with my cap at 60Hz and causing this voltage rise of about 6 to 8
times!!
Instead of shunts, these things have a small gap in the core which
provides
enough leakeage inductance to limit the current.
In fact, I'm just doing the test again right now - with the cap
connected I
set the variac for an NST output of 800V. I removed the cap and I'm only
getting about 85V. This must mean that I was actually getting about 10kV
across my cap, no wonder I got the big sparks! this is a rise of nearly
10x! Help!!! does this mean my xfrmr is useless for Tesla work? I can't
see that my RC filter (desingned for the Tesla frequency) is going to
stop
this!"
Normal behavior, don't sweat it. The Q of a typical secondary appears
to be at least 10 and perhaps more. For the 12 kV, 60 ma transformers I
use here the leakage reactance is about 200000 ohms (12000/.06), and the
measured resistance of the secondary is about 5500 ohms. If it weren't
for core loss this would give a Q of the order of 36, which could
clearly lead to insulation failure without a safety gap if the core
didn't saturate.
Ed