Agreed, Carl. I was originally running a Hipotronics .1 uFd, 50 kV
rated unit that
was right "good looking", but it died to an
internal short/flashover, without the fanfare
of case rupture and spewing of its hot, oily guts. The Maxwell seems
darn near inde-
structable, though, at least for the type of abuse that the Green
Monster goves it.
I think Joe Mastroianni did have some of the 37667 (.03 uFd, 35 kV
rated) Max-
wells to fail on him, but after discussing
this issue with him, we came to realize that a 35 kV rating is running
on the razor's edge of a safe ceiling voltage rating when firing with
a 14.4 kV pig. Of course, from Joe's description
of his failure mode, it sounds as if he had an over-
current problem and not necessarily over-voltage.?
I would assume that case rupture with the coil still operating after
rupture is an over-current issue.
My Hipotronics cap never showed any external signs of failure, just
the spark output of my coil came to a sudden and abrupt stop! That
sounds more symptomatic of an over-volting incident.
Seems that I've heard that a good "rule of thumb" is to make sure that
the DC voltage rating of the pulse cap is at least 3X the maximum AC
rms voltage that you will be feeding it. Since I run the input to my 14.4
kV pig overdriven with up to 280 volts from a variac,
that means that the 50 kV rating of my original
Hipotronics cap was cutting it too close, too.
BTW, Cameron Prince has a big SG driven coil that is very similar to
mine in power con-
sumption and performance and he also had
his original Hipotronics .1 uFd, 50 uFd cap
to bite the bullet in his, too. He too now utilizes one of those 31885
.1 uFd, 75 kV rated Maxwells in his coil and he too likes ;^)
Happy sparking,
David Rieben
----- Original Message ----- From: "Carl Noggle" <cn@xxxxx>
To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
Sent: Wednesday, February 01, 2012 10:10 AM
Subject: Re: [TCML] cap rms current
These Maxwell caps are bulletproof. I bought a 0.05uF, 50kV cap
about 20 years ago knowing that it had a leak around the top
terminals, and it finally gave up the ghost about two weeks ago,
after much service in my TC, 1800 watts from three 9/60s in
parallel. I got a nonleaking one at the same time and it is still
going strong.
trying to start/move the thread
Jay,
I have a cat # 31885, .1 ufd, 75 kV rated Maxwell
that I believe is also rated at a "mere" 25 amps RMS,
but I run my Green Monster coil (10 kVA pig fed
and overdriven up to 20 kVA) with it as the sole
primary cap and I have never been able to notice
any external warmth on it above ambient temp,
even after several minutes of non-stop 15 kVA +
running.
David Rieben
---
Assuming your pig is 14.4kV, at 15kVA it's roughly 1 Amp RMS "into
the coil"
Now, the question comes up of "what's the effective RMS current in
the cap". I like to back of the envelope these kinds of things by
doing an energy balance kind of computation.
Let's, say, for argument that your coil runs at 100kHz, and takes
about 10 cycles to go from "break" to "quench", or, call it 100
microseconds.
So, during a 8.3 millisecond half cycle, the cap is "charging" at 1
amp (from the pig) for most of it, and then, for 0.1 millisecond it
is discharging ALL that stored energy.
That is, in round numbers, it spends 99% of the time charging and 1%
of the time discharging.
We know that energy goes as the square of either voltage or current,
so the RMS current during discharge is going to be 1/10th of the RMS
current during charging (since we already know that the energy flow
ratio is 100:1)
therefore, I would assume that you're running an effective RMS
current of somewhere between 10-15 Amps, well within the rating of
your Maxwell cap.
Somewhere back in the archives, there's a better calculation of
this. I integrated a variety of damped sinusoids (linear and
exponential decrement). It would be pretty easy for someone to set
up an excel spreadsheet and do it by numerical integration, as well.
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