Thanks Bart, the penny has now dropped over the formula. I, for some unknown
reason, was thinking "**2" meant it was squared twice, as I am more used to
" ^2". Checked a few sites, but missed your page (hang head in shame)
I will have to keep the variac max limited to 250v I think, and close the
safety a bit. Safety gap setting on pigs is always going to be hit & miss
unless you have access to an adjustable HV source.
I have recently increased the MMC to 100nF to get resonant charging as
Richie advocates at his post at
http://www.pupman.com/listarchives/2001/June/msg00679.html
Unfortunately, it was achieved by altering the MMC from 9 x 19 in series to
12 x 18 in series (38Kv down to 36Kv, but 71nF up to 100nF) in an effort to
reduce capacitor costs; it was while doing the work I discovered the blown
cap.
Not a great standoff difference, but clearly not the direction I should have
been aiming for! (Although I haven't tried the coil yet)
Looks like smoke may be seen at the upcoming Cambridge Teslathon.
Phil
-----Original Message-----
From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On Behalf
Of bartb
Sent: 16 September 2010 01:47
To: Tesla Coil Mailing List
Subject: Re: [TCML] CD942 Capacitor failure
Hi Phil,
Regarding power delivered across the gap: P = bps * 1/2*C*Vp^2.
Regarding MMC equations for evaluation, try this:
http://www.classictesla.com/hot-streamer/TeslaCoils/MMCInfo/MMCPower4.html
Regarding 200bps, the temp rise is still only 0.84°C and rms current is
18 amps. I doubt this is the problem. Certainly heating internally can cause
breakdown, but I suspect it is still a voltage issue. Not sure how your
going to identify that, but for the one failure I had on an mmc I referred
to in my last post, I sided with voltage and added a couple more caps in
each series string raising the standoff value and had no further
occurrences.
Take care,
Bart
Phil Tuck wrote:
Gary, Bart et al
Sorry, grave error - I should have mentioned it is 50Hz @ 200 bps. (UK)
I have tried with the formula P = BPS * 1/2 CV**2 but I'm obviously
putting a bracket or a 'squaring' action in the wrong place.
Can one of you oblige?
Regards
Phil
www.hvtesla.com
-----Original Message-----
From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
Behalf Of bartb
Sent: 15 September 2010 01:46
To: Tesla Coil Mailing List
Subject: Re: [TCML] CD942 Capacitor failure
Hi Phil,
Interesting post. I have had only one 942C failure similar in
description (blackened hole half way down). I never did figure out the
cause. Looking at your cap string (as the current is the same in each
string) then roughly 14 amps which is not bad. However, that is based
on
120 bps. If your are running higher bps (say 240), then the rms
current increases to about 20 amps as does heating. So I'm wondering
if anything along the lines of bps has changed as it could be a possible
cause.
Take care,
Bart
Phil Tuck wrote:
Hello,
I have suffered several CD942 cap failures lately and I am wondering
on the cause. Each time it has just been the one isolated cap. I
would not have thought that my voltage was excessive for these caps,
but after consulting with another coiler, I am now wondering about a
primary
current issue.
I was running a SRSG on a pig rated at 11.55Kv but running at 12Kv at
times.
Secondary current is around 400 to 450ma depending on my ballast setting.
I was using nine strings of 19 in series giving 71nF @ 38Kv. Damage
to the caps was an exterior blackened hole where the blowout had
occurred. I unwound the last cap' to find where the damage had
occurred, this turned out to be about half way down into the cap in
its middle layers. The damage at this point being a circular hole
about
0.25-inch diameter.
Safety gap was set at 9mm or 0.354 inches at the time with the coil
running very smoothly for 3 minutes or more while producing some nice
streamers. The dv/dt time should not have been exceeded either as the
res frequency is 92 KHz.
The primary was tapped at about 8 turns I think, but he primary coil
itself only ever feels slightly warm while the wiring temperature
seems to remain unchanged.
Admittedly, none of the caps are now new, as I have used them on
previous set-ups before, but I would not say they have ever been
abused, as the MMC voltage ratings they have been used in have always
been
adequate.
As none of the damage was from proximity issues, it can only really
be down to voltage or current I suppose. Thoughts anyone?
Phil
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