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RE: [TCML] CD942 Capacitor failure
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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>>
>>
>>
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