RE: [TCML] CD942 Capacitor failure

["Phil Tuck" <phil@xxxxxxxxxxx>]

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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