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RE: [TCML] CD942 Capacitor failure



Gary,
Sorry forgot to answer the heat question.
This time I only noticed the damage several months after it would have
occurred, but I usually feel the caps exterior after a run (allowing for
obvious safety issues) and they are never noticeably warm. 
After my last decent run several months ago of 3.5 mins I noticed only a
slight temperature rise to the exterior. I would have thought that
damagingly hot inside would at least mean a noticeable heat rise to the
exterior. I think this is the second cap I have now lost to unknown causes,
hence my concern. 

PS: I have lost three others through stupidity though. This was a poorly
constructed SRSG with power arcing, while another was a bit of construction
wire left behind in the MMC that managed to short a single cap across the
charged MMC. The noise was like a rifle! 


Phil

-----Original Message-----
From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On Behalf
Of Gary Lau
Sent: 15 September 2010 00:28
To: Tesla Coil Mailing List
Subject: Re: [TCML] CD942 Capacitor failure

Did you happen to notice if the failed caps were at all warm immediately
after failing?

If I try to calculate the power throughput with P = BPS * 1/2 CV**2, I get:
P = 120 * .5 * 71E-9 * (12000*1.414)**2 = 1.2KW.  If my assumption of 120BPS
is correct, it doesn't seem to be pulling too much power with such a low
break rate.

Regards, Gary Lau
MA, USA

On Tue, Sep 14, 2010 at 2:41 PM, Phil Tuck <phil@xxxxxxxxxxx> 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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