Re: [TCML] CD942 Capacitor failure

[Gary Lau <glau1024@xxxxxxxxx>]

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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