Re: Cap AC voltage ratings

Tesla List wrote:

> Original Poster: Gary Lau  13-Mar-1999 2038 <lau-at-hdecad.ENET.dec-dot-com>
> I have begun making a new cap using strings of 16 Wima FKP-1 .015uF/1600VDC
> polypropylene caps.  The thing that concerns me is voltage ratings.
> While the DC rating seems to be no problem (16 x 1600 = 25.6KV), I'm
> concerned with how much the mfgr derates the DC rating for AC.  My caps
> are AC rated for only 500VAC each (16 x 500 = 8KV).  I believe the two
> reasons for the derating are 1) to allow for the 1.414X RMS to peak
> conversion, and 2) to limit the temperature rise due to ESR dielectric
> heating.  The graphs I have seen show a derating curve, being a function
> of applied frequency, and I would expect this.  The unexpected thing
> however is that the curves do not converge to a simple derating of 1.4X
> at a sufficiently low frequency, but instead, never allow over the rated
> AC voltage, regardless of frequency.
> While the DC rating of the caps may not be in danger, the AC rating
> appears to be a problem.  Or is the AC rating that low only if given a
> continuous duty cycle?  While the tank oscillations in TC use occur at
> very low duty cycles, the 60Hz component is continuous.  But then, we
> typically don't run our coils for more than a few minutes at a time.
> So, does anyone have a comforting explanation that will make be believe I
> won't be blowing by caps due to violating their AC ratings?
> Gary Lau
> Waltham, MA USA


Violate AC ratings at your own peril.  Most polypropelenes and made for snubber
service, which is pretty rough service with nastey high current pulses being
the norm.  I would let their AC rating be my limit for peak volts off a Tesla
supply transformer.  Thus, a cap of 8KVAC by their standard, I would limit to a
neon transformer of no more than 5KV rms secondary rating.  All of this assumes
a naturally robust polyprop cap. designed snubber service.  I have made up many
such series strings and all worked brilliantly.

DC rating on most caps is usually set at 1/2 the dielectric breakdown level and
AC ratings are often 1/3 the DC rating making a rated 10KVDC breakdown
dielectric a 5KV DC cap or a 1.5-2KVAC rated unit.

Richard Hull, TCBOR