RE: MMC voltage rating

• To: tesla-at-pupman-dot-com
• Subject: RE: MMC voltage rating
• From: "Lau, Gary" <Gary.Lau-at-compaq-dot-com> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
• Date: Wed, 07 Jul 1999 11:39:40 -0600
• Approved: twftesla-at-uswest-dot-net
• Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

I'm also puzzled by the frequently suggested advice about multiplying the
peak voltage by a factor like two to arrive at a "safe" voltage rating.
While the peak-to-peak voltage seen by a cap may indeed be 2.0 or 1.8 times
the peak AC voltage, at no point in time does the actual voltage across the
dielectric exceed the peak voltage.  I don't see how the peak-to-peak
voltage is at all relevant to the voltage rating.

Having said that however, I suspect that a cap's AC voltage rating does
indeed need to be modified from it's DC rating.  The Wima data sheets on my
web site clearly indicate that the effective dielectric strength diminishes
with increasing operating frequency.  For PP film, at 200KHz, the effective
strength is only about 30% of its DC value.  Terry, perhaps this is why your
DC tests went so well but under AC conditions, you're seeing these air
pockets developing?  Plus, there is this mysterious "ionization inception
voltage" thing.

Regards, Gary Lau
Waltham, MA USA

		Original Poster: Finn Hammer <f-hammer-at-post5.tele.dk> 

		I have been pondering this for awhile, it gets more and more
important
		to ask:

		Since I have, say 20 kV on the piggs secondary, the peak
voltage would
		be 28800 volts, right?
		If I set the gaps to fire at this voltage, the cap will be
charged to
		this voltage, before the gaps fire.

		But after this, doesn`t the primary swing opposite with
around 80% of
		this voltage?

		So, shouldn`t We design the caps around a P-P voltage of
(PIG P-P Vout *
		1.8) in this case, 28800*1.8 = 51840 V.

		Just wondring, Finn

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