Re. Cap AC voltage ratings

• To: tesla-at-pupman-dot-com
• Subject: Re. Cap AC voltage ratings
• From: Gary Lau 15-Mar-1999 2241 <lau-at-hdecad.ENET.dec-dot-com> (by way of Terry Fritz <twf-at-verinet-dot-com>)
• Date: Tue, 16 Mar 1999 11:51:52 -0700
• Approved: twf-at-verinet-dot-com

I think I'm gonna be sorry I dug into this...

I just got a catalog for the WIMA polypropylene capacitors I'm planning
on using for a new tank capacitor.  I had planned on using series strings
of 16 of the 1600VDC caps.  (16 x 1600 = 25.6KV)  But the caps were also
marked 500 VAC.

The catalog explains this.  There are two issues here.

1) The AC voltage rating seems to be a fixed maximum AC level that is
independant of frequency (though it's not clear why), to avoid ionization:
      "Furthermore the RMS voltage derived from the peak voltage shall
       not be greater than the nomimal AC voltage rating of the
       capacitor to avoid the ionization inception level:  Vrms<=VAC(rated)"

2) The dielectric strength is a direct function of operating frequency.
The catalog has a graph plotting the normalized dielectric strength of
polypropylene film vs. frequency.  
At    10Hz, the strength is 100%.
At   100Hz, the strength is  81%.
At    1KHz, the strength is  65%.
At   10KHz, the strength is  48%.
At  100KHz, the strength is  32%.
At  200KHz, the strength is  30%.
So, if I use a 1000VDC cap at 200KHz, it's only good for 300V, peak to
peak!  The low tank duty cycle doesn't enter into this.

This dielectric derating part really worries me, despite the apparent
success of several list members using these parts.  I'm just gonna have
to pretend I didn't see this...

Gary Lau
Waltham, MA USA

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