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Re: Re. Cap AC voltage ratings

We use WIMA caps at work and we push them very HARD!  I wouldn't worry to much
about them failing.  When they get to hot to touch its time to shut down the
power for awhile.  Otherwise they will do fine.  WIMA makes some of the
toughest caps around.  They will start failing when the internal dissipation
gets them to about 80C and above.

        Terry



At 10:58 PM 3/15/99 -0500, you wrote:
>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