Hello,
The kVAr (kilovoltamperes reactive) is expressed
as rms voltage over the capacitor x rms current through the capacitor.
Assuming very low losses in the capacitor, the phase angle of the resultant is
very close
to -90 degrees (power factor very close to 0)
kVAr ratings have their most significance where a capacitor forms part of a
tuned circuit or a phase compensating network, and as a blocking capacitor the
RF current rating and the DC voltage rating are more interesting.
Current ratings can be approximated by taking the expression for reactive power
in the capacitor;
S = I**2/(2*pi*f*C) and solving for I = square root(S/2*pi*f*C). Units are f in
Hz, C in Farads, S in VAr.
The frequency used should be the frequency where the kVAr rating is specfied.
As frequency goes up, the I**2*R losses increase approximately as the square
root of frequency,
so maximum current tolerable due to power dissipation will in theory decrease
approximately as 1/f.
By all probability, you will find that at frequencies above the MF broadcast
band, the current rating will
be at least an order of magnitude above what reasonably can be demanded even
for an 8169 running
at full ratings.
Plugging in some numbers:
Assuming an 8169 running flat out; 7kW output at, say, 7 MHz:
Ep = 7 kV DC Ip = 1.7 A DC, Rp= 2300 ohms
The output RF current fundamental component will then be approx.1.8 A rms
Assuming a blocking capacitance of 1000 pF and a 30 kVAr rating:
S = 1.8**2/(2*pi*7.0E6*1000E-12) = 74 VAr, safely below 30000 VAr...
or expressing in another way, ignoring current limitations;
I max = square root(30000/(2*pi*7E6*1000E-12)) = 825 A, a value which is far
exceeding the I**2*R loss limited maximum current through the capacitor.
For some interesting graphs about RF capacitor performance, visit
http://www.morgan-electroceramics.co.uk/pdfs/discrf10kv.pdf
Hope this helps and 73/
Karl-Arne Markstrom
SM0AOM
----- Original Message -----
From: <qrp73@talk21.com>
To: <amps@contesting.com>
Sent: Saturday, January 04, 2003 10:27 AM
Subject: [Amps] KVAR Caps Rating
>
> Could someone explain how the Kvar rating of doorknobs caps
> are calculated. I have two caps one with a 30KVAR rating and one of 60 KVAR
> rating. Is it just E X I? What would the RF current ratings of these caps
> be?. One is 10KV and the other 12kv. I suppose the current would be
> calculated at one frequency, since the RF current rating would vary with
> frequency. I want use this cap as plate blocker for a 8169, not sure if its
> up to the job. I Dont want to take the chance blowing things up. These caps
> were probably broadcast band transmiter caps so the RF I rating would low at
> higher frequencies.
>
> Thanks
> Greg
>
> Greg
>
> Greg
>
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