[TCML] cap rms current

[Jim Lux <jimlux@xxxxxxxxxxxxx>]

trying to start/move the thread

Jay,

I have a cat # 31885, .1 ufd, 75 kV rated Maxwell
that I believe is also rated at a "mere" 25 amps RMS,
but I run my Green Monster coil (10 kVA pig fed
and overdriven up to 20 kVA) with it as the sole
primary cap and I have never been able to notice
any external warmth on it above ambient temp,
even after several minutes of non-stop 15 kVA +
running.

David Rieben


---

Assuming your pig is 14.4kV, at 15kVA it's roughly 1 Amp RMS "into the coil"

Now, the question comes up of "what's the effective RMS current in the 
cap".  I like to back of the envelope these kinds of things by doing an 
energy balance kind of computation.

Let's, say, for argument that your coil runs at 100kHz, and takes about 
10 cycles to go from "break" to "quench", or, call it 100 microseconds.

So, during a 8.3 millisecond half cycle, the cap is "charging" at 1 amp 
(from the pig) for most of it, and then, for 0.1 millisecond it is 
discharging ALL that stored energy.

That is, in round numbers, it spends 99% of the time charging and 1% of 
the time discharging.

We know that energy goes as the square of either voltage or current, so 
the RMS current during discharge is going to be 1/10th of the RMS 
current during charging (since we already know that the energy flow 
ratio is 100:1)

therefore, I would assume that you're running an effective RMS current 
of somewhere between 10-15 Amps, well within the rating of your Maxwell cap.


Somewhere back in the archives, there's a better calculation of this.  I 
integrated a variety of damped sinusoids (linear and exponential 
decrement).  It would be pretty easy for someone to set up an excel 
spreadsheet and do it by numerical integration, as well.
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