Not so according to JavaMMC
(http://www.classictesla.com/java/javammc/javammc.html).
I've been going through this same exercise for a
PT-based coil I'm scraping together, and actually
meant to ask about this.
Indeed, peak current appears to grow in proportion to
the square root of total C. So, if you double C by
doubling the number of strings in your MMC, the peak
current *per string* goes down. However, most folks
seem to worry about Irms, *not* Ipeak, when it comes
to frying capacitors in MMCs. The kicker appears to
be this: If you double the total C of your MMC by
doubling the number of strings, JavaMMC reports that
Irms ***per string*** is also doubled! Ouch!! Can
somebody (Bart?) explain how this RMS calculation is
done?
So, Irms ***per string*** appears to scale in
proportion to total C. The next really interesting
thing JavaMMC reports is that Irms scales in
proportion to the *square root* of BPS (spark gap
break rate).
Since the coil power is given by:
P = BPS * 0.5 * C * V^2
If you hold V constant, then you've got a couple of
choices left for increasing your coil power. If you
doubled your power by doubling C, you also double your
Irms ***per string***, which seems really bad!! If
you double your power by doubling BPS, your Irms
***per string*** only goes up about 40% as opposed to
100%. Considering those figures alone, it would seem
as though increasing BPS is the better choice. Of
course, I've heard folks suggest that break rate only
gets you so far, but perhaps going from 120 to 240 is
better than going from .03uF to .06uF?
Let's see. From JavaMMC:
If you use 3 strings of 15 "Geek Group" caps (good for
13A RMS, supposedly) to get .03uF, run 15kVAC at
120BPS, then your Irms per string is 7.34A. That's
good. If you double C by using *6* strings for .06uF,
Irms per string *doubles* to 14.67A. That's not so
good. If instead you had doubled the break rate to
240BPS for the same "coil power", your Irms per string
would have increased only to 10.38A. Still under the
max. Seems "better".
All this seems to suggest to me that increasing your
total C is not a very MMC friendly thing to do
(despite an intuition that tells me that more of
<thing> in parallel means less current per <thing>)
and that increasing V and BPS are the cooler ways
(literally) to get more power. Is that correct,
though? I fully admit to not understanding how
JavaMMC gets its Irms figure, and would really like to
know!
Regards,
Aaron, N7OE
--- Tesla list <tesla@xxxxxxxxxx> wrote:
> Original poster: "Gerry Reynolds"
> <gerryreynolds@xxxxxxxxxxxxx>
>
> Hi Ben,
>
> Just design your MMC so the Ipeak, Irms, and Vdc
> specs are not
> violated. The bigger PIG will require more
> capacitance meaning more
> strings in parallel. Thus the current capacity of
> the MMC goes up as
> the number of strings is increased.
>
<snip>
>
> Gerry R.
>