Re: MMC currents (was Re: pole pig beginner)

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "J. Aaron Holmes" <jaholmes@xxxxxxxxxxxxxxxxxx>

Good deal, Bart.  Thanks for clearing that up!  I was
beginning to wonder how MMCs would scale at all! :-))
As I'm about to undertake my first MMC adventure (just
bought about 50 "Geek Group" caps), I wanted to make
sure I had some intuitive sense of why it wasn't going
to blow up before I started soldering :-)  Always
makes me feel better.

Can't wait to try out the fix!  BTW, thanks for all
these cool programs!!!  I've wasted away in front of
them some nights, living vicariously through the
numbers ;-))

Regards,
Aaron, N7OE


--- Tesla list <tesla@xxxxxxxxxx> wrote:

> Original poster: "Barton B. Anderson"
> <bartb@xxxxxxxxxxxxxxxx>
>
> Hi Aaron,
>
> The equation used is for total primary current, not
> current per
> string (mis-labeled). I'll have to fix that and
> upload a new version.
> I may as well display both string and total current
> for giggles.
>
> Very good catch! I'm surprised this wasn't brought
> up much earlier.
>
> The equation for Irms per string:  Irms =
> sqrt(Wo/Rcap)
>
> I unfortunately multiplied the number of strings to
> Irms for primary
> current but didn't change the value id.
> Like I said, I'll show both values when I fix it
> (should be tonight).
>
> Thanks for making us aware,
> Bart
>
>
>
> Tesla list wrote:
>
> >Original poster: "J. Aaron Holmes"
> <jaholmes@xxxxxxxxxxxxxxxxxx>
> >
> >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
>
>
>
>