Re: MMC size: (Was: Re: Success with PP Multi-Mini Cap)

To: tesla-at-pupman-dot-com
Subject: Re: MMC size: (Was: Re: Success with PP Multi-Mini Cap)
From: "Reinhard Walter Buchner" <rw.buchner-at-verbund-dot-net> (by way of Terry Fritz <twf-at-verinet-dot-com>)
Date: Sun, 11 Apr 1999 10:39:55 -0600
Approved: twf-at-verinet-dot-com

Hi Steve, all

"Original Poster: "Steve Young" <youngs-at-konnections-dot-com>
To crosslink or not to crosslink, that is the question.  Seems like it
would be better to NOT crosslink them, for survivability reasons.  If in
one string some caps short, then the voltage across the remainder of the
string will increase, leading to premature failure of them as well.  But
only one string would get affected.  On the other hand, if the strings
are crosslinked, then shorted caps in one string will increase the
voltage across the remaining caps in ALL the strings, which could
result in many caps in many strings getting blown out."

This really depends on what happens when a cap in a string dies.......

IF it shorts, you are correct in your statement. However, if it shorts,
then all caps in this row (not string) are (as you stated) shorted. This
means these can´t die (no voltage across them). The shorted cap
effectively protects the rest. While all chains see a higher voltage
because one row (!) is shorted out, the maximum number of caps that
can die are the number in one string. What I mean: if you have 15
caps in a string and say, 30 strings, no more than 15 caps can die.

If the cap doesn´t short, but open circuits, then nothing much happens.
The total capacitance is reduced by 1/N/R (N = number of caps in a
string, R = number of rows). The dead cap is simply "removed" from
the MMC circuit. If the cap open circuits, chances are that it will arc
inside. This means the losses go up radically and the spark output
of the TC will drop like "a Mig-29 with a dead engine." One of the
benifits of crosslinking would indeed be, that you only need the "N"
number of equalizing resistors. Depending on internal design of the
single caps used in the MMC, it might be, that the ESR is reduced
(if the string number exceeds the number of caps in a string). So,
you see, it ain´t that easy........ (:o)).

I think the MMC is more robust than a lot seem to think. While I
am now running peak AC = DC rating, my final setup will be
using 15 caps in a string. If one of the fifteen should short, the
rest will survive (then 14 vs. the 12, I am using now). I would
assume a (partially) shorted cap will run hot, so one should
be able to spot it.

Still thinking whether or not to cross link...............

Coiler greets from germany,
Reinhard