MMC potting: (Was Re: UK MMC bulk order)
Hi Dave, all (E)MMC designers,
> Original Poster: "David Dean" <deano-at-corridor-dot-net>
> BTW my mmc consists of two strings of 11 for a total of 10nf. I put
>the strings side by side inside a 1 1/2" O.D. polypropylene tube
>12" long. perfect fit. Potted with epoxy resin. A 1/4" X 1 1/2" brass
>bolt serves as a stud on each end. Much smaller than the equivalent
>rolled poly in oil.
I have a few comments / questions about your MMC construction:
1.) You say, you are using two strings. This means each string
will have to take 50% of the primary amperage. Depending on
design (of the coil) this might lead to severe overstressing of
the caps. Do you know how high your peak and rms primary
current is? What power are you running?
2.) You write, that you potted them in epoxy resin. Why? I
can see NO advantage in doing this. However, there are a
number of disadvantages doing so:
a.) Internal heating of the caps canīt be removed. The epoxy
will act as an insulator.
b.) This setup is unrepairable. One of the MMCīs advantages
is easy repair (in case something blows). By potting them, you
defeat this advantage.
c.) The epoxy will do nothing for corona suppression, simply
because you wonīt experience any ;o) (outside) corona. As
for internal corona (partial discharges), the epoxy does
nothing to prevent this.
d.) You donīt say what kind of NST you are using, but I will
assume 15kV. This means you have 15/11kV (or about
1360V) per cap. If you are using the "standard" lead spacing
of 22 - 37.5mm, you wonīt be experiencing any flashover
e.) Voltage versatility. Your setup also defeats the voltage
versatility of the MMC design (as you canīt change the
number of caps in an expoxied string).
I have noticed that most MMC coilers are mounting their caps
on fancy PCBs. Why? I think my design has several advantages
over this type of construction. I use a piece of wood (it is a
"shelf" in my coiling table), that serves as a base for my MMC.
My caps (rectangular & radial) are placed upside down (leads
facing up) on this base. The leads are angled at 90° and simply
soldered together. This makes for a nice, flat package and it
keeps the lead length to a minimum. Eq-Rs can simply be
soldered on across each cap. There is no space between the
small sides of the cap. Each cap string is "friction-fastend" to
the baseboard with a strip of 1/2" wide and 1/4" thick plywood.
The caps are simply held by friction. There is no need for any
kind of glueing, etc. I can turn the baseboard upside down and
none of the caps will fall out. In practice, this will never happen
anyway (because I wonīt be turning the coiling table upside
down ;o}). Yet, I can easily remove a complete string, by just
unsoldering the two end leads and pulling the whole string out.
This wood strip also provides my caps with an air space
around each string. This, in turn, allows for convection cooling.
Main connections (on the left and right side of the MMC) are
made with 1/2" wide strips of beryllium copper. Around the
whole MMC is a frame, which is slightly higher than the
individual MKP caps. To protect it from dust, etc, the top is
covered with a piece of plexiglass. However, I would ONLY
recommend such a cover, if the MMC design will not be
generating considerable amounts of heat (i.e: for non EMMC
design ONLY). The PCB does nothing for performance or
looks, but it does complicate (and make it more expensive)
the design, esp if repairs are necessary.
Here is a pictorial of my MMC design. Please use a fixed
font to view:
F stands for the frame.
B stands for the bus connectors
C stands for the individual caps
W stands for the clamping wood strips
Coiler greets from germany,