Re: MMC potting: (Was Re: UK MMC bulk order)
From: Tesla List <tesla-at-pupman-dot-com>
To: tesla-at-pupman-dot-com <tesla-at-pupman-dot-com>
Date: Thursday, July 08, 1999 4:49 AM
Subject: MMC potting: (Was Re: UK MMC bulk order)
Original Poster: "Reinhard Walter Buchner" <rw.buchner-at-verbund-dot-net>
> 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?
Please correct me if I am wrong, but it seems to me that the amperage
through one string would be exactly the same if there was only one string as
it would be if there were a hundred in parralel. The amperage through a
string would be dependent on the capacitance of the string. the voltage it
was charged to just before it was to be discharged, and the resistance (make
that impedance) of the primary circuit.
Now if I had 22 caps in series for each string, the capacity would be half,
the current would be half. and I would need twice as many strings. Or four
times as many caps to reach the same value.
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:
Because I have epoxy resin sitting one the shelf. I do not have any perf
board or any pc board material lying around, and would have to drive 60
miles or more during the week when I am supposed to be working to get any,
or else order some and have to wait for UPS to take their sweet time to get
the stuff to me. Or I could go to Radio Shack and buy some of their sucky
stuff that would be to small to use for such a thing anyway. Or I could
build some kind of fancy apparatus like Terry did but I am just to lazy to
do that. Or I could copy your design, which I like, BTW, but it would not
incorperate well into my design. I need the cap mounted vertically.
a.) Internal heating of the caps canīt be removed. The epoxy
will act as an insulator.
I don't buy that. Epoxy has a fairly high thermal mass. It absorbs a lot of
heat. It conducts it rather slowly. It acts not as an insulator so much as
like a thermal flywheel. Let me give you an analogy. If you build a house
out of concret and cover it with six feet of dirt, the heat of summer will
be reaching the structure at about the same time as the cold of winter is
outside. The cold of winter will reach the structure at about the same time
as the heat of summer is outside. Silicone rubber acts the same way.
If I were planning to run this thing for hours I might worry about it. But
just a few short bursts from time to time, I think it will work O.K.
Most of the reports I have read on this list describe the heating as not
noticeable to bairly noticable.
If I am wrong, time will tell, and I will let you know.
b.) This setup is unrepairable. One of the MMCīs advantages
is easy repair (in case something blows). By potting them, you
defeat this advantage.
It is really easier to unpot something from epoxy than the tar in NSTs. All
you need is an oxyacetalyne torch with a small tip, a stiff brush, alittle
skill and some patience. You play the flame across the epoxy and it turns
all crumbly kind of like dried out jello.9 If you heat it too much it
changes color, gets darker.) Then brush the crumbs away, and flash with the
flame again. I have used this method to unpot timer modules and solid stat
relays so I could repair them in the field in emergancies when replacement
parts were not available or would take too long to get. I have done it many
times just to see what is inside the little magic black boxes.
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.
I don't know what you mean by internal corona. If you mean inside the caps,
then there is nothing I could do about that anyway. If you mean corona at
the points where the leads are soldered together, I would not expect too
much there anyway as i twisted the leads of the caps together and then
wrapped the resistor leads around them, soldered and snipped the exess off
so that makes some ball shaped blobs which
would not emit much anyway. As far as the epoxy causing a problem with
corona, I have seen too many high voltage parts that were potted in epoxy
work fine and last a long time to worry about that.
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
15kV 30ma Franceformer that is in very poor condition. I don't expect it to
last very long. Hopefully it will last long enough to see if this EMMC thing
is really worth the trouble/cost. If it doesn't, oh well. I'll just crank up
the pig. And I didn't have any flashover when I fired the single string
across a spark gap with one NST. (yes I know that that is very hard on both
the cap and NST but I don't care, I did it anyway. It made a very loud bang.
I was quite happy, and just put it on the bench. You already know the rest
of the story.)
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 just can not think of any reason why I would want to do that. I sure don't
want to make it any shorter, I'll just let someone else find out how many Es
can be strung in front of MMC. And as far as making it longer, it is a
question of economics. Longer strings means less capacity, which means more
strings which means more caps. I chose 11 because .056/11 is within 2% of
.005. The torerance on the caps is 5%, so I just call it .005uf. A nice
round number. I like nice round numbers. I chose two strings because 10
nanofarads is the smallest cap that I can tune my 4" coil to without going
to an unreasonable number of turns on my primary. With my 16.4 nanofarad
rolled poly cap the pri. tunes at 7.5 turns. With 10 nanofarads I figgure
the tune point will be closer to 10. Someday, soon I hope, I will get time
to make a new primary. The cat knocked one leg out from under the tripod I
had the coil mounted to and crash bang. The baseplate which holds the
primary disk and the adjustable secondary disk broke into three pieces. This
was early in January. I haven't made any sparks since New Years Eve. In
March when I was given 5 junk NSTs and found two worked, I jury rigged a
mount on the end of a piece of 12" PVC and hooked everything up to the NSTs
and turned it on but no sparks. The spark gap fired about 5 or six times and
the NST that had been out in the weather for a few years died instantly. The
one that still works is in about the same sad shape as far as how much tar
has leaked out of the case, but at least it had a cover over it. So I think
moisture may have played a part in that. Since then I have built a SRSG out
of and into an old GE A/C condenser case. The 1/4 hp 1125 rpm 230V perminant
split capacitor fan motor got six flats on the stator to sync at 1200 RPM.
It is mounted in the original position in the original mount. The shroud and
venturi for the fan are a ready made fragmentation shield. So now we get to
the particular advantage of my EMMC potted in a pipe. The cap will be a
structural member to support the primary as the primary will have to be
elevated above the case to keep too much energy from being absorbed by the
case. That will reduce the amount of primary "plumbing" required which will
reduce the stray inductance. And if it were to work out well, then I could
build more and bigger caps and run it equadrive and have the caps be the
only structural members holding up the primary, with no plumbing between the
sparl gap and the primary because the caps would be the plumbing, and if I
made the primary a double flat pancake using mutual inductance as the (fine)
tuning method, there would be even less stray inductance, resistance, I^2R
loss etc. ! And I don' have any busses to introduce losses in this cap. The
leads from the end caps are twisted together and soldered into the slots in
the heads of the machine screws which are the terminals.
As I understand it, you are not running any resistors across your caps. I
would be very interested to know if you have found the same thing I did when
I got shocked by the middle of the string when it did not have resistors on
it. As I said, I found some pretty high voltages on some of the caps, and I
found that the polarity of the voltages was sort of random. Some one way,
some the other. I wonder if this might be happening while the caps are
running in a coil. If so, then the caps might be seeing higher voltage
stress than they would if the resistor were there. Just a thought. Anyway, I
will always use resistors on any caps that I build in the future no mater
what design I use if the caps have more than two plates.
Hope this answers your questions.