[TCML] Considering a new coil build...
drieben at comcast.net
Tue Mar 24 16:26:58 MST 2009
Welcome back! ;^) I'm sure that you will get
numerous repsonses to your posting but I will
share a few of my observations.
First of all, MMCs could definitely work on a
coil of the magnitude that you propose but the
purchase and soldering together of several
hundred MMC caps (which is pret ty much
what would be required for a p ig powered
coil system) would prove to be impractical-
ly expensive and laborous when compared
to a single unit commercial pulse capacitor.
MMCs are great and highly recommended
for small to medium sized NST driven coils
but when you step up to piggy power, their
practicality starts to become more of an is-
Second, with your variacs, "I" would leave
the 3 of them intact although I would opt to be
able to manually turn the voltage control and
remove the motor drive. To be able to use one
as a current control, you must cut through its
core to gap it and if you're not careful, you'll
just end ruining a perfectly good variac. That
has been my personal experience. You can
use a 225 amp stick welder for very good
voltage and current control. You may also
consider constructiong your own ballast out
of building wire and an iron core from a large
discarded transformer. I used the two piece
core from an old x-ray transformer and wound
each "leg" with paralleled coils of #8 building
wire, about 105 turns each . You would have
to be sure that your core material has sufficient
cross sectional area to handle the power that
you will be controlling.
Your wire size for you secondary is pretty
much right in the "ballpark" of what you
would want but you should really opt for
a significantly larger secondary form than
8"x30". As you mention, sonotube is one
option for a larger diameter secondary
form. Also, I went to a local plastics suppli-
er and ask for 12" PVC duct pipe. They had
it but I think they would only sell it in the 10
ft. lengths and it was a bit pricey. However,
a good coiling friend of mine was also looking
for a secondary former so we both chipped
in on the former and cut it in half. You should
definitely go with a 12" or even larger secondary
form to be able to utilize the greater induct-
ance which is a good thing for longer sparks.
Most suggest keeping the secondary aspect
ratio in the 4:1 to 5:1 range so a 48" to 60"
of 12" form would would be great. Of course,
you'll need even more wire for this coil size and
you may find it to be more practical to go for
the larger 12" spool of magnet wire (~80 to
100 lbs.) as opposed to the small 6" spools
(~8 to 11 lbs.) You may check into a local
distributor of wire or find a motor repair
shop if you're in a larger city to see if you can
purchase some wire from them.
For the primary coil, the copper tubing is a
great idae although I would opt for 1/2" tu-
bing since you're building a piggy powered
system. Some would also suggest flat cop-
per strip as it would allow for a much smaller
outer diameter of the primary coil which can
help to reduce primary coil strikes.
IMHO, the G-10 with tungsten electrodes
ARSG is the way to go. However, I would
opt for at least a 10 " diameter disc as an 8"
disc would render standoff from the shaft/
hub quite a challenge at these voltage/power
levels. "I" use an 11 1/2" dia. x 1/2" thick G-10
disc with (8) 3/8" x 1 1/2" long tungsten flying
electrodes and (4) 1/2" dia. x 3" long tungsten
stationary electrodes. I also use a DC motor
from a treadmill as the drive motor and recti-
fy the 120 VAC to DC and use a variac for
speed control. These types of motors are
quite powerful for their relatively small size
and lend theirselves very easily to infinite
speed comtrol via a variac.
You could use one of the 8x24 toroids as a
shield toroid under a larger main toroid. I would
go for a toroid cord diamter of at least 10", if
not 12" and a major diameter of at least 40"
for the main toroid for better field control and
better E-field shielding. I think you'll find that
a single (or even stacked) 8x24 toroids would
really limit your coil's performance for the 10
kVA pig power that you have available.
RFI/EMI filters do help reduce the transients
produced by the coil's primary circuit that tend
to get back into the power supply lines. Just
make sure that the filter is up to the task for
the current that will be drawn by your coil.
Good luck with your project and keep us in-
formed of your progress.
----- Original Message -----
From: "Travis Tabbal" <travis at tabbal.net>
To: "Tesla Coil Mailing List" <tesla at pupman.com>
Sent: Tuesday, March 24, 2009 2:29:02 PM GMT -06:00 US/Canada Central
Subject: [TCML] Considering a new coil build...
It's been a while, but I miss Tesla coiling. :)
I had purchased a number of useful parts before stopping years back, so
here's what I have and what I'm thinking. I'd love some thoughts from you
Power source: 10KVA Pig 14.4KV
Control: 3 giant variacs (I forget the rating). I planned to use 2 to
control voltage and modify one for inductive current control. They are
currently stacked with a motor drive for the brushes. I planed to remove the
motor and one of the variacs. The other 2 will remain ganged up and I'll
move the shaft by hand with a big plastic wheel or something. There may be
better ways to handle this bit. I might be able to obtain an old arc welder
for the ballast.
Cap: MMC. To be determined. Ideas welcome.
Secondary: 8" diameter ~30" long. What do people use as a form for these big
coils? I was thinking of drying some Sonotube or other cardboard tube and
coating with laminating epoxy or even just polyurathane. Or I could get
ambitious and try to make a fiberglass tube by adding a release agent to the
sonotube so I can slip the fiberglass off after curing. Considering AWG20
magnet wire. Works out to about 1000 turns. Need a source for that much wire
in one spool. The 11lb spools on EBay and such are too small. I need about
15lb if I'm figuring right.
Primary: Copper tube spiral. Probably .25". Need to run the calcs to get the
number of turns.
Gap: Async RSG. I'm thinking to try to build one with .25" G-10 and tungsten
electrodes. About 8" diameter. Construction tips welcome. Particularly
holding the flying electrodes in place. I'd like to have it be speed
controled, so I can vary break rate around a bit. Even if it's not realtime
Topload: I bought 2 8"x24" spun toroids in a group buy years ago. Still in
the box. It's a shame not to use them. They are so pretty. :)
I also have some RFI filters for the pig I think I bought on the list.
Note that I don't have a machine shop, though I do have a decent tool
collection both power and hand types. So I can't do fancy stuff like
machining electrode holders from bar stock. :) I can drill and tap it with
some acuracy, but nothing too special. No lathe or mill. :)
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