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Re: My new coil

In a message dated 97-02-13 04:18:08 EST, you write:

<< 
 I have just recently completed the construction of my second coil.
 
 The coil consists of a 10" secondary wound on HDPE pipe 950 turns of
 1.02mm enameled copper wire.   The secondary consists of 11 turns of
 3/8"
 copper pipe, capacitors are home made poly/foil construction 0.03uF.
 
 Powered by 11KV 5KVA single phase pole transformer using a home made
 rotary spark gap based on a brush type motor with top speed of 15,000
 rpm.
 
 The toroid is pathetic, being the same diameter as the coil, I
 could'nt wait to try the coil out so last week end I fired her up.
 
 The first time of firing this coil was also the first time of using a
 pole transformer, so I thought it best to do things slowly!
 
 By turning to rotor on the gap so that two electrodes aligned I powered
 the
 coil up using a variac.   At about 10% the gap fires, and soon becomes
 red
 hot, not much good for coil firing but OK for tuning up using a
 flurescent
 tube.   Having found a good tune (turn 9), the rotary gap was started at
 low
 revs.
 
 With more power being applied to the pole transformer large bangs and
 rough
 coil running the variac began to smoke and fuses blew.   12" sparks for
 the
 top of the coil - went to bed.
 
 Using a pole transformer for the first time was scarey!   Good job I had 
 practiced on a neon transformer first.   I always unplug HV supply when
 making
 adjustments even if the variac is at 0%.   I got a nasty shock from my
 neon
 when supply still connected and variac at 0%.  I later measured the
 output 
 of the variac - it was 2 volts, stepped up through the neon made
 150volts.
 
 With a fresh mind the next morning I repaired the variac (stuck brushes)
 replaced the fuses, and used a 100 meter extension lead as resistive
 balast.
 
 This time I ran the spark gap at
 about 40% and coil at 40%.   Things ran much smoother, no banging, and
 no
 blowen fuses.   Getting 2' arcs.   Increced power to coil to 100%. 
 Sparks
 now up to 3'.    Increaced power to spark gap to 80%.   The spark length
 increaced significantly 4-5' the color of sparks changed from purple to
 white/blue, noise increaced (need ear protection) - must be getting
 better
 quenching from the gap as speed increases.
 
 Then desaster!  Vibrations on the spark gap cause the electrodes to
 strike
 shatering the rotor and bent the stationary electrodes.   Pieces of
 spark gap
 shot across the garage.
 
 After re-building the sparkgap (better desgn this time) I can not get
 longer
 sparks than 4-5'.   Partly because the ceiling of the garage is in the
 way.
 Everything is being run at 100% power now.
 
 Things I plan to do to increase spark length:
 
 1)      More electrodes on the rotary spark gap.   Currently 2 increase
 to 4
 2)      Build a toroid
 3)      Add more capacitance to the tank circuit in order to match new
         secondary resonance when new toroid fitted.
 4)      Connect second pole transformer (5KVA)
 
 Any tips on getting better/longer sparks are very welcome
 
 Julian Green
  >>

Julian,

Congratulations.  Sounds like a winner.  I did some quick calculations on
your system.  Secondary inductance should be about 49 mh, self capacitance
about 18.8 pf.  It should resonate at about 134 khz with a 10 pf toroid.  If
the primary is 12" I.D., it should be about 72 microheneries.  The math says
it should tune about turn 9 right now.  You should be able to go up to almost
30 pf in toroid capacitance and still just barely tune at turn 11.  This
would be about 100 khz.  Using the 80% rule (lose 20% of the toroid
capacitance for shading), you should be able to go up to a 35 to 38" x 5 or
6" toroid with your present capacitor.

I would definitely get some inductance for ballasting.  As you found out,
with no ballast, a pig wants to draw infinite current.  Your added resistive
ballast certainly helps but they are hard to control with no inductance.  I
use a welder on mine and set the current low (about 10 amps primary current)
for tuning then turn it up when everything is set.  Even then, there is still
some inductance in the system.  I don't remember the exact number but I think
it is around 5 mh with the welder wide open and the welding leads shorted.

Your secondary can certainly handle a much larger toroid and it should
perform very well with a toroid as I described or maybe even up to 40 or 48"
with more capacitance.    Is it possible to add a few turns to your primary?
 This may be easier unless you have more caps available.  With 11 kv in, you
may get to where the toroid is too big to break out and would require higher
voltage in.

Ed Sonderman