Re: Cap break down voltages / Rotary Question

            Re: Cap break down voltages / Rotary Question
            Tue, 04 Mar 1997 04:16:18 -0800
            julian <julian-at-glosilk.demon.co.uk>
            GlowSilk Computer Services Ltd.

Thanks Ed Sonderman, Richard Hull for your advise.  Home made caps at
10KV RMS are about right.

Running a pole pig without balast.

Well this is not exactly true, The pole pig is connected via a 100m
extension leed and the wire resistance provides some balasting (5 ohms).

My pole pig is 5KVA at 11KV.

When running the coil I run my rotary spark gap at high speed which
gives very smooth running.  At lower speeds I can feel the kick back
from the pole pig as vibrations in the variac.

I use an 8A variac protected with 13A fast blow fuses. These fuses blow
if smooth running is not achieved.

This weekend I took Eds advise and built a larger toroid.   I had some
stainless steel flu duct that was being dumped at a nearby building
site.   There was sufficient to make a 24" toroid.   Adding this to my
10" x 36" meter coil, I needed extra primary capacitance.

So with the primary tapped at turn 9 and 0.05uF the spark length
increased from 36" to 40" possibly more as the garage size is now
limiting spark length.   Arcs keep striking the ceiling and tripping the
house lights.

To give me more head room I then used my 7" x 21" coil with the same
toroid almost with the same tune point I measured 45" arcs.   I then ran
the coil on full power, no increase in spark length due to near by
objects, but the sparks are very bright, and I am sure would go further
given the room.

During this run the spark gap took a hit.  For some odd reason the
strike caused the electrodes on the rotary spark gap to colide, and self
destruction followed.

I would like to ask the list if the gap design I have is any good.  
Basically I have a high speed motor (10,000 rpm) and a rotor disk made
from fiber glass PCB copper clad board.   Two electrodes are mounted on
the disk which are electrically connected by the copper covering.  
These electrodes pass two stationary electrodes with a clearance of
about 0.5mm.   I find that when the gap is running at operating speed
the 6" disk streches and the 0.5mm gap closes almost to the point of

I calculate that with 1/4" electrodes the dwell time:

velocity of electrodes is
   6*3.14*10000/60 = 3140 inches per second

dwell time is
   0.25/3140 = 79uS

Flaming round problems don't seem to be a problem.  (No visable signs)

The remaining rotor disk following the failure has lost an electrode. 
The electrode which is 1/8" diameter has ripped a grove from the point
it was mounted to the outer edge of the disk.  Is it possible that I am
running the motor to the point of failure of fiber glass?

Comments Welcome

Julian Green