Continued Problems (fwd)
---------- Forwarded message ----------
Date: Wed, 29 Apr 1998 12:52:12 EDT
From: Esondrmn <Esondrmn-at-aol-dot-com>
Subject: Continued Problems
Sorry, this is going to be a long one.
I fired up my 6" coil last night for the first time this year. I am still
Quick overview: Secondary 6" dia, 991 turns of #22 awg, 29.1 mh. Primary 3/8"
copper tubing wound in a 30 degree saucer shape, 3/8" spacing between turns,
I.D. 8", O.D. 26", 5.25" tall, 14 turns total, 88 uh. Secondary is mounted
so the lowest turn is 3.5" up from the first turn of the primary - right at
about turn 7 or 8. Mark Rzeszotarski (sure hope I got that right Mark)
calculated the coupling to be about .167, so I wouldn't think it is coupled
too tightly. Best tune is at turn 11.75 with a 5" x 40" toroid.
Capacitor is a .025 uf, 15 kv Condenser Products unit with a spark gap mounted
directly across it made of #10 solid copper wire with 1.0" dia loops on the
ends. Gap is set to 5/8". Power is from a 5 kva 14.4 kv pole pig and is
supplied across the spark gap which consists of a rotary with a static gap in
series with it. Current is limited to the pole pig via a Miller Thunderbolt
welder with an adjustable resistor stack in series with the welder.
I built this coil about two years ago and scaled things up gradually from neon
sign transformer to the pole pig. I was quite happy with the performance,
getting nice white hot bolts up to 80" long at about 8 kva input. Then the
first C. P. cap blew up. I replaced it and within not too long, lost the
second one. Several folks on this list suggested I install a spark gap across
the cap unless I have lots of money - which I don't so I did. Now, I can't
input much power without firing the cap safety gap. Last fall, last time I
had this coil fired up, Malcolm, Dr. Cox and a few other helpful folks
suggested I had a bad case of 60 hz resonance. I have to ask a question here
- are there systems out there that can be run at say 10 kva with 15 kv in and
not fire a .25" safety gap across the cap?
For this run, I made a few modifications in the primary set up. I use RG-213
for H.V. feed lines to the coil and I stripped all the shields off these leads
- at the suggestion of Dr. Cox. I also moved the welder and resistive ballast
from the input side of the variac to the output (between the variac and the
pole pig primary). I didn't expect this to fix the problem but thought it
might be worth a try. The primary H.V. comes from the pig, goes to a safety
gap, each lead goes to two 300 ohm 100 watt resistors in series ( total of 600
ohms in each leg) then out to the coil via about 20 feet of RG - 213, again
with the copper braid stripped off. The net result of these changes is
basically zero. The coil still performs as it did before. Maybe someone can
give me some advice based on the symptoms here.
With the welder set at minimum current (maximum inductance I would guess) and
the series resistance at 4.8 ohms, I can only open up the variac to about 75%
and the cap safety gap fires. Primary pole pig current is only 8 amps. Wife
with her ear to the welder says it growls. In all these tests the asynch
rotory is running at about 400 to 500 bps.
Changing the welder setting to maximum current works much better. I can now
decrease the series resistance to 4.0 ohms, open the variac to full voltage,
primary current is now 16 amps and the cap safety gap does not fire. From
this set up, decreasing the series resistance any further or increasing the
rotary speed causes the safety gap to fire.
I tried running with resistive ballast only and can get the primary current up
to 16 amps but I get a real nasty popping sound from the rotary - sounds like
something is getting hurt and makes you want to turn it down quickly.
So, where do I go from here? Do I still have 60 hz resonance? Is this
caused by the welder, and if so how do I fix it? The primary interconnections
are made with 3/8" copper tubing. Malcolm suggested maybe the Q of the
primary is too high. I could try replacing the copper tubing interconnections
with wire as the coil was originally built - which should add resistance and
reduce the Q.
What if I use resistive ballast only - what is the destructive sounding
popping noises coming from the rotary? The rotary consists of 8 electrodes on
a .50" thick lexan disc, 10" diameter (I think) with one set of stationary
electrodes and a R.Q. static gap in series that is electrically folder in half
which would be three .028 gaps in series and in parallel with three more.
Ideas and suggestions would be most welcome.