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Rotary Spark Gaps

 * Original msg to: Esondrmn-at-aol-dot-com

Clipped from my archives 11/15/93:

The next stage employed in spark gap technologies is placing a
rotary gap in the circuit. The rotary gap is a mechanical spark
gap usually consisting of revolving disk with electrodes mounted
on the rim. The rotor is spun and the electrodes move in relation
to a set of stationary electrodes nearby. As a moving electrode
comes near a stationary electrode, the gap fires. As it moves
away the arc is stretched and broken. The rotary gap offers the
sophisticated coiler the opportunity to control the pulse in the
tank circuit. A properly designed rotary gap can control the
break rate (bps) and the dwell time.

Rotary gaps are run in two modes, synchronous and asynchronous. 
A synchronous gap runs at a fixed speed and is constructed so
that the gap fires in direct relation to the 60 cycle waveform of
the line feed to the capacitors. The point in the waveform where
the gaps are closest can be changed by rotating the synchronous
motor housing or by altering the disk position on the motor
shaft. By carefully matching the output of the supply transformer
to the value of capacitance in the tank circuit, then running  
a properly set up synchronous gap, it is possible to have the gap
fire only at the voltage peaks of the 60 cycle input current.

This technique allows the tank circuit to fire only on the
maximum voltage peaks and delivers the pulse from a fully charged
capacitor each time the gap fires. If properly engineered,
synchronous spark gap systems will deliver the largest EMFs to
the secondary coil. They are however, the most finicky, and
difficult to engineer of any spark gap, and require sophisticated
test equipment to set up.

Asynchronous gaps are more common. They work quite well and are
much easier to run. Fixed or variable speed motors may be used,
though variable speed gaps give the builder the most experimental
leeway. Break rates need to be in excess of 400 bps, and I have
found that breaks rates around 450-480 bps give the best
discharge on my large coil. Since the gap is firing more often
than the 60 cycle waveform switches polarity, more power can be
fed into the tank circuit, as the capacitors can be charged and
discharged more rapidly. Though this system will increase the
amount of spark from the secondary, sparks are generally not as
long as with synchronous gaps.

At higher powers (over 5 kVA) even a rotary gap will not deliver
the quench times required for excellent performance unless it is
very large. If the arc in the spark gap hangs too long (NOT
quenched), it leaves the tank circuit electrically closed. With
the gap still firing energy will backflow from the secondary into
the primary and create continued oscillation in the tank circuit.
The secondary is then supplying energy to maintain the arc in the
spark gap. As power levels build, so does the pressure on the
spark gap. Engineering more sophisticated gap systems is the only
solution in large 1/4 wave coils and Magnifiers.

The easiest solution at 5 kVA is to add a static gap in series
with the rotary. By messing with the gap settings it is not
difficult to develop a gap system that fires smoothly and
quenches well. As power levels increase though static gaps will
be overwhelmed. More sophisticated gaps are required to replace
the static series gaps. Magnetic or airblast gaps must be used in
conjunction with the rotary gap to remove the strain on the
rotary and get the quench times back down.

Somewhere in here I need to cover the Q of spark gaps. Not all
spark gaps have the same Q. I have found that using large series
static gaps with lots of electrodes; the Q of the gap system
decreases as the quench time decreases! Try to avoid static gap
designs with more than 6 - 8 electrodes in series.

As my power levels went up, and my spark gap Qs went down, I
experimented with options to regain performance. I found that by
running static gaps in a combination of series/parallel gave me
good quench times and I regained some lost Q from the arc having
to make so many series jumps. The idea was to split the arc down
into two or three equal paths, reducing the current traveling
each set of series gaps. In this fashion I was able to achieve
excellent quench times with a small rotary running around 5 kVA.

The lesson learned was too many gaps in series kills the Q of a
spark gap. By adding gaps in parallel, and reducing the number of
gaps in series, some Q was regained while power levels increased.
This is a valuable hint in spark gap designs.

Another factor that should be brought into this discussion is the
effects of cooling the electrodes. To start with, I have never
run even a simple static gap without some airflow. My first few
really good static gaps were constructed inside of PVC pipe
sections with a 5" muffin fan on top. The fan did not supply
sufficient air to disrupt the arc, but did assist in removing hot
ions, and cooling the electrodes down. This allows for longer run
times. As my work progressed I realized that reducing the
electrode temperature, while not actually quenching the gap,
reduces the amount of metal ions introduced into the arc, and
makes the gap easier to quench with an airblast or magnets.

I am going to cut this off here. I feel I have covered most of
the basics, and thrown a few ideas out into the cyberspace. I
would be more than happy to expand on spark gap technologies at
any time should somebody have any specific questions, comments,
problems, or corrections. Remember, armchair debate is no
substitute for actually going out an experimenting with a few
live systems, and I am always hoping someone will tell me a
better way to do it!

One final safety note. Spark gaps are loud, and emit a lot
of hard UV radiation. Wear hearing protection as required, and 
never stare at an operating spark gap without welding goggles.
To examine the arc on large coils, a sun observation filter
on a small telescope will tell you if your gaps are quenching.

Richard Quick

... If all else fails... Throw another megavolt across it!