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Re: Rotary Gap info needed
Ron,
Here are some tidbits of info, not complete at all. You may want
to print out or save this posting, because there's some info here
that might not be easily found at any website.
A rotary gap is a set of spinning electrodes and fixed electrodes
that cause a capacitor to discharge at a certain time based on
the number of electrodes and motor rpm. A rotary will generally
run cooler than a fixed gap because of the whipping action of the
air.
It is commonly believed that a rotary forces quenching by stretching
the spark, but this is generally not the case. Rather the energy is
dissipated while the electrodes are still aligned, or even before they
align fully. The gap stops firing once the energy is gone (rung down).
If the electrodes are too wide, or the rpm too slow, or the capacitor
too small however, the gap may refire while the electrodes are still
aligned causing inefficient operation. Generally motors of at least
1750 rpm are used. Some folks bring their rotaries up to 5000 or
even 7000 rpm. The balancing and strength becomes much more
critical and important the higher the rpm of course.
The electrode gap must be set close enough so the coil fires
steadily, so the gap spacing will depend on the voltage and the
number of series gaps.
An async gap fires randomly in regard to the 60Hz ac cycle, but a
sync gap will fire at a certain spot on the ac cycle all the time. An
async gap may fail to fire when the ac is at a low point (near the ac
zero crossing). So whatever BPS you calc, the true BPS will be
lower than that in an async gap. The same thing can happen using
a sync gap at higher break rates. At 120 BPS, the sync gap will
fire on every ac half cycle however. A sync gap which runs at a low
BPS needs to have some method of adjusting the phase of the
firing to the phase of the ac. The phasing is generally accomplished
by rotating the whole motor, or by placing the fixed electrodes onto
an arm that can be pivoted. Another way to adjust the phase is to
shift the position of the rotor relative to the motor shaft. Methods
of shifting the phase with capacitors and variacs are now being
explored.
There are certain async speeds which may cause a pulsing type
spark because the speed is close to synchronous, this is to be
avoided.
You can use a variable speed motor or a one speed motor for an
async gap. The variable speed motor gives more flexibilty when
running the coil. For instance if your cap is too small, you can
then raise the break rate to get more power throughput. But if
you use a one speed motor, then you have to add electrodes to
raise the break rate which is more work. Generally induction
motors are single speed, although some are special multi-speed
types, and the speed can also be slowed with a variac, but they
don't respond all that well. Universal motors such as those in a
vacuum cleaner are variable speed types (they have brushes)
In my work, I've seen the best efficiency at 120 BPB sync, for both
NST, and PT powered systems up to 2000 watts. I can't say what
will happen at higher powers. Sync systems are more critical
regarding design of the coil; the cap must be the right size, etc.
A very small rotary can use a 1/50 HP motor , I just built one:
http://members.aol-dot-com/FutureT/TT-42index.html
But generally a larger motor of 1/10 to 1 HP is used. It all depends
on the size of the rotor, the weight of the electrodes, the amount of
air drag, etc.
The rotor should be made from an insulating material such as G10
phenolic which is very strong. A material such as Lexan can be
used, but can't be permitted to get hot because it will melt and the
electrodes will fly out and kill someone. I often use steel bolts for
the electrodes, but copper is probably better, and tungsten is best
from a wear point of view. Don't use Lucite for the rotor, it's too
brittle. You can put a lexan or wooden guard around the rotor for
safety.
Sync gaps are esp useful when used with NST's to get the most
power and spark length out of them. I also like to use sync gaps
with PT's. My new TT-42 coil gives 42" sparks max from a 12/30
NST.
Induction motors can be modified by grinding flats on the armature
(actually the squirrel cage rotor), to make it synchronous.
Some folks use a metal disc for their rotary, and they mount it on
a shaft and an insulated base, and drive it using a rubber belt and
pulleys so it's insulated. This is generally more work then just
attaching the rotor to the motor, but it permits a different speed to
be obtained from the motor.
Some folks (including myself) have experimented with offset spacings
of the rotor spinning electrodes. More work is being done in this
area.
Generally, there are two series spinning gaps at work in the rotary,
but more series gaps can be used. More series gaps may help
the quenching a little, but will tend to increase the gap losses.
Generally it's better to decrease losses than to worry about quenching.
Rotaries are usually used in high power coils over 2 or 3kW, but are
also commonly used in small NST powered coils, or in special
research type coils. Async rotaries tend to destroy NST's BTW.
A rotary can be built at almost zero cost using commonly available
materials.
I hope this helps.
Cheers,
John Freau