# Re: Rotary Spark Gap.

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Sent: 	Friday, December 05, 1997 12:27 AM
To: 	Tesla List
Subject: 	Re: Rotary Spark Gap.

>Could you elaborate on syncronous vs non-syncronous?
>"Adjusted to the right phase?" (240 vs 120??)
>"frequency at either 1800 ot 3600 rpm."

A syncronous motor rotates in sync with the frequency of the line current
(for US 60Hz).  A 1800 rpm motor's shaft rotates exactly once for every
two cycles of the AC supply.  Similarly, a 3600 rpm motor shaft goes
around once every cycle.  3600 rpm = 60 rps = 60Hz.

This is useful, since the tank circuit capacitor charges to maximum (+ or
-) every half cycle (albeit 90 degrees or so out of phase with the
transformer).  When the rotating gap's disc is sync'd up so that the
moving electrode passes between the static electrodes at exactly the
maximum point (this is "in phase"), you get the cap discharging from it's
maximum voltage.  Higher voltage = more current to the primary = more
power = bigger spark.  The rotary gap can quench huge amounts of power,
since the electrodes are physically moved far apart from each other very
rapidly after the spark initially jumps.

>What about using a variable speed
>motor?

Variable speed motors are used in non-syncronous gaps, for achieving the
optimal number of breaks per second.

>I'm starting with a 15kV, 30ma neon.  But hope someday to build bigger.  The
>RSG seems to be a big investment  (the biggest for me so far..) .  Can it be
>used on a wide range of coil sizes?  Would a airblast gap suffice for the
>neon?

IMHO a sync rotary gap is entirely unnecessary for use with small
neon-powered coils.  Rotaries are used because they can quench large
amounts of power (2kVA+) effectively.  Your 450VA supply will do fine to
start with a multiple static gap, or an air blast type gap.  John Freau
has shown that the quenching speed possible with a syncronous gap will
improve even neon powered systems, but the proper and safe design of such
a rotary gap is quite costly and definitely non trivial.

On the other hand, a rotary gap could be very useful when you move up to

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