Re: Rotary Sparkgap
Subject: Re: Rotary Sparkgap
From: MSR7-at-PO.CWRU.EDU (Mark S. Rzeszotarski, Ph.D.)
Date: Mon, 19 Feb 1996 11:50:12 -0500
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>In a message dated 96-02-16 15:12:56 EST, Kevin M. Conkey writes:
>>Subject: Rotary Sparkgap
>> have seen recently that it is preferred to have a high rpm spark gap.
>Ed Sonderman replied:
>It sounds like a great rotary gap to me. I would definitely fire it up and
>try it. What are you using for primary capacitor and transformer? If the
>transformer will put out several hundred milliamps, you can crank the break
>rate way up and get much more power out of the coil. Might as well fire the
>gap as soon as the capacitor is fully charged.
>This would be a great question for Mark Rzeszotarski who seems to be our
>resident scientist/mathamatician. With say 14,400 volts in at 500 ma how
>long does it take to fully charge a .025 mfd capacitor? And how long after
>the gap fires do we want to wait for the primary to ring down a ways (10
>cycles?) before we fire the gap again? If I remember the math on this, one
>time constant would be about 1ms so to fully charge the cap we would need
>about 5ms. This means we should not fire the gap any faster than every 5 ms.
> If I calculate my gap speed correctly, I am firing the gap (at least
>bringing the electrodes in a position to fire) every .75 ms. Something is
>wrong here, maybe Mark can figure it out.
Thanks for the compliment. However, I have no hands-on practical
experience with rotary gaps yet. I have read quite a bit, however, so I'll
comment on this to stimulate some more discussion. The charging time is
related to the RC time constant of the capacitor, transformer internal
resistance, wire resistance, and current limit of the transformer due to
shunting. This PROBABLY has little effect on performance for most coils.
Rather, it is a problem of getting the gaps to fire at the right time, and
You want the rotary spark gap to fire a number of times in each half
cycle of the A.C. input sine wave. Let's say you have a rotary gap with
very close contacts in series with several series stationary gaps to provide
some total distance D between the gaps. This means that the gap won't fire
unless the voltage on the capacitor is at least great enough to arc across
this distance D. If D is selected so the gap won't fire until the voltage
is 1/2 of peak transformer voltage, then the rotary can potentially fire any
time the A.C. sinusoid rises above 1/2 peak voltage, or below -1/2 peak
voltage, for the negative half cycle. Ideally, you want many gap firings
while there is high voltage available for charging the capacitor and also
able to fire the gaps.
Let's consider an example: If we use the 1/2 amplitude example,
this corresponds to a sinusoid with an angle of 30 degrees. As the A.C.
sinusoid sweeps from angle 0 degrees to 360 degrees there is sufficient
voltage for the spark gap to fire if a rotary contacts its stationary pin
during angles 30-150 degrees, and during angles 210-330 degrees for the
negative half cycle. 60 cycle A.C. has a period of 16.7 milliseconds.
During 66.7% of this time period, the voltage across the capacitor is
sufficient to fire the spark gaps. Hence, about 11 milliseconds of time is
available for sparking to occur in each full cycle, and about 5.56
milliseconds is available during each half cycle. If we run our rotary at
100 breaks per second (BPS), a contact is possible every 10 milliseconds.
Unless we time things just right, we will not even fire once every cycle.
If we raise the rotary speed to 600 BPS, now a contact is available every
1.7 milliseconds, and we could potentially fire the gap 3 times each half
cycle, if we happen to catch things just right. (A graph would be useful
here in a future FAQ on this!)
This is why people use high break rates (>600 BPS) and sometimes go
to synchronrous motors so the timing of the gap firings can be made to match
the A.C. cycle perfectly. The trick is to adjust the break rates and the
total gap distance so that you get maximum energy transfer to the coil.
>I know I was running my rotary at 667 bps and increased the performance quite
>a bit by going up to 1333 bps (16 electrodes running at 5000 rpm).
This makes perfect sense in terms of firing more gaps during the
time when voltage is sufficient to arc across the gaps.
Mark S. Rzeszotarski, Ph.D.