High Voltage Forum
Tesla coils => Spark Gap Tesla Coils (SGTC) => Topic started by: thedoc298 on May 09, 2019, 03:25:01 AM
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Im starting to dive into the bits of Synchronous Verses Asynchronous for my spark gap. Some say the best is Asynchronous and some say Synchronous. From what I have read the Synchronous would lock on at 120, 240 ....and such. But how is Asynchronous lined up to fire at the correct time, I don't seem to fully comprehend the Asynchronous spark gap and the pro's and con's of both.
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The short answer to your question is that there are no fixed firing points for an asynchronous RSG. The firing points will wander slowly through the sine wave as the motor RPM drifts with time.
The primary advantage of a ARSG (asynchronous) over a SRSG (synchronous) is simplicity. In order to make a SRSG, you need a synchronous motor powerful enough to spin the disk. These are usually made from regular non-synchronous induction motors by grinding either 2 flat spots (for a 3450 RPM motor) or 4 flat spots (for a 1725 RPM motor) on the armature. This will then make the motor synchronous at either 3600 RPM or 1800 RPM. Then you have to phase the motor disk so that the spark gap electrodes fire at the peak points of the sine wave (60 Hz or 50 Hz) when charging the capacitor for 120 BPS (60 Hz) or fire at the peak sine wave points and the zero-crossing points for 240 BPS (60 Hz). The reason for doing all this is to make the firing points correspond to the maximum charge on the capacitor. This results in the maximum power delivered to the primary coil per bang.
The ARSG is much simpler. It just uses a regular induction motor with no modifications. The firing points happen at arbitrary points along the capacitor-charging sine wave. Thus the amount of power per bang delivered to the primary coil changes and drifts with time as the motor RPM wanders. With only 2 or 4 flying electrodes, you can even experience variation in streamer length and sound volume as the firing wanders along the sine wave.
With greater than 4 flying electrodes the difference between the SRSG and ARSG diminishes and the synchronism is of little value.
I built my SRSG from a 1/2 HP 3450 RPM induction motor by grinding 2 flats 180 degrees apart on the armature. It now runs synchronously at 3600 RPM. I set the firing points using an optical detector on the disk along with an oscilloscope to monitor the firing points. I rotated the motor until I got the firing points that I wanted. Then I locked down the motor. I use 4 flying electrodes so I get 240 BPS firing at 0 degrees, 90 degrees, 180 degrees, and 270 degrees on the sine wave.
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Thanks very much for the clear explanation.