PowerLabs Rotary Spark Gap


 After a most enlightening conversation over the #Tesla_Coil IRC chat channel with the expert Tesla Coil builder Terry Fritz, it became clear to me that, if the POWERLABS Tesla Coils were ever to achieve their maximum possible efficiency, a synchronous rotary spark gap would be necessary; As efficient as multi section vacuum quenched linear static gaps may be (they are, in fact, considered by many to be the the most efficient Tesla Coil static gap), they still lack control over the firing cycle: In theory the transformer would fully charge the capacitor twice during the AC cycle: Once at the positive peak of the sine wave, and the second time at its negative peak; and in theory, the gap would be set so that it would only fire at these maximum (peak) values… But reality is other, and as oscilloscope waveforms show, partial ionization, localized electrode vaporization, gap heating, and several other factors cause static gaps to fire erratically and somewhat unpredictably, leading to the need of choosing smaller tank transformers and sometimes even leading to over voltage of neon sign transformers. Also, with static gaps running at high powers, continuous duty quickly causes performance to deteriorate once the gap begins to overheat.
The synchronous rotary spark gap represents the leading edge of high voltage power switching technology. It is capable of firing each time at exactly the same place in the AC cycle, as its rotational speed is locked in synchrony with the AC sine wave feeding it. By utilizing such gaps in a Tesla Coil system, it is possible to choose the largest possible tank capacitor, and be sure that the transformer will have it fully charged every time the electrodes line up. With the SRSG, it is possible to obtain the true maximum performance a Tesla Coil system can deliver.
Below you will find the fully documented POWERLABS SRSG construction effort:

The Motor:

Synchronous motor with gearbox and starter capacitor installedSynchronous motors are very different from those found in electro domestic appliances or on electric tools. Back in the old days they were used on some sewing machines or even on reel-to-reel tape decks, but they are virtually impossible to obtain nowadays. Or so a thought, for after several months looking for one in surplus shops and electronic stores, I was about to try my chances milling grooves on the armature of a non synchronous motor in an attempt to make it suit my needs. The process is not overly complicated but it would be rather difficult to do without specialized tools. Well, to my surprise I happened to run across not one, but rather two boxes full with at least 30 such motors! The price made it all the more attractive: Being sold as junk, those motors were going for $25 a piece! I didn’t think twice before buying one. This particular model has the following ratings:
115V at 0.65A (62W) input. 25W power output
Constant Duty cycle, thermally protected. 1900RPM.

Disassembled motorIt was impossible to determine what kind of machine it came from; however, the fact that it had a 1:36 gearbox attached to it seems to indicate it was meant for some kind of slow, repetitive work… Maybe draw curtains for a stage? Beats me… Well, being useless for my needs, the gearbox and its attachment plate were thrown away, and the motor was disassembled, as can be seen to the right. It is interesting to note that the armature (the shiny metal cylinder below the coils) consists merely of a metal cylinder! This is odd for someone who is used to rotating magnets found on DC or on rotating electromagnets on AC motors. On close inspection both the armature and the coils were found to be rusted, and covered with some sort of black solidified dirt which also found itself all over the other motor parts. Hence everything was carefully dismantled and cleaned with water and soap (for the non electrical parts) or ethanol (for the coils). Finally everything was given a metal coat and the ball bearings were re-greased with some light engine grease. Here is what the motor looks like now, reassembled. Its dimensions are approximately 12 X 10cm, and I reckon it weights around one kilogram.

Cleaned and reassembled Synchronous Motor

The Disk:

Because this is a rather low power motor, everything will have to be kept as light as possible, and yet sturdy enough to handle up to 5kW. I am currently considering a 15cm (6″) diameter, 8mm (1/4″) thick polycarbonate disk with eight 3cm (1.2″) long, 6mm (1/4″) diameter thoriated tungsten electrodes press-fitted into it. Static electrodes will probably be two, made from the same material but mounted on tall heat sinked brass blocks. The entire assembly should be polycarbonate (bullet proof polymer), depending on the price of that material.