Sunday, August 16, 2009

The Rotary Spark Gap for a Tesla Coil

You are looking at an Oriental Motor 1800 rpm synchronous motor, a G-10 fiberglass complete rotor thats under 4 inches! (about 90 mm) with a matching solid copper 18 gauge back plate disk that has been drilled and balanced, a mandrel designed for this rotor and cw & ccw operation, (2) motor run capacitors , (4) each flying tungsten electrodes press fit into the rotor and (2) 1 inch static electrodes with solid copper holders. Some of the pictures you see are examples of the type of Rotary Synchronous Spark Gaps you can build with these, and your added components. When I started into building Tesla Coils, it wasn't long before I wanted a Rotary Gap for my Neon Transformer based coil. In over 100 hours of searching, I found the parts I needed were hard to come by. When I finally did and switched to a rotary spark gap, the increase in performance was incredible!
To get a true synchronous gap I needed a motor that operated as a salient pole type synchronous motor. My only option seemed to be to convert a standard type motor by "grinding or machining the armature" of a standard motor. Not really up for that, I wanted to "buy" a motor that would work "out of the box". I had read about these motors from the Oriental Motor Company called "Reaction Motors" that operate as a "Salient Pole". Hundreds of synchronous spark gaps have been successfully made from them. The suggested surplus model was unavailable and the new replacement in small quantities were $234.00 each. The companies I was directed to (Herbach and others) were all sold out of the surplus version.
After much searching, I found a small lot of these motors that are Brand New Old Stock. I am keeping some for my own use and selling the rest here. I have made over 100 of these and still invest about 2 1/2 hours of time per rotor. I recently added pictures of the latest rotor design, lighter in weight and I have guys using them at 20KV at 75ma in Australia. Also pictured is a new motor run capacitor. I have been looking for a capacitor like this for these motors for over a year. Smaller, the right voltage, value and non-OILCAN look, even has a mounting bracket built in! I had also had so many requests for flying and static tungsten electrodes I decided to include those as well.
Asynchronous motors work fine If you have a "Pole Pig" transformer. If your using a "Neon Sign" transformer they can be really rough on it and may destroy it. The motors I'm selling are rated at 115VAC. With the under 4" rotor and the 4 flying electrodes and 2 static electrodes pictured, (which gives 120 BPS, ideal for most neon sign transformer coils) sync lock is achieved at approximately 90 volts (see picture). You are buying only the items listed above, you will need to supply a base and mount the motor, etc.
The mandrel makes the sync adjustment easy, just align the rotor and re tighten. Its been great talking to all the coilers out there, what a great group of folks from all over the world. These motors will run on 50Hz or 60Hz at 1500 rpm or 1800 rpm for our friends in other countries. I have sold quite a few in the UK and Australia with good feedback from the users. They just use a variac to reduce the voltage. Check out my website,

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