You must have a Synchronous motor. The motor needs to rotate 180 degrees so you can adjust it while the rotor is turning so you can find the peaks. It works but it is too much work and too much $$$ for what you get. There is a better way. If I remember correctly Lexan will get you killed that thing will explode at 3600 RPMs.
You need to build a Variable Speed Vacuum Fan Spark Gap here are some photos of mine.
https://i67.photobucket.com/albums/h292/mikeweaver/gap1.jpg
https://i67.photobucket.com/albums/h292/mikeweaver/gap2.jpg
https://i67.photobucket.com/albums/h292/mikeweaver/gap3.jpg
https://i67.photobucket.com/albums/h292/mikeweaver/gaps5.jpg
It can be used from 400 watts to 15,000. watts. You use a variac to adjust the motor speed. As the motor speeds up you can see the output sparks of the Tesla Coil get noticable longer and longer and longer. When the motor reaches a certain speed the sparks output stops getting longer. If you speed the motor up faster the sparks start getting shorter. You can dial in maximum spark length in about 6 or 7 seconds.
This is my 10" Tesla Coil. It makes 12 ft long sparks. The sphere is 3 ft diameter. If I put my TC in the center of the back yard it will produce a 27 ft circle of sparks and arcs in the yard. This is running on about 12KW. Power Transformer is 14,000. Volts secondary, primary is 240 VAC 60 amps.
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This is several 1000s sparks.
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This is my 4" and 6" tesla coils
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This is the 4", 6" and a tiny little 2" tesla coil.
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More photos.
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This is my 6" TC 5 ft sparks form 3 neons.
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This is my 4" TC 24" sparks from 1 neon
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