Synchronous DC motor

While trying to get more power out of my Neon-Sign Tesla Coil, I looked into using a synchronous rotary spark gap (SRSG). The SRSG is used to synchronize firing of the spark gap to the 60Hz mains frequency. With these synchronized, it can be adjusted so that the spark gap fires at every Vpeak of the mains (when the capacitors are charged to the highest potential).

The one downside to a SRSG is that they require a synchronous AC motor, which are expensive and somewhat hard to come by. I had a nice DC servo motor sitting around, so I looked into a way to synchronize it to the mains frequency.

 This is a rather crude test bench that I created. The “power supply“ consisting of a big heavy transformer, bridge rectifier, and smoothing cap are on the right.

Here’s a better shot of the motor and proximity sensor.

The ‘brains’ of the circuit, A PIC16f877A.

I found some app notes for implanting a PID control system on a PIC, and started programming.

For validating the circuit, I hooked up a “timing light” to shine on the rotating disk.  I wired up a 1W LED to a zero-crossing detector triggered by the mains power. This would flash the led very briefly every time the mains voltage crossed 0V. I also drew a couple of ‘timing marks’ on the disk. If the motor was actually synchronized to the mains, the timing marks should appear to stand still. This can be seen in a couple of short movies. Link1 Link2 (mov format, right click and save as)

After some tweaking, I achieved a nice stable synchronous dc motor. Unfortunately, the components I used have a hard time functioning when exposed to the massive electro magnetic fields generated by a Tesla coil. As soon as I started putting power to the Tesla coil, the motor just started going nuts. I might have been able to shield the components, but I decided to move on to the <Off-Line Tesla coil> which replaces the spark gap all together.