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Hello Greg, When answering someone, I usually like to leave a few sentences of their remarks but as you can see above, your post disappeared when I opened your diagram and that was all I was left with. Bit of a pain but never mind.
This circuit for a motor speed control or light dimmer on this website is similar to yours and simpler.
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There is description on how it works. Perhaps you can study the diagram with description and then get a better idea how your circuit works, since some areas are similar.I have built that particular controller for a DC motor in an old mig welder and it works very well. Much better than the original motor controller which justed varied the voltage to the motor. Just thought I would mention that if all you were looking for was a simple motor controller.
Regards, John Crighton Hornsby