Okay, this evening I made a small sketch that set the duty cycle to 64, 128, 192, 255, and back down, for a few seconds each point. The power supply has three terminals connected to a potentiometer, and two terminals for PWM. Here are some of the things I observed:
With the pot turned to zero, the PWM signals did nothing.
With the pot turned up anywhere from just barely turning, or almost ready to turn, the PWM settings caused it to start, and run faster at each level. Unless the pot was turned up pretty high, the PWM signals would not get it turning beyond about 8000 rpm, and if turned up high enough, the higher PWM signals actually made it slow down.
If the spindle is turning fast enough, it generates power that feeds back through the LCD, which gets darker (more pixels turning dark) until it’s unreadable, just solid blocks. If the power is off on the arduino, and I run the spindle up with the pot, it will also start to light up the LCD - I have not pushed it to full power like this.
With the pot disconnected, and only the PWM wires connected, nothing happens at 64, it turns at 1100 rpm at 128, 2400 at 192, and 4400 at 255. It won’t go any faster than that.
The voltage output to the motor varies over a range of maybe +/- 10vdc as the PWM cycles, centered around whatever voltage the rheostat is set to. I didn’t measure the output when I had the pot disconnected.
Anyway, that’s just some observations. I’m not sure what to make of them yet. I post them here in case someone can decipher the behavior and give me an idea of how to get it all working.