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Hey Bones!
c*RPM*current = pwm_duty
Let's say the current is large and the rpm is very very small. Then the voltage across the motor is going to be very small (almost a short circuit)
Now let's say the motor is turning almost infinitely fast. Then just when the current is about to get going in the coils one way, the brushes make it turn around and flow the other way, so the net result is that very little is flowing.
That's just a plausibility argument as to why current and rpm are inversely proportional to each other, and why RPM and pwm_duty are directly proportional to each other. I actually don't know why it's true in the case of series DC motors. But let's pretend that it is!
Then to limit the RPM, you just limit pwm_duty/current in software. hurray!
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