That's for a two-wire PM DC motor right? How does measuring the motor current determine how much voltage to apply?
I always thought that circular shaped circuit board inside the motor was a voltage regulator, with an adjustment pot below the hole to trim the speed.
Just as a sanity check, I connected an ordinary DC PM motor to a variable-voltage supply here, the RPM is proportional to set voltage. The RPM doesn't change by much when loading the spindle. When the loading on the spindle makes the current exceed the power supply limit, then the revs drop. I suspect the voltage is dropping.