As I understand your intended system, if the current were at a set level and the pump load were suddenly reduced the motor would speed up, increasing the tacho output. To counter the speed increase you would use the tacho signal to reduce the current from the converter significantly. That reduction would be your indication of dry-running.
I don't see how the motor would speed up, because we are talking about the pump "suddenly" running dry ("sudden" being the operative word here)....ie complete and sudden, severe load removal......the current loop wouldn't be able to react immediately, so at the instant of sudden load removal, there would be miles too much current flowing in the motor coils.....this would literally slingshot the rotor round at a heck of a speed......and the control loop wouldn't be able to keep up with this, and so it would pulse motor coils at the wrong times, and so the motor would just end up running unsmoothly, rather than actually speeding up.........and so the current loop wouldn't detect the increase in speed that it needs to detect in order to reduce the current from the buckboost.....and so the system would in fact, end up increasing current....obviously not wanted.
I can assure you that the essential method that we are using here is the method used by all high power BLDC motors.....that is, not PWM'ing the bridge transistors in order to control motor current......if you do it like that, then that is ok for small low power motors, but its no good for big motors because you have to have a current sense resistor downstream of the motor coils which gets used to control the current in the motor coils....and you oviously have to wire to this sense resistor, and wiring down to sense resistors is not practical with high power motors, as you get too much wiring inductance, therefore, with high power motors, you have an upstream variable current or voltage source, and you vary that in conjunction with the motor speed feedback.
I say the upstream regulator can either be variable voltage or current, but either way, you have to put current limiting circuitry in there, so why not just use that to regulate the current and make it a current regulated upstream power source?
I guarantee all that doing high frequency PWM of motor coils to control coil current is never done with high power BLDC motors...thus the method we are doing is not at all unusual.
do you agree?
I am still not sure how the RVCO pin voltage affects the commutation frequency?
I thought the VCO was to do with "Locking" the coil commutation gate pulses to the back-EMF signals?.....ie, not to do with speed setting?, which is surely the job of the SPEEDSET pin of the ML4425?