What exactly does Close Loop Speed Control System control?

Question

I'm working on a project about controlling Brush-less DC Motor using PI(Proportional Integral) Close Loop Speed Control Method.
3 Phases Motor is driven by 6 MOSFETs, Rotor speed is sensed and feedback to the control IC, this IC will compare the reference speed (target speed) with the sensed speed and adjust the control of the 6 MOSFETs.
Schematic is as below:

Calculation and controlling can be done inside the IC itself, but I want to know what signal the PI Close Loop Speed Control will control to archive the target speed? Eg: when rotor speed is lower than ref speed, the controller will increase the Duty cycle of PWM or will it increase the phase switching frequency?
Thank you all for reading.

DKNguyen · Accepted Answer

What it is actually controlling is the duty cycle, but the phase switching/commutation frequency follows the RPM so it will increases as the RPM increases since it must be synchronized with rotor or the motor will lock up.
It is wrong to think that the driver directly controls the commutation frequency. It does increase but the driver doesn't directly control it by dialing in a number. It just commutates whenever the rotor reaches particular angular positions so in reality the RPM (or more accurately how frequently the rotor passes a commutation point) controls the commutation switching frequency. And RPM is affected by both applied load and duty cycle.

What exactly does Close Loop Speed Control System control?

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