On the diagram for DC Current Modes and FOC Current modes, it is indicated that only two phase currents are measured. How do we control the current in the third phase without measuring the current?
The FOC current mode describes that we measure the current in all three phases but the diagram shows current measurement in only two.
The current of the third is the negative sum at any given moment of the other two, so you only need two.
That’s called Kirchhoff’s Current Law (KCL).
Just my 2 cents, as I had to explore this recently:
Althought you need only 2 current measurements, with low side current sensing it’s good practice to use 3 shunts so you get to pick the 2 phases with lowest PWM duty cycles(highest low side mosfet on time). This ensures you have enough time for the current measurement and can reach higher PWM duty cycles.
Hoverboard mainboards only measure 2 phase currents, so you need to limit the maximum duty cycle.