Shunts for sensorless FOC

[abudabit]

This is my first time using the 2 shunt method of sensorless field oriented control (3 phase wye). Do I place the shunts between the source and ground for 2 of the 3 phases low side mosfets? Or do I place the shunts between where the high side and lowside mosfets connect to the winding? All of the documentation I'm going by is vague on this.

I attached a schematic I made since I'm probably describing the locations horribly. Is R1 + R2 correct or R3 + R4?

I saw a few schematics of the shunts between low side source and ground (R1 + R2) but that seemed odd since they said Ia + Ib + Ic = 0... shouldn't there be a net current from the drains of the highsides to the sources of the lowsides?

 

[cswilson]

You need to place them in series with the windings. Your instincts that two "low-side" sensors can't detect everything that is going on in a 3-phase motor is correct.

(If you have sensors on all of the low-side legs AND you have a switching pattern that does not permit current to flow from a top-side resistor through a top-side flyback diode, it might be possible, but you would have to transform the measurements into phase currents, which is not trivial.)

The problem with in-line resistors is the necessary level-switching and isolation. At 220V and up, I have always gone with Hall sensors instead.

Curt Wilson
Delta Tau Data Systems