Brushless controller parts

So you have the "logic" part that runs at 0-5V. The controller is a microprocessor that can read out the current monitors and respond to them by changing the voltages by changing the connection of the FETs (one to ground, one to voltage). This brings voltage to the motor windings, and makes it torn.

For FOC, the microcontroller sets FETs so that the voltages on the coils become the output of the controller, which uses them to generate sine wave outputs at the motor voltage.

One thing I don't understand is where the current is sunk in FOC control. Where does it flow ? Wouldn't the neutral point in the motor simply increase to some average of the voltages across the coils ?

Is this accurate ?

  • \$\begingroup\$ You don't need the DAC. Typically the modulation of effective voltage at the motor and hence current through it is done by PWM. \$\endgroup\$
    – Phil G
    Oct 24, 2018 at 14:58
  • \$\begingroup\$ What if you want to drive the fets with sinusoidal currents + high frequency tune for sensorless positional operation ? \$\endgroup\$ Oct 25, 2018 at 0:09
  • \$\begingroup\$ To get sinusoidal drive, you use a sine lookup table to derive from the electrical angle the amplitude values of each phase, then update the PWM output at each period. There's lots of diagrams here robotics.stackexchange.com/questions/261/… \$\endgroup\$
    – Phil G
    Oct 25, 2018 at 14:13

1 Answer 1


Yes it is basically correct. The connection to the motor could be more clear, which may be why you are confused about where the current goes. I found a nice picture on the web that's a schematic representation of delta and Y connections. With a brushless (or any 3-phase device), you're always driving at least two terminals at different voltages, so there's always current being driven in at least two coils.

In a typical BLDC setup, two terminals would be driven, one high and one low, and the third would be floating. In a sensorless motor, that third terminal would have its voltage monitored, which would be used for commutation. In a sensored motor, the shaft position relative to the phases is read directly. In a BLAC all three terminals are driven, and if the motor is sensorless the current consumed in each leg of the driver is used to determine commutation.

  • \$\begingroup\$ Would this still be true in a FOC driven motor ? \$\endgroup\$ Oct 25, 2018 at 0:02
  • \$\begingroup\$ Same thing. What I said is independent of whether it's a FOC motor or not. \$\endgroup\$
    – TimWescott
    Oct 25, 2018 at 3:02
  • \$\begingroup\$ The idea for FOC seems to be to use SPWM for sinusoidal current generation, but how would you use that to generate a high frequency signal to inject ? \$\endgroup\$ Oct 25, 2018 at 9:07

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