PMSM Example in MBDT: Questions about specific implementations

[aniket_bagade]

Hi team,

 I was going through example of closed loop FOC of PMSM using hall sensor given in example section of MBDT library.

there in Fast control loop sub system actual value of current( in ampere) is compare with reference value( in ampere) and error term is given to PI controller but output of PI controller is voltage which is between -1 to 1.i.e output of PI is per unit value not actual value in terms of Voltage. I think output of Pi should be actual value of voltage not Per unit value. have you divide output of PI by base value of voltage? I could not find anything like this .or is PI gain taking care of this conversion.

2) at what point you are sampling currents is it at peak of triangular carrier or at zero crossing of triangular carrier?

[dumitru-daniel_]

Hi Aniket, 

output of PI controller is voltage which is between -1 to 1.i.e output of PI is per unit value not actual value in terms of Voltage

The main reason for normalizing the output of the PI controller to [-1:+1] is to take benefits of the AMMCLIB Floating point blocks that required the inputs to be normalized.

You can compute the actual voltage as PI controller output, or you can take that into consideration when you compute the KP/KI gains. The actual voltage in V units in not needed (perhaps only for vizualization) hence if you include the normalization in the controller gain computations you will save 2 divisions operations (controller_out/U_MAX)

2) at what point you are sampling currents is it at peak of triangular carrier or at zero crossing of triangular carrier?

The current shunts are placed in series with the lower transistors, hence in order to pass the current thru the shunt, you need the lower transistor to be in conduction for a minimal amount of time. Since the PWM is center align, then the lower transistor will be open at the peak of the triangular carrier. The drawback of this method is that you can't apply 100% of the available DC bus voltage (you need the lower transistor to open). To overcome this drawback - we use 3rd harmonic injection.

Hope this helps!

Daniel