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From what I understand Field Oriented Control is meant to be used with PMSM as PMSM has sinusoidal back EMF due to distributed stator windings.

On the other hand BLDC's have concentrated stator windings resulting in a trapezoidal back EMF.

Six Step commutation uses strictly two activated stator windings, 1 deactivated winding (lets assume 3 stator 1 pole pair motor). Whereas FOC uses variable current through all the coils to create a stator magnetic field that is always orthogonal to the rotor.

Does the concentrated structure of the BLDC stator windings make FOC performance worse, and if so why and how does it make it worse?

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You can supply sinusoidal voltages to a motor with trapezoidal back emf. The reason this is typically not done is because you will get torque ripple.

Imagine you have a DC motor. \begin{equation} \tau=ki \end{equation} Now apply a rectified 3 phase sine wave current to the motor to get the average torque. \begin{equation} \tau_{avg}=\frac{3\sqrt{3}}{2\pi}ki_{peak}\\ \end{equation} Now imagine that there will be some min and max current applied from the rectified sine wave current. This will generate the torque ripple. https://www.electronics-tutorials.ws/power/three-phase-rectification.html.

If you are doing some sort of speed control and your load has high inertia, the torque ripple might not matter. If you are doing position or torque control, then I would not recommend.

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In reality, BLDC motors don't have truly trapezoidal back EMF. The inductance of the motor smooths the waveform out, resulting in the back EMF being much more sinusoidal. Trapezoidal, sinusoidal and FOC are all common and legitimate commutation strategies for off the shelf BLDC motors.

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