While designing my own BLDC controller I got curious about how the big boys do it and opened up a commercially available one (Maxpos 50/5). Most things were quite straightforward, except a 11.3uH series inductor in series between the the current measuring resistor of each half bridge and the motor phases connector. I have never seen anything like this ever before, and am very curious as to what the function of it might be. Any ideas?
The inductance is present to smooth the phase current. If there was no inductance in the motor at all, the phase current would look like the PWM voltage waveform. This would cause excessive eddy current losses in the core, potential EMC issues, and will also reduce overall torque production efficiency, as the torque output is the average of the current waveform, whereas the resistive losses are the RMS of that same waveform.
In most motor designs there is sufficient inductance present in the motor anyway, but if the PWM frequency is low, or the motor is a coreless design, then external inductance may be required to sufficiently smooth the current.