I'm designing some custom hardware for a 15 kW 400V BLDC/PMSM InstaSPIN system using the CCS050M12CM2 six-pack SiC MOSFET power module. Because the power module has the drains of all the low side FETs bussed together internally, it's not possible to use low-side shunts for current feedback. Instead I plan to use hall effect sensors in the motor phase lines.
When using isolated phase current transducers rather than low-side shunts, the problem of current measurement during high PWM duty cycle disappears. Is there then any benefit at all to having hardware support for 3 sensors rather than 2? I know I can set USER_NUM_CURRENT_SENSORS to 2 and then the stock instaspin library will happily solve for the third current using Ia+Ib+Ic = 0. The only possible advantage I can think of is the reliability enhancement via redundancy - in principle one with 3 sensors one could write code to check that sum for consistency to try to detect and exclude a failed sensor. But that seems rather arbitrary since those sensors are usually pretty robust and there are plenty of other single failure points in the system.
In case it's relevant, precise control at low speeds is unimportant for this application (it's driving a propeller) and as I understand it, FAST is mostly relying on voltage feedback rather than current feedback at higher speeds - though I'd love to understand that better if somebody has an explanation or reference.
Before I lay out this board with only 2 sensors, did I miss something? :)
Thanks, Henry
