I have studied several application notes and technical literature regarding field oriented control of three phase induction motors. Based on that I have encountered that there are two main approaches as far as timing of the control software.
One approach that I have found is that the free running timer periodically starts A/D conversions of two phase currents and dc bus voltage. The frequency of the A/D conversions is several times greater then the pwm frequency. The interrupt service routine associated with the end of conversion then contains whole current control loop and at the end updates the duty cycles for the pwm.
Another approach that I have found is that the A/D conversions of two phase currents and dc bus voltage are synchronized with the pwm duty cycle reload events. The frequency of the A/D conversions was usually one half of the pwm frequency. Again the interrupt service routine related to the end of converstion contains the current control loop. This approach repeated in more cases then the first one.
In my opinion the first approach is better because it reduces the transport delay of the control loop and ultimately increases phase margin of the control loop. Thus I do not understand why the second approach is more frequent. I have probably missed something. Does anybody know the answer?