Hi there,

Yes you can do that, however if there is any slight phase shift in any of the three phases, the crossover angles (and hence the timing!)will change. This is the biggest problem with using just timing to fire the thyristors in the inverter. The easiest way to ensure that this does not happen is to use a PLL (phased locked loop - a lot easier than you might think!) synchronised with phase AB (ph C can be derived mathematically so don't need to worry about this and we can add some dead-time to allow for any phase shift) thus ensuring any phase shift can be detected.

The beauty of a PLL is that it does not matter what the input frequency is as all you are doing is detecting the crossover angles and syncing with them (you can use an analogue comparator to do this with about 3/4 discrete components. Use the PIC to count output pulses of the comparator and then fire your thyristors! And hey presto active braking/regen!

Let me know if this is of any help, if you require more in-depth info just say so and I can provide timing diagrams and I could probably put a simple circuit diagram together, I will however need to know what its application is so that I can implement some extra protection in the way of delays etc. for you.

There are other simpler PIC only ways of doing it by detecting the frequency of ph AB and just timing it, but these are scary if you are dealing with high-voltages.

Neil