I think that turn-off IS a concern if there is a large inductive load. At the moment when the current aproaches and crosses zero, conduction of triac would stop. However, there is a substantial amount of momentary voltage which results in a large dU/dt across the triac, which could in turn, in conjuction with some other factors, result in the autofiring of the triac. But it could be avoided with a snubber network across the triac.
I agree that the triac should be triggered at the voltage zero-crossing across it, that also means at the current zero-crossing because the triac appears to be purely resistive. But the question is HOW?
When the triac is in a stationary blocked state, there is a full voltage across it, however,when in conduction, a voltage across it is about 2 Volts. Also, there are the transients in between. So, the voltage across the triac could be readily used when it is blocked, i.e. at the initial turn-ON. But than the voltage across it falls to merely a volt or two, which means that the voltage at the sensing point will span over very wide range. However, it could be managed if that is a right way.
Does anybody sees any problem here?
Thanks