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If the voltage between phases is 400V RMS it's peak will be 565V which is starting to get close to the triac rating, it would be advisable to use a higher voltage one. Don't forget that as well as the phase signal there will be noise and spikes from other equipment on the wires and these can increase the instantaneous voltage between phases.
If the voltage rating is OK, the triac should not turn on without one of two conditions being present: an appropriate voltage on the trigger or, a change in voltage faster than the triac can withstand. It is possible that noise is causing fast enough spikes to cause false triggering, especially with your inductive load. It would be a good idea to place a snubber network across the triac to minimize them.
Brian.
If the voltage rating is OK, the triac should not turn on without one of two conditions being present: an appropriate voltage on the trigger or, a change in voltage faster than the triac can withstand. It is possible that noise is causing fast enough spikes to cause false triggering, especially with your inductive load. It would be a good idea to place a snubber network across the triac to minimize them.
Brian.