I am controlling fan speed using Triac phase control with a circuit similar to this. The problem that i am facing is that my triac burned out after using it for 2 3 days and after burning it will remain switched on continuously without gate trigger. I concluded that i am triggering it in 1- and 3+ quadrant resulting in slow degradation, and eventual shorting of MT1 and MT2.
Referred following document for 3+ quadrant switching details. (http://www.nxp.com/documents/application_note/AN_GOLDEN_RULES.pdf