Transformer Input Voltage

I have no answer.

A triac is not necessarily a good choice for this if you have 3 phase or 2 phases to fire (2 separate legs of single phase) as the entire unit fires on at each cycle, so it is not true phase angle control. To do it correctly you need a full inverse parallel SCR set for each phase / leg. Using a triac will result in a high DC component in your input which will in turn increase the losses in your transformer and heat it up beyond what it normally would. If the transformer is lightly loaded you may be able to get away with it because the extra heat is going to be proportional to current, but that is usually not worth taking the chance.

Thanks, i think two SCRs ( one in each input wire) is the same as one Triac on one wire.

Zaghlool

"...i think two SCRs ( one in each input wire) is the same as one Triac on one wire."

Yes, if you are only controlling one wire. But into a transformer primary you are usually going to control both.