What will happen if we excite a Synchronous Machine with AC?
You'll see smoke I reckon. Think of a transformer with an AC voltage applied to both primary and secondary windings - unless this is totally matching the turns ratio there'll be a lot of smoke.

Imagine a synchronous motor run like an induction motor - you can easily do this by shorting the rotor slip rings. When first powered, the motor generates tons of torque because the induction into the rotor is 50Hz and this generates a field that causes the motor to start to turn. As it turns the "slip frequency" gets less and less until it is a few Hz - it reaches equilibrium somewhere close to (but below) synchronous speed. 

If you held the rotor fixed mechanically so it couldn't turn, the rotor induction would be massive and at 50Hz - this is what your question is asking - it would burn.