When a synchronous generator supplies electric power to a load,the armature current creates a magnetic flux wave in the air-gap which rotates at synchronous speed. This flux reacts with the flux created by the field current and electromechanical torque results from the tendency of these two magnetic fields to align.In a generator,this torque opposes rotation and a mechanical torque from the prime mover must be applied to the shaft to sustain rotation.This is the mechanism by which the generator converts mechanical energy to electrical energy.