Armature reaction in any machine is the effect of armature flux on main flux. In alternator, rotor is energized through DC supply and made rotated through external prime mover. As stator houses 3 phase conductors, stator coil cut this rotor field flux . When load is connected to 3 phase load, load draws current from the alternator. This current in turn generate its own flux which counters or oppose the main rotor field flux. This effect of armature flux on rotor flux is called armature reaction in alternator.