I quote from my book-
When a synchronous generator's rotor is spun, a voltage EA is induced in the generator's stator windings. If a load is attached to the terminals ofthe generator, a current flows. But a three-phase stator current flow will produce a magnetic field of its own in the machine. This stator magnetic field distorts the original rotor magnetic field, changing the resulting phase vo ltage. This effect is called armature reaction because the armature (stator) current affects the magnetic field which produced it in the first place."
When the rotor magnetic filed BR crosses the coil EA is at its maximum value. So, BR and EA coincides. A lagging load is attached with the generator. So the current will lag. Assuming IA where it is in the picture. My question is, why is BS is perpendicular to IA. It should be perpendicular to the plane the coil is on, in other words, it should be perpendicular to EA applying the right hand rule. Align all your fingers to the direction of current flow, the thumb will show the magnetic field direction.