Armature Reaction Reactance
Armature reaction refers to the influence of the armature flux on the field flux in the air gap when the stator windings are connected across a load.
If Ff is the field mmf in the generator under no load, then the generated voltage Eg must lag Ff by 90o. Per phase armature current Ia produces armature mmf Fa which is in phase with Ia . The effective mmf is Fr.
The armature mmf Fa will induced an emf Ear in the armature winding. Ear is called the armature reaction emf. This emf will lag its mmf by 90o. Hence the resultant armature voltage is the vector sum of the no-load voltage Eg and armature reaction emf Ear .
From the observations of the phasor diagrams for lagging and leading power factors, that the resultant mmf Fr is smaller or larger depending on the power factor. As a result the terminal voltage Vt is larger or smaller than the no-load induced emf when the power factor is leading or lagging.
Since the armature reaction emf Ear lags the armature mmf Fa or Ia by 90o, so it can be expressed as
Where Xar is called armature reaction reactance.