Armature Reaction Reactance - Synchronous Generators

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 F_f is the field mmf in the generator under no load, then the generated voltage E_g must lag F_f by 90^o. Per phase armature current I_a produces armature mmf F_a which is in phase with I_a . The effective mmf is F_r.

The armature mmf F_a will induced an emf E_ar in the armature winding. E_ar is called the armature reaction emf. This emf will lag its mmf by 90^o. Hence the resultant armature voltage is the vector sum of the no-load voltage E_g and armature reaction emf E_ar .

From the observations of the phasor diagrams for lagging and leading power factors, that the resultant mmf F_r is smaller or larger depending on the power factor. As a result the terminal voltage V_t is larger or smaller than the no-load induced emf when the power factor is leading or lagging.

Since the armature reaction emf E_ar lags the armature mmf F_a or I_a by 90^o, so it can be expressed as

Where X_ar is called armature reaction reactance.