Armature reaction in synchronous generator

Armature reaction is the effect of the armature (stator) mmf, produced by the load current, on the main field flux of the machine. When load current flows in the three-phase stator winding, it creates its own mmf, which rotates at synchronous speed in step with the rotor poles and distorts, weakens or strengthens the main flux. Its effect depends on the angle ψ between the load current I_a and the induced emf E (the internal power-factor angle).

The armature flux is in phase with the armature current, while E lags the main field flux by 90°. So when I_a is in phase with E, the armature flux is 90° (electrical) away from the main field flux.

Resistive (unity pf) load

• I_a is in phase with E.
• The armature mmf is at right angles to the main field. Hence the armature reaction is cross-magnetising.
• Effect: the field is distorted. In a generator, the flux density is reduced at the leading pole tip and increased at the trailing tip. With magnetic saturation the net flux falls slightly, so the terminal voltage drops a little (also because of the armature resistance drop).

 Resistive load: Fa is 90° to Ff

        Ff (main field)
         ^
         |
 --------+----> Fa (armature mmf)

Inductive (lagging pf) load

• I_a lags E by up to 90°.
• The armature mmf opposes the main field flux, so the armature reaction is demagnetising (purely so at zero pf lagging).
• Effect: the net flux falls and the terminal voltage falls strongly, so the field excitation must be increased to keep the voltage constant.

 Lagging load: Fa opposes Ff

   Ff ---->          Fa  <----
   (Net flux = Ff - Fa, weakened)

Capacitive (leading pf) load

• I_a leads E by up to 90°.
• The armature mmf adds to the main field, so the armature reaction is magnetising (purely so at zero pf leading).
• Effect: the net flux increases and the terminal voltage may rise above the no-load value (negative regulation). The excitation must be reduced.

 Leading load: Fa aids Ff

   Ff ---->  Fa ---->
   (Net flux = Ff + Fa, strengthened)

For a general pf, the armature mmf has a demagnetising or magnetising (direct-axis) component F_a sin ψ and a cross-magnetising (quadrature-axis) component F_a cos ψ. In calculations, the reaction is accounted for by the armature-reaction reactance X_a, and X_s = X_a + X_l is the synchronous reactance.