Armature reaction in synchronous generator

Armature reaction in a synchronous generator is the effect of the armature mmf $F_a$ (produced by load current) on the main field mmf $F_f$. Armature current in the three-phase winding produces a field that rotates at synchronous speed, so it is stationary with respect to the rotor poles. The result depends on the angle $\psi$ between the armature current and the induced emf $E$.

1. Resistive load (unity pf)

Current $I$ is in phase with $E$. The armature mmf $F_a$ lags $F_f$ by $90^\circ$, so it acts across the pole axis.

• Cross-magnetising armature reaction.
• The flux is distorted: the trailing pole tip is strengthened, the leading tip weakened; the net flux is roughly the same (slightly reduced by saturation).
• Terminal voltage drops slightly.

  Ff -->   Fa (down, 90 deg)   Result: Fr = Ff + Fa
   N-----S      |                 (shifted)

2. Inductive load (lagging pf)

Current $I$ lags $E$ by up to $90^\circ$. For zero-pf lagging, $F_a$ is directly opposite to $F_f$.

• Demagnetising (partly cross-magnetising for general lagging).
• Net flux is reduced, so the induced emf falls and $V$ drops significantly.
• To keep $V$ constant the field current must be increased.

  Ff -->        <-- Fa (zero pf lag)
  Fr = Ff - Fa

3. Capacitive load (leading pf)

Current $I$ leads $E$ by up to $90^\circ$. For zero-pf leading, $F_a$ is in the same direction as $F_f$.

• Magnetising (partly cross-magnetising for general leading).
• Net flux increases, the induced emf rises, so $V$ may exceed $E$.
• The field current must be decreased to hold voltage.

  Ff -->   --> Fa (zero pf lead)
  Fr = Ff + Fa