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Synchronous generator working; armature reaction

Short answer

Armature reaction is the effect of stator mmf on the main field flux. For resistive load it is cross-magnetising, for inductive load demagnetising, and for capacitive load magnetising, respectively causing slight drop, large drop, or rise in terminal voltage.

How does three phase synchronous generator work? What do you mean by armature reaction? Comment on the results of different load power factor on armature flux.

Working of a three-phase synchronous generator (alternator)

  1. The rotor carries a DC-excited field winding (the poles). The prime mover turns the rotor at the constant speed NsN_s.
  2. The rotating flux cuts the three-phase armature (stator) windings, which are placed 120∘120^\circ (electrical) apart. By Faraday's law, an emf is induced in each phase:
E=4.44 f ϕ TphKpKdwithf=PNs120E = 4.44\,f\,\phi\,T_{ph}K_pK_d \quad\text{with}\quad f = \frac{PN_s}{120}
  1. The three emfs are equal in magnitude and displaced by 120∘120^\circ in time, giving a balanced three-phase supply.
  2. When a load is connected, a three-phase armature current flows, and the terminal voltage is V⃗=E⃗−I⃗(Ra+jXs)\vec V = \vec E - \vec I(R_a + jX_s).

Armature reaction

Armature reaction is the effect of the armature (stator) mmf, produced by the load current, on the main field flux (its magnitude and distribution). The armature current produces a field rotating at synchronous speed, which is stationary relative to the rotor, and it either opposes, aids or distorts the main flux.

Effect of load power factor

Load pfAngle of II with EEArmature reactionEffect on flux and VV
UnityII in phase with EECross-magnetisingFlux distorted (weakens one pole tip, strengthens the other); slight fall in VV
Zero laggingII lags EE by 90∘90^\circFully demagnetisingMain flux reduced, VV falls a lot
Zero leadingII leads EE by 90∘90^\circFully magnetisingMain flux increased, VV rises
General laggingbetween 0 and 90∘90^\circPartly cross, partly demagnetisingVV falls
General leadingbetween 0 and 90∘90^\circPartly cross, partly magnetisingVV may rise
 Zero pf lagging: Fa opposes Ff      Zero pf leading: Fa aids Ff
     Ff --->   <--- Fa                   Ff --->   ---> Fa
     Fr = Ff - Fa                        Fr = Ff + Fa

The armature reaction is represented by a reactance XaX_a, which together with the leakage reactance gives the synchronous reactance Xs=Xa+XlX_s = X_a + X_l.

✓ Answer checked by Claude Opus 5.5 (high effort)

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