Neglecting armature resistance Ra (and hence Cu loss), then power developed (Pd) by an alternator is equal to the power output (Pout). Fig. shows the phasor diagram of the salient-pole synchronous generator. The per phase power output of the alternator is
The total power developed would be three times the above power. The following points may be noted:
(i) If there is no saliency, Xd = Xq
This is the power developed by cylindrical rotor machine.
(ii) The second term in eq. of pd above introduces the effect of salient poles. It represents the reluctance power i.e., power due to saliency.
(iii) If E0 = 0, then,
The power obtained with zero excitation is called reluctance power. It should be noted that reluctance power is independent of the excitation.