Equivalent Circuit of an Alternator

From the previous discussion it is clear that in all there are three important parameters of armature winding namely armature resistance R_a, leakage reactance X_L and armature reaction reactance X_ar. If E_ph is induced e.m.f. per phase on no load condition then on load it changes to E' due to armature reaction as shown in the equivalent circuit. As current flows through the armature, there are two voltage drops across R_a and X_L as I_a R_a and respectively. Hence finally terminal voltage V_t is less than E' by the amount equal to the drops across R_a and X_L.

Fig. 1  Equivalent circuit

       In practice, the leakage reactance X_L and the armature reaction reactance X_arare combined to get synchronous reactance X_s.

       Hence the equivalent circuit of an alternator gets modified as shown in the Fig. 1.

Fig. 2  Equivalent circuit of an alternator

       Thus in the equivalent circuit shown,

       E_ph = induced e.m.f. per phase on no load

       V_tph = terminal voltage per phase on load

       I_aph  = armature resistance per phase

       Z_s = synchronous impedance per phase