Equivalent Circuit of an Alternator

Equivalent Circuit of an Alternator

From the above discussion it is clear that in all 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 Ia flows through the armature, there are two voltage drops across R_a and X_L as I_a Ra and I_a X_L respectively. Hence finally terminal voltage V_t is less than E’ by the amount equal to the drops across R_a and X_L

In practice, the leakage reactance X_L and the armature reaction reactance Xar are combined to get synchronous reactance X_S.

Hence the equivalent circuit of an alternator gets modified as shown in the Fig. 2.7.2.

Thus in the equivalent circuit shown,

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

V_{t ph} = Terminal voltage per phase on load

I_aph = Armature resistance per phase

Z_S = Synchronous impedance per phase

Review Question

1. Draw the circuit model of a cylindrical pole alternator and explain.