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_{ar}are 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
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