What is meant by synchronous impedance of an Alternator?

The complex addition of resistance, R and synchronous reactance , jXs can be represented together by a single complex impedance Zs called synchronous impedance.

In complex form Zs = (R + jXs )
In polar form Zs =I Zs I< Ѳ
and where the I Zs I=√(R²+X²s)
and Ѳ=Tan-1 (Xs/R)