What is synchronous impedance and how is it determined?

Synchronous impedance (Zs) represents the combined effect of armature resistance, leakage reactance, and armature reaction in synchronous machines. It is the impedance that appears to limit current flow during normal operation. Determination uses open-circuit test (get Voc at various field currents) and short-circuit test (get Isc at same field currents). Zs = Voc/Isc at the same excitation. Synchronous impedance varies with saturation; unsaturated value is used for short-circuit calculations, saturated for voltage regulation.