The rotating field within the generator's air gap, while at onload operation, is determined by the superposition of the inductor field and the exciter field. This interaction represents the Armature Reaction, and it is varying with the load's character and the constructive type of the synchronous generator. We can distinguish: 
usually does not modify the value of the flux 
Analitical calculation of the armature reaction consist in the determination of the value of the field current equivalent of the armature's load currents, which leads to the field ampereturns' modification. Operational equations, in stationary conditions (steady state), of a hydro generator are: 
U_{f0} + U_{ead} + U_{eaq} = U + R * I + j * X * I

θ_{rez} = θ_{f} + θ'_{ad} + θ'_{aq}

Due to different magnetic reluctances across of the two axes we take into account the separate actions of the two ampereturns. The load current I should be decomposed into two components I_{q} and I_{d}. 
I_{q} = I * cosφ, I_{d} = I * sinφ

Synchronous reactances characterize the armature reaction phenomenon: 
X_{q} = X_{aq} + X, X_{d} = X_{ad} + X 
where:
 X_{aq} = transversal reaction reactance,
 X_{ad} = longitudinal reaction reactance.
Phasorial diagrams are built using the operational equation.
The angle φ, between the polar e.m.f. phasor U_{e0} and the terminal voltage phasor U, is called load angle. It represents the spatial shift of the polar star before its position from idle running. 
