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The rotating field within the generator's air gap, while at on-load 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:
REACTION
REASON
CHARACTER
Transversal
resistive load
usually does not modify the value of the flux
Longitudinal
inductive load
demagnetizing
capacitive load
magnetizing
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 ampere-turns' modification.
Operational equations, in stationary conditions (steady state), of a hydro generator are:
Field:
Uf = Rf * If
Armature:
Uf0 + Uead + Ueaq = U + R * I + j * X * I
θrez = θf + θ'ad + θ'aq
Due to different magnetic reluctances across of the two axes we take into account the separate actions of the two ampere-turns. The load current I should be decomposed into two components Iq and Id.
Iq = I * cosφ, Id = I * sinφ
Synchronous reactances characterize the armature reaction phenomenon:
Xq = Xaq + X, Xd = Xad + X
where:
  • Xaq = transversal reaction reactance,
  • Xad = longitudinal reaction reactance.
Phasorial diagrams are built using the operational equation.

The angle φ, between the polar e.m.f. phasor Ue0 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.
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About HG
Armature Reaction
Excitation Systems
Paralleling
Loading
Design
Electrical Braking
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