Question

what is meant by ( armature reaction) how its effect taken into account in the equivalent circuit of AC generator ?

Answer 1

Answer) The effect of armature winding mmf or flux on the main working flux created by field poles is called the armature reaction. The armature field is produced by the armature conductors when current flows through them. And the main field is produced by the magnetic poles.When the current flows through the armature winding of the an alternator, a flux is produced by the resulting MMF. This armature flux reacts with the main pole flux, causing the resultant flux to become either less than or more than the original main field flux.Armature winding in an electrical machine is the winding which carries the load current. Under no-load condition, the armature current is zero. But as the machine is loaded, load current flows through the armature winding and creates magnetic flux.

The synchronous reactance is an equivalent reactance the effects of which are supposed to reproduce the combined effects of both the armature leakage reactance and the armature reaction. The alternator is supposed to have no armature reaction at all, but is supposed to possess an armature reactance in excess of its true leakage reactance. When the synchronous reactance is combined vectorially with the armature resistance, a quantity called the synchronous impedance is obtained as shown in figure.

The armature winding has one more reactance called armature reaction reactance in addition to leakage reactance and resistance. Considering all the three parameters the equivalent circuit of a synchronous generator can be written as shown below. The sum of leakage reactance and armature reaction reactance is called synchronous reactance Xs. Under this condition impedance of the armature winding is called the synchronous impedance Zs. Hence synchronous reactance Xs = Xl + Xa per phase and synchronous impedance Zs = Ra + j Xs per phase As the armature reaction reactance is dependent on armature current so is synchronous reactance and hence synchronous impedance is dependent on armature current or load current.