Armature reaction in synchronous machines

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.

The effect of Armature (stator) flux on the flux produced by the rotor field poles is called Armature Reaction.

The armature reaction in synchronous machine affects the main field flux very differently for different power factor .for the sake of simplicity , we shall consider the following three extreme condition namely , Unity power factor , zero power factor lagging and zero power factor leading. The result can be generalized for different power factors met in practice.

For the purpose of this discussion, power factor will be defined as the cosine of the angle between the armature phase current and the induced emf in the armature conductor in that phase.

Armature reaction at unity power factor:

Armature Reaction is mostly cross-magnetizing though it has a small demagnetizing component . when a generating machine supplies unity power factor .

Armature reaction at zero power factor lagging:

Armature Reaction is demagnetizing. when a generating machine supplies zero power factor lagging current.

Synchronous machine in generating mode  supplies a lagging power factor current, the armature reaction has both demagnetizing and cross-magnetizing components.

Synchronous machine in motoring mode  Draws a lagging power factor current, the armature reaction has both magnetizing and cross-magnetizing components.

Armature reaction at zero power factor leading:

Armature Reaction is magnetizing. when a generating machine supplies zero power factor leading current .

Synchronous machine in generating mode  supplies a leading power factor current, the armature reaction has both magnetizing and cross-magnetizing components.

Synchronous machine in motoring mode  Draws a leading power factor current, the armature reaction has both demagnetizing and cross-magnetizing components

In a motoring machine the armature reaction MMF and flux are in phase opposition to the armature current, the nature of armature reaction is just the reverse.