What phenomenon occurs when the armature's magnetic field distorts the magnetic neutral in a generator?

Armature Reaction refers to the phenomenon where the magnetic field produced by the armature current in a generator distorts the main magnetic field. This distortion leads to a change in the effective magnetic neutral axis of the machine, which is the point where there is no induced electromotive force (EMF). The armature's magnetic field interacts with the field created by the field windings, causing the neutral plane to shift.

This shift can lead to various operational complications, such as sparking at the brushes and reduced efficiency of the generator. Understanding armature reaction is crucial for proper generator design and operation, as it helps engineers take measures to mitigate its effects, ensuring stable performance.

The other concepts in the question, while related to electrical machines, do not specifically describe this interaction between the armature and the magnetic field. Core Saturation pertains to the condition where a magnetic core can no longer increase its magnetization despite an increase in current. Phase Shift relates to the difference in phase angle between the current and voltage in an AC circuit. Commutation refers to the process of switching the direction of current in the armature windings, primarily in a DC machine, to maintain unidirectional current output.