what is meant by armature reaction & where its happen ?

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what is meant by armature reaction & where its happen ?..

We know that all current-carrying conductors produce
magnetic fields. The magnetic field produced by current in
the armature of a dc generator affects the flux pattern and
distorts the main field. This distortion causes a shift in
the neutral plane, which affects commutation. This change
in the neutral plane and the reaction of the magnetic field
is called ARMATURE REACTION.

We know that for proper commutation, the coil short-
circuited by the brushes must be in the neutral plane.
Consider the operation of a simple two-pole dc generator,
shown in figure 1-8. View A of the figure shows the field
poles and the main magnetic field. The armature is shown in
a simplified view in views B and C with the cross section
of its coil represented as little circles. The symbols
within the circles represent arrows. The dot represents the
point of the arrow coming toward you, and the cross
represents the tail, or feathered end, going away from you.
When the armature rotates clockwise, the sides of the coil
to the left will have current flowing toward you, as
indicated by the dot. The side of the coil to the right
will have current flowing away from you, as indicated by
the cross. The field generated around each side of the coil
is shown in view B of figure 1-8. This field increases in
strength for each wire in the armature coil, and sets up a
magnetic field almost perpendicular to the main field.