1-8
Figure 1-7.—Commutation of a dc generator.
Q10. What causes sparking between the brushes and the commutator?
ARMATURE REACTION
From previous study, you 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.
You 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.
