stationary field coil. A simple dc generator is illustrated
in figure 6-12. The commutator in basic generator is a
ring split into two segments, which are insulated from
each other. The two ends of the armature coil are
connected to opposite commutator segments. The dial
and pointer in figure 6-12 represent a galvanometer
with zero in the middle of the scale. The pointer will
move in either direction, measuring small amounts of
current. Notice how the two brushes are mounted on
opposite sides of the commutator to permit each brush
to be in contact with a different segment as the armature
In view A of figure 6-12, the armature is rotating
clockwise; the black side of the armature coil is moving
toward the North Pole, and the white side is moving
toward the South Pole. At this very instant, both sides of
the armature coil are parallel to the lines of flux. No
lines are cut; therefore, no voltage or electromotive
force (emf) is induced into the armature coil.
In view B, the armature coil has rotated 90 degrees.
Both sides of the armature coil are cutting maximum
lines of flux, so maximum voltage is induced into the
armature coil, causing maximum current to flow as
indicated by the galvanometer.
In views A and C (0 and 180 electrical degrees), no
lines of flux are cut; therefore, no voltage is induced
into the armature coil. There is no current flow, as
indicated by the galvanometer. In views B and D, both
sides of the armature coil are cutting maximum lines of
flux, thereby inducing maximum voltage.
In studying figure 6-12, note carefully that when
the black side of the armature coil is under the influence
C O M M U TATO R
of the North Pole, the current flow is OUT. When the
black side of the armature coil comes under the
influence of the South Pole, shown in view D, the
current flow is IN. As the armature rotates at certain
positions (0 and 180 electrical degrees), the current
flow will change direction within the armature coil.
This changing direction of the current is referred to as
G A LVA N O M E T E R
alternating current (ac). When current was flowing
OUT, the segment attached to the black side of the
armature coil was making contact with the negative (-)
brush, but when the current starts to flow IN, the
Figure 6-12.--Generating direct-current electrical energy.
segment switches to the positive (+) brush. This
switching action of the commutator segments changes
The direction of current flow in the armature coil
the ac within the armature coil to dc within the external
during rotation can be determined by using the
circuit. Current always flows in the same direction
left-hand rule for generators. The left-hand rule, illus-
through the galvanometer.