connected to each other and to the commutator
segments in such a way that the voltage impulses
overlap and produce a steady output voltage. This may
be compared to the overlapping of power impulses in an
The armature core (fig. 6-14) on which the
armature windings or coils are mounted is made of
laminated soft iron. The core is laminated to reduce
eddy currents, and is made of soft iron for permeability.
The purpose of the field windings (field core) (fig.
6-14) is to increase the strength of the magnetic field so
more current will be induced in the armature windings
as the armature rotates. In most generators the field
windings are connected in parallel with the armature
Figure 6-13.--Left-hand rule for generators.
winding (that is, across the brushes); this arrangement
trated in figure 6-13, is applied as follows: Extended the
is called a shunt-field winding. Figure 6-14 represents a
left hand so the THUMB points in the direction of
shunt-wound generator with only one armature coil.
conductor movement (side of armature coil), and the
About 8 to 12 percent of the total current generated by
FOREFINGER points in the direction of magnetic flux
the armature is shunted through the field coils for produc-
(north to south). By pointing the MIDDLE FINGER 90
ing the magnetic field under normal load conditions.
degrees from the forefinger, it will point in the direction
Dc generators (figs. 6-15 and 6-16) consist of four
of current flow within the conductor.
main subassemblies or components: the frame and field
The simple generator circuit shown earlier in figure
assembly, the armature, the commutator end head
6-12 was for explanation purposes only. The output of
assembly, and the drive end head. The frame and field
that generator would be a very low dc voltage. In
assembly consists of the pole shoe, the field coils, and
reality, a generator must have several loops or turns of
the frame, which supports the remaining main
wire in each armature coil for sufficient voltage to be
components of the generator. The field coils supply the
induced into the coil (fig. 6-14). Likewise, the armature
magnetic field, which is necessary to generate
must have several coils distributed evenly around the
armature core. The coils or armature windings are
lines of flux of the field coils and hold the field coils in
place. The armature consists of a laminated iron core
fixed to a shaft and the copper windings, which are
wound in slots in the core. The ends of these windings
are connected to the commutator, which consists of a
number of copper segments that are insulated from
each other and from the core and shaft.
The commutator end head is a cast-iron plate that
closes one end of the frame and supports one of the
armature bearings. The commutator end head also
supports the brush holders and brushes that contact the
armature. The drive end head closes the drive end of the
frame and also supports a bearing for the armature
Dc generators vary in design because of the
different electrical and mechanical characteristics
desired for particular installations. The size, type of
mounting, type of drive, or the voltage and current
output differ, but all dc generators include the four
components just described.
Figure 6-14.--A shunt-wound generator.