Figure 2-12.Sectional view of a dc generator.
electromagnets or poles magnetized by current
flowing through the field coils. Soft iron pole pieces
(or pole shoes) are contained in the field frame that
forms the magnetic circuit between the poles.
Although generators may be designed to have any even
number of poles, two- and four- pole frames are the
most common. The field coils are connected in series.
In the two-pole type frame, the magnetic circuit flows
through only a part of the armature core; therefore. the
armature must be constructed according to the number
of field poles because current is generated when the
coil (winding on the armature) moves across each
The current is collected from the armature coils by
brushes (usually made of carbon) that make rubbing
contact with a commutator. The commutator consists
of a series of insulated copper segments mounted on
one end of the armature, each segment connecting to
one or more armature coils. The armature coils are
connected to the external circuits (battery, lights, or
ignition) through the commutator and brushes. Current
induced in the armature coils thus is able to flow to the
There are two types of field circuits, determined by
the point at which the field circuit is grounded, which
are as follows:
One circuit, referred to as the "A" circuit, shunts
the field current from the insulated brushes
through the field winding grounding externally
at the regulator.
In the other, the "B" circuit, the field current is
shunted from the armature series winding in the
regulator to the generator field windings,
grounding internally within the generator.
The three basic design factors that determine
generator output are (1) the speed of armature rotation,
(2) the number of armature conductors, and (3) the
strength of the magnetic field. Any of these design
factors could be used to control the generator voltage
and current. However, the simplest method is to
determine the strength of the magnetic field and thus
limit the voltage and current output of the generator.