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 magnetic circuit.

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 external circuits.

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. 2-12