Lesson 1/Learning Event 1
When the generator is assembled, the armature is placed inside the housing between the pole shoes.
A drive-end head cover is mounted on one end of the housing. The drive-end head supports one
end of the armature. A commutator-end head cover goes on the other end of the housing and
supports the other end of the armature. It also serves as a mount for the brushes. One brush is
grounded to the commutator-end head. The other brush is connected by a wire to the armature
terminal on the generator housing. On waterproof generators, both of these terminals are enclosed
in a waterproof outlet.
Operation
Several things are needed for this type generator to operate properly. One thing we must have is a
magnetic field. In a previous lesson, you learned that soft iron could be magnetized, but when the
magnetizing force is removed, soft iron quickly loses most of its magnetism. Notice we said
MOST. When the magnetizing force is removed, soft iron will retain a slight amount of
magnetism. This is called residual magnetism. Let's say for now that the pole shoes do contain
residual magnetism.
Now let's see what takes place when this generator is put into operation. When the armature is
turned, the armature coils will cut the weak magnetic field produced by the residual magnetism
retained by the pole shoes. This sets up a small voltage (usually 1 to 1 1/2 volts) across the
brushes, which makes, in this particular case, the upper brush positive (+) and the lower brush
negative (-). This voltage is enough to cause a small amount of current to flow from the negative
brush through the field windings around the pole shoes. It then flows out the field terminal,
through the external (outside) circuit, and back through the armature terminal and positive brush to
the armature. When part of the current picked up by the brushes is sent through the field windings,
the generator is said to be shunt (parallel) wound. All military wheeled vehicle DC generators are
shunt wound.
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