DC Generators:

        In a dc generator, the ac output from a rotating armature is
converted to pulsating dc by using a commutator in place of 
slip rings. The rotating coil and the magnetic field are the same for
both AC and DC generators. The difference is in the method used for
removing the voltage induced in the armature. In an AC generator the 
armature coil is attached to slip rings which make contact with brushes.
In a DC generator the armature coil is attached to a commutator which
also makes contact with brushes.

         A commutator is basically a slip ring which is split into two
or more parts. These parts are called commutator Segments. The
segments are insulated from each other and from the shaft. Each end of
the armature coil is attached to one of the segments. The brushes make
contact to opposite sides of the commutator segments. As the coil 
rotates the brushes bridge the segments at one point in the cycle, the
point at which the coil is parallel with the magnetic field, at this
point there is no voltage induced in the coil loop. This area is called
the commutating or Neutral plane. The bushes are always set so
that they touch two commutator segments while passing through the 
neutral plane. If the brushes were to bridge the commutator segments
while an emf was being induced in the coil, a heavy current would flow
in the coil. Since the coil and brushes have a very low resistance,
the armature coil would be damaged unless commutation takes place in
the neutral plane.

        Pulsating DC is obtained from a DC generator because the 
connections from the armature coil are reversed each half cycle. With
the armature coil in the neutral plane, coil rotation produces no 
armature output, as the armature rotates past the neutral plane, the
coil conductors begin to cut across the flux lines, and current begins
to flow in the armature circuit. The current increases steadily until
the armature coil is moving directly across the face of the magnets. 
At this time the coil conductors are cutting the flux lines at the
maximum rate. The armature output begins to decrease until it reaches
a minimum, which is at the neutral plane (parallel with the magnets).
At this time the commutator reverses the electrical connection, keeping
the current flowing in the same direction or polarity, producing a
pulsating dc or two half cycles instead of the full cycle forward and
reverse polarity of the ac generator.
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