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TM 9-8000
armature winding, the magnetic field produced by the
the field winding will depend upon the number of
field windings is approximately straight through the
armature coils spanned by the brushes that collect the
armature from one pole piece to the other. The voltage
field current.  Thus, moving the third brush in the
generated by each armature coil is then practically
direction of the armature rotation increases the average
uniform during the time the coil is under the pole pieces.
current delivered to the shunt-field winding  and,
consequently, the output of the generator. Moving the
(2) As the generator speed and current increase, the
brush  against  the  direction  of  armature  rotation
armature winding acts like a solenoid coil to produce a
decreases the output. When this brush is moved, care
cross-magnetic field.  The magnetic whirl around the
should be taken to see that it makes perfect contact with
armature winding distorts the magnetic field produced by
the commutator.
the shunt-field windings so that the magnetism is not
distributed equally under the pole pieces.  With this
(3) Because the third-brush generator depends upon
distortion of the magnetic field, the armature coils no
the current flowing through the armature winding to
longer generate a uniform voltage while passing under
produce the field distortion necessary for regulation, it is
the different parts of the pole.  Although the voltage
obvious that it is current-regulated internally (as distinct
across the main brushes remains nearly the same, a
from external current regulation).  Therefore, it must
greater proportion of this voltage is generated by the
have a complete circuit available through the battery at
coils between the positive brush and the third brush than
all times. Otherwise, regulation would be destroyed and
was generated between them when little current was
excessive field currents would burn out the generator
flowing through the armature winding. This is due to the
windings. The generator terminals must be grounded in
distortion of the magnetic field, which crowds more
case the third- brush generator is disconnected from the
magnetic lines of force between the positive and the third
(3) The coils that connect the commutator between the
13-16. Control of Third-Brush Generator. A fuse is
negative and the third brush are in the region of the
sometimes provided in the field circuit to guard against
weakened field and generate a lower proportion of the
the possibility of the third-brush generator burning up.
voltage.  The result is a dropping off of the voltage
When used, it is placed either in the generator end plate
between the negative and third brushes, which is applied
or in the regulator control unit. If the battery becomes
to the shunt-field winding, thereby weakening the field
disconnected, there is a rise in voltage at the generator.
strength. As the field strength decreases with increased
This, in turn, sends an abnormally heavy current through
generator current, the result will be an automatic
the field winding and this field current burns out the fuse.
regulation of the current output.
As soon as the fuse is blown, the field circuit is open and
no current can flow through it.  The generator then
c. Output.
merely turns, producing practically no voltage, and does
no harm.  The third-brush generator provides current
(1)  One  of  the  outstanding  characteristics  of
regulation only and does not take battery voltage into
generators with third-brush regulation is that the output of
consideration. In fact, a fully charged battery that has a
the generator increases gradually up to an intermediate
high voltage actually will get more current from a third-
speed.  After this, due to obvious field distortion, the
brush generator than a battery that is completely
output falls off as the speed continues to increase. At
discharged, because the high voltage holds up the
high generator speeds, the output is approximately one-
voltage at the generator, makes the field stronger, and
half its maximum value.
causes the generator output to increase. This, combined
with the varying demands of radio sets and other current-
(2) In practically all generators that have third-brush
consuming  devices,  necessitates  more  accurate
regulation, provision is made for changing the output to
regulation than a third-brush generator alone can give.
suit the conditions under which the generator is
operated. This can be done by moving the position of
a. Switch Control. Practically all systems of regulation
the third brush on the commutator. The average voltage
provide a means for inserting a resistance in series with
applied to
the third-brush field.


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