limit current output to a safe value, or to provide voltage
The rheostat is connected to the shunt-winding circuit of
regulation, the armature pressure can be adjusted to
the voltage regulator. Its purpose is to permit adjustment
vary the resistance.
of the voltage regulator setting. When all resistance in
the rheostat is cut out (by turning the knob), the full
c. Current Regulation. To limit current, or to provide
generator voltage is imposed in the shunt winding. But
when some of the rheostat resistance is cut in, less than
current regulation, the carbon-pile regulator has a heavy
full generator voltage is imposed on the shunt winding
winding through which all current from the generator
(part of it being in the rheostat). In the latter case,
must pass. This winding produces a magnetic pull as
generator voltage must go higher before voltage
current asses through it, which opposes the armature
regulation commences. Thus, accurate setting of the
spring pressure. When the output reaches the value for
regulator can be made.
which the generator is rated, the magnetic pull
overcomes the spring pressure sufficiently to reduce the
pressure on the carbon disks and thereby increase the
resistance of the pile. This increased resistance, which
is in the generator field circuit, prevents any further
Third-brush regulation is much simpler in operation and
increase of output.
less expensive to manufacture than other methods of
control. However, it can be used only for relatively small
d. Voltage Regulation. A winding is incorporated in
and specialized applications. Generators with this type
the carbon-pile voltage regulator to regulate voltage.
of control have an extra brush called the third brush,
This shunt winding is connected across the generator so
located between the two main brushes.
that generator voltage is forced on it. When this voltage
reaches the value for which the regulator is set, the
a. Arrangement. Arrangement of a typical two-pole,
winding produces enough magnetic pull on the armature
third-brush generator is shown in figure 3-13. One end
to reduce the armature spring pressure. This causes the
of the shunt-field winding is connected to the third brush,
resistance of the voltage-regulator carbon pile to
the other end is grounded. Only a part of the total
increase. The increased resistance, which is in the
voltage generated is supplied to the field by the third
generator field circuit, prevents any further generator
voltage increase and thereby reduces generator output.
(1) When the generator is running at a low speed and
little or no current is flowing in the
Figure 13-13. Third-Brush Regulation.