exceeding its rated maximum current output. When the
generator field coils and helps to dissipate the surge of
generator current output reaches the value for which the
induced voltage that occurs in the field coils when
current regulator is set, current flow through the series
either the voltage or the current regulator operates. This
winding of the current regulator creates a magnetic
induced voltage surge is due to the sudden drop of field
field strong enough to overcome the tension of the
current and the resultant decrease in strength of the
armature spring.
surge reduces arcing at the voltage and current
When the armature is pulled down, the current
regulator contact points.
regulator contacts open. This inserts R1 in series with
the generator field coil circuit, thereby decreasing
Operation
generator field voltage and current, and reducing the
generator output current. This reduction is accom-
The charging system shown in figure 6-33 is in a
panied by a weakening of the magnetic field of the
static condition. Notice that the reverse current relay
series winding of the current regulator, which allows
contacts are open. The current regulator and voltage
the contact points to close. When the contact points
regulator contacts are closed. With the contacts of the
close, R1 is removed from the field coil circuit. Field
two regulators closed, the generator field circuit has
voltage and current will increase, and generator output
minimum resistance. As the generator is driven and
current will increase. This cycle is repeated as many as
speed increases, the output voltage builds up because of
50 times per second to limit the generator output
the unrestricted increase of field voltage and current.
current to a set value.
When the generator output voltage builds up higher
When the generator speed decreases to a point
than the battery voltage, the current through the shunt
where generator output voltage becomes less than that
winding of the reverse current relay creates a magnetic
of the battery, current from the battery flows back to the
field strong enough to overcome the tension of the
generator, through the series winding of the reverse
armature spring and close the contacts. With the
current relay. This opposes the magnetic field created
contacts closed, the generator is connected to the
by the shunt winding of the reverse current relay. This
battery. The generator output flows through the battery,
allows the armature to return to its original position,
the series winding of the reverse current relay, and back
opening the contact points and breaking the circuit
to the generator armature, adding strength to the
between the generator and the battery. The battery is the
magnetic field created by the shunt winding.
only source of power.
Once the generator output voltage reaches the value
The regulator assembly provides control of the
for which the voltage regulator is adjusted, the
generator under all conditions. The current regulator or
magnetic field created by the winding of the voltage
the voltage regulator may be operating at any one time,
regulator overcomes the tension of the armature spring
but in no case do they both operate at the same time.
and pulls the armature down, opening the contacts. This
inserts R1 in series with the generator field coil. The
TWO-UNIT VIBRATING-CONTACT AC
increased resistance in the field circuit lowers the field
REGULATOR
voltage and current. This results in a weaker magnetic
field and a decrease in the generator output.
While the regulator for an ac generator (alternator)
performs the same function as does the regulator for a
A surge of voltage is induced in the field winding
dc generator, there are differences in its makeup just as
by the collapsing magnetic field. This voltage is
there are differences between the alternator and the dc
dissipated across R2. The reduction in generator output
generator.
voltage is accompanied by a weakening of the magnetic
field of the voltage regulator winding, which allows the
A reverse current relay is not required in an ac
contacts to close. When the contacts close, R1 is
charging system. The one-way action of the diodes in
removed from the field circuit. Field voltage and
the alternator prevents reverse current flow. The bat-
current increase, and generator output voltage
tery cannot discharge through the alternator. Also, a
increases. This cycle may be repeated as many as 200
current regulator is not required. The design
times per second (hence the name vibrating contact) to
characteristics of the alternator maintain current within
maintain a constant voltage in the charging system.
the limits of the alternator throughout its operating
range. An alternator is sometimes referred to as a
When electrical load requirements are high, the
constant current machine.
current regulator operates to keep the generator from
6-28