amplifier is an internal power supply. You can
2. Field rectifier assembly mounted in the
adjust the control current from the SWBD by the
generator enclosure air path
MANUAL MODE VOLT ADJ knob or the
3. Motor-driven rheostat mounted on the
GEN VOLTAGE RAISE-LOWER switch (with
associated SWBD for manual voltage
the VOLT REG MODE switch in the MAN
control
position). With the EPCC in manual control,
the control current may also be varied through
4. Mode select rotary switch mounted on the
the VOLT LOWER-OFF-RAISE switch at the
associated SWBD
EPCC.
The GCU provides generator field excitation
In automatic operation (fig. 3-30, view B), the
at 100 amperes at 150 volts dc at full load. Voltage
voltage regulator output supplies control current
control is in automatic or manual modes.
to the magnetic amplifier fields. An internal
motor-driven rheostat sets the required voltage.
Control for this motor is from the SWBD for local
GENERATOR FIELD EXCITATION.--
operation and from the EPCC for remote
Excitation power for the generator field is
operation.
supplied by the generator output. It is controlled
by a 3-phase magnetic amplifier. Different values
of dc flowing in a control winding provide
VOLTAGE REGULATOR.--The voltage
different levels of saturation in the magnetic
regulator in auto operation compares generator
amplifier. This controls the output of the magnetic
voltage with a reference voltage to provide an
amplifier to the generator field.
error signal (fig. 3-30, view B). This error signal
is amplified and applied to the magnetic amplifier
Another source of field excitation comes from
control winding. This changes the output of the
three CTs. This is rectified by a 3-phase, full-wave
magnetic amplifier. This, in turn, provides field
bridge in the field rectifier assembly. Since the
current to set the output voltage of the generator.
source of field excitation for the magnetic
The reference voltage is adjustable through the
amplifier comes from the generator output, a
motor-driven rheostat in the static exciter/voltage
short circuit on the system will cause the voltage
regulator assembly.
to collapse. This results in a loss of excitation
voltage. The excitation source from the CTs can
A line current signal is brought in from
supply enough excitation to the generator field
the three paralleling CTs to the field forcing
under short circuit conditions to keep the
rectifier. This provides two functions in automatic
generator output at a minimum 320 percent of
mode.
rated current. In this way the overcurrent devices
can sense the short circuit. They can trip the
1. When an individual generator is on line,
generator breaker to clear the fault.
this current signal acts to compensate for load
changes. When load increases, this signal will call
On initial start-up of the generator, the
for an increase in excitation. This relieves the
magnetic amplifier has little or no excitation
voltage regulator of having to make the entire
voltage. To assure that the generator voltage will
correction with its error signal. This load
build up, another source of excitation must be
compensation increases the accuracy of voltage
used. Excitation is supplied by the PMA on the
regulation.
generator shaft extension. It is rectified through
2. When two generators are operating in
a 3-phase, full-wave bridge. The output voltage
parallel, their voltages are equal. Therefore, any
of this excitation source is less than the normal
adjustments in the excitation of individual
output of the magnetic amplifier at 450-volt
machines can only change the power factor of
generator output. It is automatically removed by
both machines. This creates circulating reactive
a blocking diode once the magnetic amplifier
currents between machines. In this case, the
output takes over. This function is called field
current signal brought in from the paralleling CT
flashing.
will help regulate the division of reactive line
current. This reduces circulating current between
Under manual operation (fig. 3-30, view A),
machines.
the source of control current for the magnetic
3-42
