GENERATOR FIELD EXCITATION.--
are provided to automatically switch regulators
Main generator field excitation is supplied by a
if a regulator fails. This prevents an overvoltage
brushless exciter assembly. The brushless exciter
condition. An indicating light on the face of the
assembly has three main parts: stator, rotor, and
GCU will illuminate when a regulator fails. A
rectifier assembly. The rotor and rectifier
RESET push button is provided on the face of
assembly are attached to the generator shaft. They
the GCU. When depressed, it will return control
turn inside the stator that is attached to the
to the (normal) regulator. It also extinguishes the
generator frame. The operation of the exciter is
regulator failed light. Manual or automatic
similar to that of any ac generator. The exception
control may be selected at the SWBD by the
is that the rotor and stator functions have been
VOLT REG MODE-OFF/AUTO/MANUAL
reversed. When dc is passed through the exciter
control. The regulators receive their power from
field winding, lines of magnetic flux are created
t h e generator output through potential
that pass through the air gap. This creates a
transformers. Thus, on initial start-up of the
3-phase ac output from the rotor. This 3-phase
generator in automatic mode, the voltage
ac is rectified to dc by the rectifier assembly. It
regulator will have little or no excitation voltage.
is then conducted through the generator field. An
To assure that the generator voltage will build up,
advantage of using this brushless exciter over the
excitation is obtained from the PMA. A relay
brush slip ring type of generator is the greatly
internal to the regulator will divert power from
reduced maintenance.
the PMA to the generator field until voltage has
risen about 340 volts (75 percent of rated). Then,
VOLTAGE REGULATOR.--Two solid-state
the relay will switch excitation control over to the
voltage regulators (normal and standby) control
regulator.
the exciter field in normal automatic operation
The source of regulator power is the generator
(fig. 3-32). Indicator lights on the face of the GCU
output. Therefore, a short circuit on the system
depict which regulator is in use. Overvoltage relays
will cause the voltage to collapse. This results in
Figure 3-32.--Model 139 voltage regulator.
3-45
