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generator isolation, bus tie, generator automatic reset, and ac bus isolation circuitry.
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I-2-26 ORIGINAL
Figure 2-9. Simplified Electrical Schematic
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If a generator fault occurs, generator isolation circuitry removes the affected generator from its buses
(L or R GEN caution and caution light). All generator faults except N2 underspeed require manual
generator reset (GEN switch cycled to OFF then NORM or RESET button pressed). Generator reset
is successful only if the out-of-tolerance or fault condition has cleared. If the generator fault remains,
bus tie circuitry allows the remaining generator to power all electrical buses. During an N2 underspeed
condition, the affected generator is automatically restored when rpm returns to normal range.
If a short or overload condition (bus or equipment fault) occurs on a bus (the R 115 vac bus, for
instance), the following sequence is initiated. The right generator attempts to power through the short
and, if unsuccessful, trips offline. The left generator is then connected to the right buses by the bus tie
circuitry, attempts to power through the short, and, if unsuccessful, also trips offline. Approximately
1 second after the dual generator outage, the generator automatic reset logic resets both generators. If
the bus or equipment fault has cleared, both generators remain online to power their respective buses.
If the bus or equipment fault remains, the right generator trips offline again, but bus isolation circuitry
now prevents the left generator from picking up the right buses (GEN TIE caution light). The left
generator remains online to power the left buses and the R 28 vdc bus. The R 115 vac bus, R 26 vac bus,
and battery charger are unpowered, and the battery runs the maintenance bus. This entire process may
take as long as 16 seconds.
For the ac bus isolation and generator automatic reset circuits to operate, the GEN TIE switch must
be in NORM, the BATT switch must be ON and the PARK BRAKE handle must not be set. With the
PARK BRAKE handle set, the generators do not reset following a dual outage.
2.4.2.3 GEN TIE Caution Light. During initial engine start (battery or external power) GEN TIE
circuitry requires a set PARK BRAKE handle to properly function. If the PARK BRAKE handle is not
set during right (first) engine start, a GEN TIE caution light comes on when the right generator comes
online. For a battery start, setting the PARK BRAKE handle and cycling the R GEN switch reties the
left and right buses and clears any avionics faults that would otherwise occur. For an external power
start, setting the PARK BRAKE handle, disconnecting external power, and cycling the GEN TIE
switch reties the left and right buses.
2.4.2.4 GEN TIE Switch. The red-guarded GEN TIE switch is located on the left console outboard of
the exterior lights panel.
RESET Resets the bus tie circuitry. Reset is accomplished by cycling the switch to RESET
then NORM.
NORM With the BATT switch ON, enables the bus tie, ac bus isolation and generator
automatic reset circuits.
If the left and right buses are isolated because of a detected fault (e.g., R
GEN caution and GEN TIE caution light), cycling the GEN TIE switch
reenergizes the faulty bus/equipment and may cause further damage or
loss of the remaining generator.
2.4.3 DC Electrical Power. DC electrical power is provided by two TRs, three dc outputs from each
PMG, the battery, and the battery charger.
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2.4.3.1 TR - Transformer-rectifiers. While each TR is powered by its respective 115 vac bus, the
output of each TR is connected in parallel, powering both the left and right 28 vdc buses and providing
primary power for the essential bus. If one TR fails, the other powers the entire dc system. There is no
cockpit warning of a single TR failure (no caution and no MSP code). TR operation is checked on
maintenance phase inspections.
2.4.3.2 PMG - Permanent Magnet Generator. The PMG in each generator provides three dc sources,
two for FCC channels and one for essential bus backup. The left PMG provides the primary power
source for FCC A (channels 1 and 2), while the right PMG provides the primary power source for FCC
B (channels 3 and 4). Regardless of generator control switch position, the PMGs come online when the
engine reaches approximately 50% N2 rpm on spool up and remain on until 20% N2 rpm on spool
down.
2.4.3.3 Battery. The primary operational use of the battery is engine start. The battery powers the
maintenance bus directly, allowing operation of the canopy, ladder, and maintenance monitor in the
absence of ac electrical power. With the BATT switch ON (first engine start), the battery also powers
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NATOPS Flight Manual 飞行手册 1(43)
