King Air C90GTi, Pilot Training Manual

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KING AIR C90GTi/C90GTx PILOT TRAINING MANUAL
2-9
FOR TRAINING PURPOSES ONLY
When the batter y switch is tur ned ON, the
batter y relay and the batter y bus tie relays
close (Figure 2-11). Battery power is routed
through the batter y relay to the triple-fed
bus, and through the batter y bus tie relay to
t h e c e n t e r b u s a n d t o b o t h s t a r t e r r e l ay s .
Neither generator bus is powered since the
generator bus ties are nor mally open, how-
ever, batter y power is available to per mit
star ting either engine.
After either engine has been star ted and the
generator switch has been moved to RESET,
the generator control unit (GCU) will bring
the generator up to voltage. Releasing the
spring-loaded switch to the center ON po-
sition closes the generator line contactor,
t h e r e by p owe r i n g t h e g e n e r a t o r b u s , a n d
closing both generator ties automatically.
This action distributes power through the
250-amp cur rent limiters and the generator
bus tie relays. Generator output will then be
routed through the center bus to per mit bat-
tery charging. In addition, the opposite gen-
erator bus and triple-fed bus will be powered
by the generator, supplying 28-VDC power
to the f ive primar y air plane buses (Figure
2-14) When both generators are operating,
each generator directly feeds its respective
generator bus.
The generator buses, hot batter y bus, and
batter y are tied together by the center bus.
The triple-fed bus is powered by the batter y
and each generator bus through 60-amp lim-
iters and through diodes providing fault iso-
lation protection between the power sources.
BUS TIE SYSTEM
The electrical system is protected from ex-
cessively high cur rent flow by the bus tie
system. Three cur rent sensors, consisting of
Hall effect devices and solid-state circuitry,
are used to sense cur rent flow through the
portion of the circuit being monitored. Two
bus tie sensors and their relays are located
between the generator buses and the center
bus, and a third is between the battery and
the center bus.
With no power applied to the aircraft elec-
t r i c a l s y s t e m , a l l t h r e e b u s t i e r e l ay s a r e
open. When the BAT switch is tur ned ON,
hot batter y bus voltage energizes the coil
circuit of the batter y bus tie relay, thereby
closing it. This action has no effect on the
generator bus ties.
A similar action occurs when a generator or
exter nal power is brought on-line. When ei-
ther generator is brought on-line, voltage
from the generator control panel energizes
the coil circuit of both generator bus tie re-
lays. This switches voltage from the L and
R GEN TIE OPEN annunciators to the re-
lays, causing the annunciators to extinguish
and the bus tie relays to close. When exter-
nal power is brought on-line, the only dif-
ference is the source of generator b us tie
coil voltage, which is the small pin of the
exter nal power receptacle. Neither genera-
tor or exter nal power affect the batter y bus
tie circuitry unless the battery switch is also
tur ned ON.
2
ELECTRIC
AL
POWER
SY
STEMS