Figure 1-17 - Electrical Power Distribution
The aircraft electrical components utilize either or both alternating
current (ac) or direct current (dc) electrical energy. Variable
frequency ac is provided by two engine-driven ac generators for normal
operation and a ram air turbine-driven, fixed-frequency, ac generator
for emergency operation. Constant frequency ac is supplied by a
hydraulic-motor-driven ac generator. The dc power requirements
are supplied by two transformer rectifiers for normal operation and two
batteries for emergency operation. Refer to electrical power
distribution. See Figure 1-17.
Auto-transformers are provided for those components requiring reduced
loads. Some of the components contain their own integrated power
reduction units for circuits that require decreased loads.
AC ELECTRICAL POWER SUPPLY
The electrical power supply is derived from two engine-driven ac
generators, a hydraulic driven ac generator and a ram air
turbine-driven generator. Their description and function follows:
ENGINE DRIVEN GENERATORS
The 20 kva engine generators, supplying 115/200 volt 3-phase variable
frequency (320 to 520 Hz) ac, constitute the main electrical power
source. Each generator is controlled by means of a voltage
regulator, protection panel, relays for automatic bus transfer and an
individual pilot operated switch. Normally the No. 1 generator
energizes the No. 1 primary and secondary ac buses and the No.2
generator energizes the No.2 ac bus and emergency ac bus. If an
over-voltage or under-voltage condition exists for either generator,
that generator is automatically removed from its respective bus and the
warning light panel illuminates to indicate which generator is
inoperative. If both generators fail, there will be no warning
light indications until the ram air turbine is extended. The
automatic bus transfer system provides six possible modes of
operation. See Figure 1-17.
The No.2 ac bus also directs power to the 120 amp transformer rectifier
which changes the ac to 28 volt dc to energize the primary dc
bus. The emergency ac bus directs power to the 20 amp transformer
rectifier that provides 28 volt dc to energize the No. 1 and No.2
battery busses for normal operation.
The electrical supply system is equipped with under-frequency relays which cut the two 20 kva generators
off the busses when the engine rpm drops below approximately 65%. Under this condition, all electrically
operated equipment will be inoperative, EXCEPT the No. 2 boost pump, the battery busses, and the busses
operated by the hydraulic driven generator. The boost pump will continue to operate at lower engine rpm
(down to approximately 40%). This feature assures sufficient boost pump pressure for high altitude air starts.
HYDRAULIC DRIVEN GENERATOR
A 2.5 kva hydraulic driven generator provides 115 volt 3-phase constant
frequency (400 Hz) ac power to the primary and secondary fixed
frequency buses whenever the No. 2 hydraulic system is
functioning. These buses energize those components requiring
fixed frequency ac power. See Figure 1-17.
If both main generators fail, the fixed frequency bus tie relay opens,
de-energizing the secondary fixed-frequency bus. In addition an
APG 502 radar power interlock relay is energized which cuts out all
generator fails, as indicated on the warning light panel, a relay
closes connecting variable frequency power from the emergency ac bus to
both fixed frequency buses. Thus, those components powered by the
fixed frequency bus that can also operate on variable frequency will be
The No. 2 hydraulic system will produce enough pressure to operate the hydraulic motor driven generator
at engine windmill speeds as low as 20% rpm. Thus the fuel flow indicator will be inoperative to provide
the pilot with fuel flow indications during air-starting procedures.
RAM AIR TURBINE DRIVEN GENERATOR
The aircraft is equipped with an extendable ram air turbine which
drives an emergency hydraulic pump and a 4.5 kva generator that
supplies 115/200 volt, 3-phase fixed frequency (400 Hz) ac power for
emergency operation. Once extended, the ram air turbine cannot be
retracted in flight. If the hydraulic driven generator is not
operating and both No. 1 and No. 2 generators fail, the ram air turbine
generator, when extended, will energize the emergency ac bus.
This in turn, energizes the primary fixed-frequency dc buses and both
battery buses through the 20 amp transformer rectifier.
The emergency ac bus will not operate the primary fixed-frequency ac bus if the hydraulic driven generator
EXTERNAL POWER SUPPLY
The aircraft is equipped with a receptacle for connecting an external
ac power source to the electrical system. This receptacle , see
Figure 1-2, is located on the lower right side of the fuselage and is
accessible through a door above the hydraulic panel. When an
external power supply is connected to the aircraft, the No. 1 and No. 2
generators automatically disconnect from their respective buses, and
all ac buses receive power from the ground power unit. A two
position switch installed in the external electrical power receptacle
will permit the LN-3 to be energized from an external power source
without energizing the aircraft main electrical power buses. This
permits energizing the LN-3 for warm-up without energizing other
equipment. In the NORMAL position external power is applied to
the main buses. In the STANDBY position the main buses are
de-energized and heat is applied to the LN-3 only. An engine
start can be accomplished with the limited ground power input.
Electrical power transfer to aircraft internal power will occur when
main generators come on the line.
DC ELECTRICAL POWER SUPPLY
The direct current requirements of the aircraft normally are supplied
from the No.2 ac bus through a 120 amp transformer rectifier.
This changes the 115/200 volt ac to 28 volt dc which is directed to the
primary dc bus. Power is drawn directly from this bus to operate
the various units shown in Figure 1-17.
The No. 1 and No. 2 dc emergency buses furnish power to units which are
considered essential for safe operation of the aircraft. Due to
this requirement, an alternative source of power to these buses is
provided in the event that power from the primary dc bus is
disrupted. Under this condition, the No. 1 and No. 2 emergency dc
buses will be connected automatically to the 20 amp transformer
rectifier unit which is connected to the ac emergency bus. When
the ram-air turbine-driven ac generator is operative (Emergency
Mode), it is important that the load on the emergency ac bus be
minimized when using the aircraft leading and trailing edge flaps since
they are powered directly from the emergency ac bus. To reduce
loads and ensure maximum flap effectiveness, the No. 1 emergency dc bus
is automatically disconnected from the 20 amp transformer rectifier as
long as the flaps are in operation, and those units which are powered
from this bus, including the UHF command radio, will be inoperative
during the period of flap operation.
EMERGENCY DC POWER SUPPLY
If the ram air turbine and both engine driven generators fail, the
batteries will furnish a supply of direct current to the battery
buses. The batteries are the only independent source of direct
current in the aircraft electrical system. Normally, the
batteries and battery buses are paralleled with the 20 ampere
transformer rectifier, and the batteries are thereby maintained in a
fully charged condition. In emergency operation, battery output
is prevented from discharging to the 20 amp transformer rectifier by
blocking rectifiers in order to conserve the limiter power supply for
those units connected directly to the battery buses. There is no
battery control switch in the cockpit, and operation of the battery
system is entirely automatic.
The circuit breaker panels, see Figure 1-8, on the left and right
cockpit consoles, contain push-to-reset, pull-out type breakers for
certain ac and dc circuits. All of the distribution circuits in
the electrical system are protected by various types of circuit
breakers. Circuit breaker panels which are not accessible during
flight, but which should be inspected before flight, are located in the
electronic compartment aft of the cockpit and in the lower electrical
bay on the right side of the fuselage.
Circuit breakers should not be pulled or reset without a thorough understanding of all the effects and results.
Pulling circuit breakers can eliminate from the system some related warning system, interlocking circuit or
cancelling signal, which could result in an undesirable reaction.
A generator switch is provided for each of the 20 kva generator
systems. These switches, see Figure 1-7, are identical and are
located on the right forward panel. The switches are powered from
the No. 2 battery bus. Each switch has three positions: ON-RESET,
OFF, and a centre NEUTRAL position which is the normal position of the
switch when covered by the guard. The switches are spring loaded
to return to the NEUTRAL position. Placing either switch up to
the ON-RESET position will return the generator to normal operation if
it has been removed from the line for any reason other than complete
generator failure. When placed down to the OFF position either
switch will energize the generator control relay which will remove the
generator from its associated bus.