TM 1-1520-238-T-6
9–78
SECTION II.
THEORY OF OPERATION
9–7.
SYSTEM DESCRIPTION
9–7
a. Electrical System.
The electrical system (fig. 9–71) generates, controls, and distributes power
throughout the aircraft.
(1) The power generation portion of the system consists of two 35 KVA ac generators, two T/Rs, and a
Ni-cad battery. The ac generators provide 3-phase, 115 VAC, 400 Hz electrical power for aircraft operation. The
T/Rs convert ac to dc electrical power.
(2) The output of each generator is controlled by a GCU which monitors the output of its respective ac
generator and corrects for undervoltage, overvoltage, or underfrequency.
(3) The power distribution portion of the system consists of two ac contactors, two dc contactors, two ac
essential buses, three dc essential buses, a dc emergency bus, and an electrical power distribution box. The
contactors provide appropriate routing of the ac and dc voltages to the buses. The electrical power distribution
box distributes the voltages through out the aircraft.
(4) If either ac generator fails, power is automatically generated from the other generator through the ac
contactor to the ac essential bus of the failed generator.
(5) If either of the T/Rs fail, power automatically is applied through a dc tie bus contactor to the dc
essential bus of the failed T/R.
(6) In the event of a complete electrical system failure, the battery provides a source of emergency power
for flight critical instruments and components.
(7) An external power receptacle is provided to allow the electrical power system to operate from an
external power source for extended periods of time without operating the APU or engines. When external power is
applied to the aircraft, the external power contactor routes the ac power to the ac essential buses 1 and 2, and to
T/R 1 and 2. The dc tie bus contactor routes dc power from T/R 1 and 2 to dc essential buses 1, 2, and 3, and the
emergency dc bus.
b. Purpose.
The electrical power system generates and distributes electrical power required to operate the
helicopter systems. The electrical power system supplies ac power to two T/Rs to produce dc for helicopter
systems requiring dc power.
c. System operation.
(1) AC Electrical Power Generation System.
The ac electrical power generation system (fig. 9–72) is
the primary source of electrical power.
(a) Two 35 KVA generators are driven by the accessory gearbox of the main transmission. The output
of the generators are 3-phase, 115/200 VAC, 400 Hz. Each generator supports an identical and redundant system
and supplies one-half of the helicopter load. Two ac contactors control connections between the generators and
the ac buses 1 and 2.
(b) The GCUs monitor and protect the generator output against overvoltage, undervoltage,
underfrequency (on ground only), and overcurrent. If a fault is detected, the GCU shuts the generator down and
the GEN 1 or GEN 2 indicator on the pilot’s caution/warning panel lights. If a generator fails, the GCU switches the
failed load to the other generator. The generators can be reset through the use of the GEN 1 and GEN 2 switches
located on the pilot ELEC PWR panel.
