AC CONTROL
GEN. VOLTS
GEN. AMPS
FREQ.
TEST
GEN. ON
ASf07047
Figure 7-47.--NC-8A mobile electric power plant.
RECEPTACLE
B
ON ENERGIZED
OFF
C
A
simultaneously, provided the total output power does
not exceed 48 kilowatts.
OFF
OFF
The unit is capable of attaining a speed of 20 mph
AMM/VOLT
PHASE SET
on smooth level terrain. It can operate at the same
AC SERVICE CABLE
speed forward or reverse. Propulsion is provided
ASf07046
t h r o u g h a 7 . 7 h o r s e p ow e r e l e c t r i c m o t o r. T h e
Figure 7-46.--AC Control panel.
propulsion motor is a reversible, variable-speed,
28-volt dc motor. The motor drives the rear wheels
The over/under relay K7 samples the voltage of the
through an automotive-type differential. The speed is
three phases. As long as the output of each phase
controlled by a foot-operated accelerator pedal. The
remains between 104-122 Vac, K7 remains energized.
direction of travel is controlled by a switch mounted on
Should the voltage exceed either limit, K7
the instrument panel.
de-energizes and contacts 10 and 11 close. This
provides a path for current flow in the fault relay
Electrical Power System
circuit, and K6 energizes. The generator field relay K5,
the governor control relay (not shown), and the output
The generator is a combination ac and dc unit (fig.
service relay K2 are turned off simultaneously. The
7-48). It has a single main shaft connected to the power
output cable is also turned off, the generator output is
plant by a driving disc. The main shaft turns at the same
interrupted, and the engine speed returns to idle.
speed as the power plant. This provides relative motion
The over/under frequency relay K8 is designed to
for the ac and dc sections of the generator. The dc
operate at 400 hertz 10 hertz. If the generator exceeds
section of the generator contains a stationary field and
either limit, K8 de-energizes and contacts 6 and 7
a rotating armature. The output of the armature is
close. This energizes the fault relay K6 as described
delivered through a commutator and brushes. Access
above. An excessive service load can trip the circuit
plates allow easy access to the brushes. The output of
breakers CB1, CB2, or CB3 with the same result.
the dc generator is controlled by a solid-state electronic
voltage regulator. It is protected from overvoltage by
NC-8A MOBILE ELECTRONIC POWER
an overvoltage relay. An overvoltage condition
PLANT
activates the fault circuit, which causes the power plant
to release from governed speed and return to idle.
The NC-8A is a mobile electric power plant that is
T h e a c s e c t i o n o f t h e g e n e r a t o r c o nv e r t s
used to service and start rotary or fixed-wing aircraft
mechanical power to electrical power by induction. It
(fig. 7-47). The prime mover is a four-cylinder,
has a rotating field and stationary output windings.
two-stroke cycle diesel engine controlled by an
Induction occurs as a result of the relative motion of the
electrohydraulic governor. The NC-8A is capable of
rotating field. The electromagnetic field is caused by a
providing 400-hertz, three-phase, 115/200 Vac,
dc current passing through the field windings. The
6 0 - k VA p o w e r a n d 2 8 V d c a t 5 0 0 a m p e r e s
rotation of the field induces the alternating current in
continuously. It can provide 28 Vdc at 750 amperes
intermittently. The ac and dc power may be supplied
the stator coils. The three-phase stator is
7-39
