Battery Contactor Closing Circuit; Alternator - Cessna 150 STANDARD Service Manual

diode and the master switch wire connect to the minus
terminal of the contactor coil. A nylon cover is in-
stalled on the terminals to prevent accidental short
circuits. Refer to figure 16-1.
16-24. REMOVAL AND INSTALLATION (Refer to
figure 16-1).
a. Open battery box and disconnect ground cable
from negative battery terminal. Pull cable clear of
battery box.
b. Remove the nut, lockwasher and the two plain
washers securing the battery cables to the battery·
contactor.
c. Remove the nut, lockwasher and the two plain
washers securing the wire which is routed to the
master switch.
d. Remove the bolt, washer and nut securing each
side of the battery contactor to the battery box. The
contactor will then be free for removal.
e. To replace the contactor, reverse this proce-
dure.
16-25. BATTERY CONTACTOR CLOSING CIRCUIT.
This circuit consists of a 5 amp fuse, a resistor and
a diode installed across the battery contactor. This
serves to shunt a small charge around the battery
contactor when the battery is too dead to energize the
contactor by itself.
16-26. GROUND SERVICE RECEPTACLE.
16-27. DESCRIPTION. A ground service receptacle
is offered as optional equipment to permit the use of
external power for cold weather starting or when per-
forming lengthy electrical maintenance.
NOTE
Before connecting an external power source,
it is important that the master switch be
turned "ON." This will close the battery
contactor and enable the battery to absorb
transient voltages which otherwise might
damage the electronic equipment.
It
will
also provide excitation of the alternator
field in the event that the battery is com-
pletely dead.
16-28. REMOVAL AND INSTALLATION (Refer to
figure 16-2).
a. Open battery box and disconnect the ground
cable from the negative terminal of the battery and
pull the cable from the battery box.
b. Remove the nuts, washers and ground strap
from the studs of the receptacle and remove the
battery cable.
c. Remove the screws and nuts holding the recep-
tacle. The receptacle will then be free from the
bracket.
d. To install a ground service receptacle, reverse
this procedure. Be sure to place the ground strap on
the negative stud of the receptacle.
16-29. ALTERNATOR POWER SYSTEM.
16-6
16-30. DESCRIPTION. The alternator system con-
sists of an engine driven alternator, a voltage regu-
lator mounted on the left hand side of the firewall and
a circuit breaker located on the instrument panel.
The system is controlled by the left hand portion of
the split rocker, master switch labeled "ALT." Be-
ginning with 1972 models an over-voltage sensor
switch and red warning light labeled "HIGH VOLTAGE
are incorporated to protect the system, (refer to para-
graph 16-36). The aircraft battery supplies the
source of power for excitation of the alternator.
16-31. ALTERNATOR.
16-32. DESCRIPTION. The alternator is three phase,
delta connected with integral silicon diode rectifiers.
The alternator is rated at 14 volts at 60 amperes con-
tinuous output. The moving center part of the alter-
nator (rotor) consists of an axial Winding with radial
interlocking poles which surround the winding. With
excitation applied to the Winding through slip rings
the pole pieces assume magnetic polarity. The rotor
is mounted in bearings and rotates inside the stator
which contains the windings in which ac is generated.
The stator windings are three-phase, delta connected
and are attached to two diode plates, each of which
contains three silicon diodes. The diode plates are
connected to accomplish full-wave, rectification of ac.
The resulting dc output is applied to the aircraft bus
and sensed by the voltage regulator. The regulator
controls the excitation applied to the alternator field
thus controlling the output of the alternator.
16-33. REMOVAL AND INSTALLATION.
a. Ensure that master SWitch is off and the negative
lead is disconnected from the battery.
b. Remove wiring from the alternator and label.
c. Remove screw and nut holding blast tube to sup-
port strap clamp, blast tube will then be free for re-
moval.
d. Remove nuts and washers from alternator mount-
ing bolts.
e. Remove alternator.
f.
To replace alternator, reverse this procedure.
16-34. ALTERNATOR FIELD CIRCUIT PROTEC-
TION. The 2-amp automatic resetting circuit breaker
located in the removable electrical switch and fuse
panel below the shock panel is provided to protect the
alternator field circuit.
16-35. ALTERNATOR REVERSE VOLTAGE DAM-
AGE. The alternator is very susceptible to reverse
polarity current because of the silicon diodes. The
diodes, having a very high resistance to reverse cur-
rent flow, are used without any cutout relay such as
used on a generator system. The alternator diodes
are arranged with their cathodes connected to the
aircraft bus bar which is positive and no back current
will flow.
If
the polarity of the battery is reversed,
the diodes will offer no resistance to the current flow.
The current rating of the diodes is exceeded and diode
failure may result.