SMALL CRAFT ELECTRICAL
(S2) in the respective position (2, 8, 15, or 30
amperes).
SYSTEMS
Energize the battery charger by placing the selector
Small craft perform an important function in the
switch (S1) in the ON position. This will cause the
daily routines of all naval vessels. When their parent
SCRs to conduct during a portion of the input cycle of
ships are at sea, they serve as duty lifeboats and also as
the step-down transformer (T1). The amount of
troop carriers or assault boats. In port, they are used for
conduction of the SCRs is controlled by the feedback
transporting stores and liberty parties and for
signals fed from the magnetic amplifier (L1). This will
establish a fixed voltage reference across the Zener
conducting other ship's business. Most small craft are
diode (CR13) through the control coil (L1), the linear
driven by a diesel engine.
resistor (R4), and the temperature compensating resistor
The electrical system covered here is representative
(R5). The R5 resistor serves to change the preset output
voltage during temperature changes by changing the
of those found on a large number of ship's boats and
current through the L1 control coil. The negative
small craft. The electrical system consists of the engine
feedback is fed to the L1 coil through the resistors (R10
starting system and the battery charging system.
through R15) and the selector switch (S3B). The
current transformer (T2) output is determined by the
resistors (R6 through R9) through the selector switch
ENGINE STARTING SYSTEM
(S2), which will determine the voltage across the
capacitor (C5) and the current through transformer T2.
The engine starting system on small boats is
When the output current exceeds the selected breakover
equipped with storage batteries (previously discussed),
voltage of the reference Zener diode (CR13), the current
a starting motor, and control circuitry.
flowing through the control coil of L1 from the black to
white leads is in such a direction as to oppose the
reference voltage. `Ibis will lower the output voltage
Starting Motor
until the excess current of the transformer (T2) is
accepted by the battery on charge and starts to
recharge.
The starting, or cranking, motor is slow-voltage, dc
series motor used to start internal combustion engines
The shorted winding of the reactor (L1) connected
by rotating the crankshafts. It is flange-mounted on the
to leads white/orange and white/yellow allows for the
engine flywheel housing and is supplied with current
respoonse time of the output of the magnetic amplifier to
from the battery. All starting motors are similar in
changes in the control signals. This increases stability
design and consist essentially of a frame, armature,
against transient signals generated by the ac supply and
brushes, field windings, and drive mechanism. The
the firing of the SCRs.
armature shaft is supported on bronze bearings equipped
The choke filter (L2) reduces the ripple of the dc
with wick oilers. The number of field poles and brushes
output caused when the SCRs fire.
varies according to the cranking requirements and the
operating voltage.
The battery chargers in use today must meet
specification MIL-C-24095B. These battery chargers
The starting motor has low resistance; it is designed
can charge 1 to 18 cells and have a maximum current
to operate under heavy load with relatively high
limit of 45 amperes.
horsepower for short periods of time. The high
SUMMARY
horsepower is accompanied by a high current that
creates considerable heat and, if operated for any
The discussion about the model 24-302-BN-1
considerable length of time, will result in failure of the
battery charger introduced you to the various
motor due to overheating. Hence the starting motor
components that make up the battery charger. Also
must be operated for not more than 30 seconds at a time
covered was the functions of the charger. Maintenance
and at about 2-minute intervals to allow the heat to
on this equipment should be accomplished according to
dissipate. The starting current on most small boats is
the prescribed instructions from the manufacturer and
over 600 amperes.
installed PMS procedures.
5-9
