GMC 1976 ZEO 6083 Maintenance Manual page 398

6Y- 1 8 ENGINE ELECTRICAL
attached to the slip ring end frame. The regulator
voltage setting is not adjustable .
The generator rotor bearings contain a supply of
lubricant sufficiently adequate to eliminate the need
for periodic lubrication . Two brushes carry current
through two slip rings to the field coil .
The stator
windings are assembled on the inside of a laminated
core that forms part of the generator frame. A rec-
tifier bridge connected to the stator windings con-
tains six diodes, and electrically changes the stator
A .C . voltages to a D.C . voltage which appears at the
generator output terminal . Generator field current is
supplied through a diode trio connected to the stator
windings . A capacitor, or condenser, mounted in the
end frame protects the rectifier bridge and diode trio
from high voltages, and suppresses radio noise.
OPERATING PRINCIPLES
A typical schematic wiring diagram of the 80-
amp integral type charging system is shown in Fig-
ure 23 . The basic operating principles are explained
as follows:
Figure 23-Schematic Diagram of Generating System
(Typical)
When ignition switch is closed, current from the
battery flows through the indicator lamp to genera-
tor No . 1 terminal, through resistor R1, diode D1,
and the base-emitter of transistor TR1 to ground,
then back to the battery. This turns on transistor
TR1, and current flows through the generator field
coil and TR1 back to the battery . The indicator lamp
then turns on . Resistor R5 carries some of the indica-
tor lamp current and is identified in Figure 23 .
With generator operating, A.C . voltages are
generated in the stator windings, and the stator sup-
plies D.C . field current through the diode trio, the
field, TR1, and then through the grounded diodes in
the rectifier bridge back to the stator . The six diodes
in the rectifier bridge change the stator A.C . voltages
to a D .C . voltage which appears between ground and
generator "BAT" terminal . As generator speed in-
creases, current is provided for charging the battery
and operating electrical accessories . Also, with the
generator operating, the same voltage appears at the
"BATT" and No . 1 terminals, and the indicator
lamp goes out to indicate the generator is producing
voltage .
The No . 2 terminal on the generator is always
connected to the battery, but the discharge current
is limited to a negligible value by the high resistances
of R2 and R3 . As generator speed and voltage in-
crease, the voltage between resistors R2 and R3 in-
creases to the point where zener diode D2 conducts .
Transistor TR2 then turns on and TR 1 turns off.
With TR1 off, field current and system voltage de-
crease, and D2 blocks current flow, causing TR1 to
turn back on . The field current and system voltage
increase . This cycle repeats many tirr es per second
to limit generator voltage to a pre-set value.
Capacitor C1 smooths out the voltage across R3,
resistor R4 prevents excessive current through TR 1
at high temperatures, and diode D3 prevents high
induced voltages in the ,field windings when TR1
turns off. Resistor R2 is a thermister which causes
the regulated voltage to vary with temperature, thus
providing the optimum voltage for charging the bat-
tery .
GENERATING SYSTEM TROUBLE
SYMPTOMS
Abnormal operation of the generating system is
usually indicated by one or more of the following
symptoms :
1 . Battery undercharged (low specific gravity of
electrolyte) .