Service Manual
Generator Operation Automatic Voltage
Regulator (cont'd)
Refer to Figure 134 for the following description of the
AVR.
To begin, assume that the end of the exciter coil
labeled
"A”
is positive. The following sequence of events
will occur:
1.
2.
3.
4.
5.
6.
7.
8.
Diode D3 will be forward biased. The arrowhead
end (anode) is positive (connected to the positive
end of the exciter coil) and the diode will conduct.
Diode Dl will not conduct as
it
is reverse biased.
The flat
end
(cathode) is connected to the positive
end of the exciter coil. SCR will not conduct until
it
has the proper voltage at its gate.
Current will flow through D3 and
R1
to the gate of
SCR1.
With current at
its
gate SCRl will turn on and
conduct because
it
is also forward biased.
Current will not flow through D4, R2 or SCR2
because they are reverse biased; (their anode
ends are connected to the negative end of the
exciter coil). Initially current will not flow through
the transistor (Tr) as
it
has not been turned on with
the proper base current.
When SCRl turns on
it
provides a very low
resistance path for current to flow from the positive
end of the exciter coil to the field coil (rotor) where
it
will build an electromagnetic field.
Return current can now flow from the field coil
(rotor) through diode D2
(it
will conduct in this
direction because
it
is forward biased), back to the
negative end of the Exciter coil.
When the
end
of the exciter coil labeled
"B”
becomes positive, diode D4 and SCR2 will become
forward biased.
A
repeat of steps 1
-
7 using D4, R2
and SCR2 will occur.
It
is important to note that the
arrangement of Dl, D2, SCRl and SCR2 form a full
wave bridge rectifier and will send DC electricity to
the field coil.
The amount of current that
is
sent to the field coil
is
controlled by the transistor (Tr) and zener diode (ZD).
The control circuit operates as follows:
1. Two sensor wires are connected to the main coil
(MC) to monitor the output voltage of the
generator.
DC by the full wave bridge rectifier
(D).
3. The electricity from the rectifier (D) Rows to a
voltage divider network made
up
of
resistors
R3
and R4. When the voltage at the connecting point
(C) reaches the breakdown level of the zener diode
2. The electricity from the sensor wires is changed to
(ZD),
the zener diode will allow current to flow to
the base (b) of the transistor (Tr).
4. The current at the base
of
the transistor will turn
the transistor
"
on
"
and will allow current to flow with
almost no resistance from point
E
to point F.
5.
Electricity from R1 and R2 that would normally be
used to turn on SCRl and SCR2 is now diverted
through the transistor and the
SCR'S
remain in the
"
off'
mode. Electricity will not flow from the anode
to cathode
of
the SCR.
6.
The amount of current flowing through the
transistor
is
very small and flows through the field
coil, creating an insignificant magnetic field and
then back to the exciter coil.
7.
When the output voltage of the main coil drops the
zener diode will turn
off
and block current to the
base of the transistor. The SCR's will turn on and
full current flow will return to the field coil.
I
GENERATOR
TESTING
CAUTION: The following tests will be
completed with the generator
running. The electricity this
generator produces can cause
death. Never touch any part
of
your body to exposed
or
uninsulated terminals
or
wiring.
Generator
67
Testing Color Code
P
Pink
G Green
R
.Red
B Black
W
White
Y
.Yellow
BI Blue
Br Brown
Or .Orange
Ltbl .Light Blue
Ltg Light Green
Ltg/R Light Green with Red Tracer
W/R .White with Red Tracer
R/B
Red with Black Tracer
Y/G Yellow with Green Tracer
R/W
Red
with White Tracer
W/B .White with Black Tracer
G/W
Green with White Tracer
BI/R .Blue with Tracer
Bl/W Blue with White Tracer
G/R Green with Red Tracer
Br/Y Brown with Yellow Tracer
G/Y
Green with Yellow Tracer