voltage has built up, GEN ON light DS20 illuminates
is activated, contacts 5 and 7 close, arming the fault
and a 28-volt dc signal is fed through diode CR3 into
relay K6.
the voltage regulator at pin C. A loss of this signal can
cause a dc voltage shutdown.
CAUTION
With the dc output cable connected to the aircraft,
Approximately 30 seconds are required to cool the
thermal control circuit within relay K3. Therefore,
distribution is accomplished by pressing the dc
in the event of a fault, the mode selector switch S29
SERVICE CABLE ON button S25. This energizes the
should be placed in the SERVO BOOST or the
dc output power relay K10, completing the circuit to dc
START/DRIVE position and the engine permitted
service output jack J6. The dc SERVICE ENERGIZED
to idle. If the unit is operating properly, this amount
light DS19 illuminates, indicating that relay K10 has
of time is required prior to going into the
energized and dc power is available for distribution.
SERVICE POWER position again.
Overvoltage relay K25 is connected to the B+
With switch S29 in the SERVICE POWER
o u t p u t o f t h e d c g e n e r a t o r G 2 . I t s e n s e s a ny
position, the ac generator field control relay K5
overvoltage not corrected by the dc voltage regulator
energizes through the normally closed contacts C2 and
VR2. When energized it, in turn, energizes lockout
C3 of relay K6. The field flash voltage is provided by
relay K12. Lockout relay K12 then de-energizes field
the permanent magnet generator (PMG) located inside
control relay K11, removing the 28-volt dc signal from
the voltage regulator's pin C and stopping generator
G1. This completes the field circuit and the voltage
output. Output power relay K10 de-energizes if dc
regulator VR1 circuit. The engine accelerates and
servicing is in progress. After an overvoltage
stabilizes at approximately 2,400 rpm by the engine
shutdown, the lockout relay K12 remains energized
governor.
until you place the dc POWER switch S5 in the OFF
Relay K3 does not energize for 5 seconds. This
(shown) position.
time delay allows the ac generator to reach its rated
AC POWER CONTROL AND DISTRIBU-
voltage and frequency and prevents the inadvertent
TION.--For the discussion of the ac power control
tripping of fault relay K6.
and distribution, refer to figure 7-45, a simplified
At the rated voltage and frequency, relays K7 and
schematic of the ac power control and distribution
K8 energize. This opens circuits to relay K6 and closes
circuit, and figure 7-46, a picture of the ac control
circuits to the ac SERVICE ON button S23 and ac GEN
panel.
ON light DS1. The ac generator is now ready for use.
Prior to ac power distribution, the engine must be
After the ac cable is connected to the aircraft,
operating and the mode selector switch S29 must be in
pressing ac SERVICE ON button S23 energizes relay
the SERVICE POWER position. The engine is
K2, completing the circuit to ac service output
controlled in the same way that it is under dc power
connector J2. The ac SERVICE ENERGIZED ON
control and distribution.
light DS2 lights, indicating that relay K2 is energized
The ac generator output is controlled by the
and ac power is available for distribution. K2 is battery
vo l t a g e r eg u l a t o r V R 1 . I t i s p r o t e c t e d f r o m
operated. Contacts No. 1 and No. 2 close to complete a
malfunction by the time delay relay K3, over/under
holding circuit. K2 remains energized until the ac
voltage relay K7, and the over/under frequency relay
SERVICE OFF button is pressed or a fault occurs.
K8. Other protective devices are the phase sequence
The phase sequence relay K9 is attached to the
relay K9, the fault lockout relay K6, and the circuit
three output phases of the ac generator. The phases
breakers CB1, CB2, and CB3.
must be in the proper sequence--A, B, and C. If two
Relay K3 is a thermal time-delay relay set to
phases are reversed, K9 will not energize and close the
energize 5 to 10 seconds after voltage is applied to the
normally open contacts 5 and 6. This prevents the use
coil. This time delay allows the ac generator to
of the output circuits, although 115 volts are available
stabilize at the rated voltage and frequency. When K3
on each phase.
7-37
