Guardian 4389 Repair Manual page 21

GENERAL INFORMATION
GENERAL
Standby electric power generators will often run unat-
tended for long periods of time. Such operating para-
meters as (a) engine oil pressure, (b) engine temper-
ature, (c) engine operating speed, and (d) engine
cranking and startup are not monitored by an operator
during automatic operation. Because engine opera-
tion will not be monitored, the use of engine protective
safety devices is required to prevent engine damage
in the event of a problem.
Prepackaged generator engines mount several
engine protective devices. These devices work in
conjunction with a circuit board, to protect the engine
against such operating faults as (a) low engine oil
pressure, (b) high temperature, (c) overspeed, and (d)
overcrank. On occurrence of any one or more of
those operating faults, circuit board action will effect
an engine shutdown.
LOW OIL PRESSURE SHUTDOWN:
See Figure 1. An oil pressure switch is mounted on
the engine oil filter adapter. This switch has normally
closed contacts that are held open by engine oil pres-
sure during cranking and startup. Should oil pressure
drop below approximately 10 psi, the switch contacts
will close. On closure of the switch contacts, a Wire
86 circuit from the circuit board will be connected to
ground. Circuit board action will then de-energize a
"run relay" (on the circuit board). The run relay's nor-
mally open contacts will then open and a 12 volts DC
power supply to a Wire 14 circuit will then be termi-
nated. This will result in closure of a fuel shutoff sole-
noid and loss of engine ignition.
HIGH OIL TEMPERATURE SHUTDOWN:
An oil temperature switch (Figure 1) is mounted on
the engine block. The thermal switch has normally
open contacts that will close if oil temperature should
exceed approximately 284° F (140° C). This will result
in the same action as a low oil pressure shutdown.
OVERSPEED SHUTDOWN:
During engine cranking and operation, the circuit
board receives AC voltage and frequency signals
from the generator engine run windings, via Wire 66A.
Should the AC frequency exceed approximately 72Hz
(4320 rpm), circuit board action will de-energize a
"run relay" (mounted on the circuit board). The relay's
contacts will open, to terminate engine ignition and
close a fuel shutoff solenoid. The engine will then
shut down. This feature protects the engine-generator
against damaging overspeeds.
NOTE: The circuit board also uses engine run
winding output to terminate engine cranking at
approximately 30 Hz (1800 rpm). In addition, the
engine run winding output is used by the circuit
board as an "engine running" signal The circuit
board will not initiate transfer of electrical loads
to the "Standby" source unless the engine is run-
ning at 30 Hz or above.
PART 1
ENGINE-GENERATOR PROTECTIVE DEVICES
LOW OIL SWITCH
Figure 1. Engine Protective Switches on an
Air-Cooled Engine
OVERCRANK SHUTDOWN:
Automatic engine cranking and startup normally
occurs when the circuit board senses that utility
source voltage has dropped below approximately 60
percent of its nominal rated voltage and remains at
that low level longer than fifteen (15) seconds. At the
end of fifteen (15) seconds, circuit board action will
energize a crank relay and a run relay (both relays
are on the circuit board). On closure of the crank relay
contacts, circuit board action will deliver 12 volts DC
to a starter contactor relay (SCR, for v-twin models)
or a starter contactor (SC, for single cylinder models).
The control contactor will energize and battery power
will be delivered to the starter motor (SM). The engine
will then crank.
During a manual startup (AUTO-OFF-MANUAL
switch at MANUAL), action is the same as during an
automatic start, except that cranking will begin imme-
diately when the switch is set to MANUAL.
Circuit board action (during both a manual and an
automatic start) will hold the crank relay energized for
15 seconds on. The relay will then de-energize for 15
seconds off. It will then energize for seven (7) sec-
onds on and de-energize for seven (7) seconds off. It
will repeat this same cycle for another 45 seconds.
If the engine has not started after approximately 90
seconds of these crank-rest cycles, cranking will auto-
matically terminate and shutdown will occur. The cir-
cuit board uses AC signals from the stator engine run
winding as an indication that the engine has started.
SECTION 1.5
HIGH TEMP SWITCH
Page 19