Discontinued, Figure 3-3.2 – Murphy Series A900 User Manual

Page 9

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(f) Overspeed Speed Switch;

1. Provides a signal to the overspeed shutdown circuit if

the engine exceeds the preset speed. The factory
setting is approximately 3894 Hz, or 1980 RPM on an
engine with 118 teeth on the flywheel ring gear. The
adjustment range is 300 Hz to 10,000 Hz.

2. A push to test switch is provided to allow testing of the

overspeed circuit without overspeeding the engine.
When switch is depressed, the set point of the speed
switch is lowered approximately 10%.

3. The push to test switch can also be used to set the

overspeed switch at approximately 110% of running
speed. Hold the switch depressed while adjusting the
setting until the overspeed trips. When switch is
released, the set point will be 10% above normal running
speed. This is the method used to make the factory
setting. With engine speed at 1800 RPM or 3540 Hz.

(g) Cooldown circuit; this circuit keeps the engine running

for five minutes after the transfer switch has removed
the load from the generator and signaled the A901-2 to
stop the engine. This feature may be selected (turned
On, standard setting is Off) with a switch on the back of
the Control / Display Module.

3-3.1.5 Seven (7) First-out Shutdown Circuits; provide first-out
indication of cause of shutdown. These circuits deactivate the
fuel relay and lockout the A901-2 until reset. Shutdown circuits
are activated by the crank disconnect circuit and are not active
when the engine is stopped. Six (6) Red Shutdown Indicating
Lights are provided. The Remote Shutdown circuit does not
have an indicator light.

(a) Overcrank; tripped by the overcrank sensing circuit which

counts the number of cranking attempts.

(b) Overspeed; tripped by overspeed switch when engine

exceeds a preset speed.

pressure drops below preset point while the engine is running.

(d) Water Temperature; tripped by the water temperature

SWICHGAGE

if engine coolant temperature exceeds the

switch setting.

(e) Spare Shutdown; tripped by an external switch. This

circuit is provided for an additional shutdown that may be
recommended by the engine supplier.

(f) Loss of Speed Signal; tripped by an internal sensing

circuit that detects the loss of the Magnetic Pickup
frequency signal for the speed switches while the engine
is still running. Since the Overspeed shutdown circuit is
activated by the frequency input, it prevents the engine
from running without overspeed protection.

(g) Remote Shutdown; this circuit is provided to shutdown

the engine generator from a remote location. No
indicating light is provided.

3-3.1.6 Nine (9) Alarm Indicating Circuits; provide indication
of cause of alarm. These circuits activate the alarm relays.
Alarm circuits are active as long as power is applied to the
A901-2. All are active when the engine is running, shutdown or
on standby except Low Oil Pressure Pre- alarm and High Water
Temperature Pre-alarm. Nine (9) Amber Alarm Indicating
Lights are provided.

(a) Low Water Temperature; operated by low water temp-

erature SWICHGAGE

(b) Air Damper Closed; operated by an external switch or by

the Overspeed Relay circuit. This circuit is provided with
a selector switch to allow the user to determine the source
of the signal.

TEST mode selector switch.

(d) Low Fuel Level; operated by level switch on fuel tank.
(e) High Battery Voltage; operated by relay contact in battery

charger.

(f) Low Battery Voltage; operated by low voltage sensing

device in battery charger.

(g) Low Oil Pressure Pre-alarm; operated by pre-alarm

switch in oil pressure SWICHGAGE

(h) High Water Temperature Pre-alarm; operated by pre-

alarm switch in water temperature SWICHGAGE

(i) Battery Charger AC Failure; operated by relay contact in

Battery Charger.

3-3.2 Relay Module. The Relay Module, figure 3-3.2, includes
all of the control, alarm and shutdown relays that are required to
make up the generator engine control. Relays are available for
operation on either 12 or 24 volt battery systems. Six (6) relays
are included for:

3-3.2.1 Fuel Relay; this relay provides two outputs, one to
energize the engine fuel solenoid and a separate circuit to
energize the battery charging alternator field.
3-3.2.2 Shutdown Relay; this relay is operated by the shutdown
circuits of the Control / Display Module and provides an output
that can be used to trip the generator circuit breaker on an
emergency shutdown.
3-3.2.3 Overspeed Relay; operates only on an Overspeed
shutdown. This relay provides an output to trip an air shut off
solenoid to close the air damper on a two-cycle engine.
3-3.2.4 Local Audible Alarm Relay; provides a contact closure

DC12V

3A Fuse

DC12V

6-1/4 in.

(159 mm)

4-1/2 in.

(114 mm)

Figure 3-3.2

DISCONTINUED

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