Discontinued – Murphy Series A900 User Manual

Page 26

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to the table for examples.

of the electronic components in the A900. Minor
variations, 1 or 2 volts, normally will have no effect on the
A900 operation.

(d) The AC voltage at terminal 29 must be a minimum of 2

VAC to drive the speed sensing circuits of the A900.

7-2.2 With power applied and the Mode Selector switch in
AUTO, the following occurs when a switch is closed from
terminal 28 to ground.
7-2.2.1 The fuel relay is energized, which opens the circuits at
terminal 5 and 7, and closes the circuits at terminal 6 and 8. This
applies battery power to the run solenoid and to the exciter
terminal of the alternator.
7-2.2.2 At the same time, the crank relay driver provides battery
voltage at terminal 17, thru the auxiliary crank disconnect relay
contact to the auxiliary starter solenoid. This output is controlled
by the crank and rest timer but it can be overridden by the crank
disconnect speed switch and / or the auxiliary crank disconnect
circuit. The output will remain on until the crank time expires or
the crank disconnect circuits detect that the engine has started.
7-2.2.3 If the engine does not start during the crank time, the
rest timer will prevent cranking until it has timed out. After the
rest time, the engine will begin cranking again. This sequence
will repeat until the engine starts or the overcrank shutdown
circuit trips.
7-2.2.4 Engine speed is detected by an AC signal from a
magnetic pickup wired to terminal 29. When this frequency
exceeds the setting of the crank disconnect speed switch,

(a) Cranking is terminated.
(b) A time delay starts timing during which the oil pressure

and water temperature shutdown circuits are locked out.

7-2.2.5 An engine run signal closes to ground at terminal 19
when the engine starts.
7-2.3 With the engine running, the six shutdown TATTLETALE

circuits are activated and will operate anytime a switch closes
from ground to their inputs.

Circuit

Input

Output

(a) Remote Stop

Terminal 33

Terminal 41

(b) Oil Pressure

Terminal 30

Terminal 39

Terminal 32

Terminal 40

(d) Overspeed

Int. Speed Switch

Terminal 38

(e) Loss of Speed Signal

Int. Circuit

Terminal 43

(f) Spare Shutdown

Terminal 11

Terminal 42

7-2.3.1 Each circuit has a indicating light and a transistor output that
closes to ground to operate an individual remote indicating light.
7-2.3.2 When any of the six shutdown functions trip, the
shutdown relay K-2, pulls in and applies battery power to
terminal 9. At the same time, the fuel relay, K-1 drops out and if
the shutdown was due to overspeed, the overspeed relay, K-3
pulls in to signal an air shut off solenoid, terminal 10 to close.
7-2.3.3 The six shutdown circuits, paragraph 7-2.3, are first out
latching type TATTLETALE

. That is, only one of the six may

trip at one time and it must be reset before the A900 will operate
again.
7-2.3.4 If one of the TATTLETALE

circuits trip, the following

occurs to shutdown the engine and generator.

(a) The TATTLETALE

light for that function turns on.

(b) Shutdown relay K-2 is energized, closing its contacts at

terminal 9. These contacts can be wired to a shunt trip
solenoid in the generator circuit breaker if required.

the run solenoid, terminal 6, and from the alternator
exciter, terminal 8. The circuits at terminal 5 and 7 will
close to reconnect the battery positive to these terminals.

(d) The run signal, terminal 19 turns off.
(e) If the shutdown is due to overspeed, the overspeed relay

operates and applies battery positive to terminal 10 to
close an air shutoff solenoid if the engine is so equipped.

7-2.3.5 If the engine is shutdown by the remote stop circuit, no
TATTLETALE

light is provided. Otherwise a TATTLETALE

light will show the cause of shutdown.
7-2.3.6 In the A900, the Loss of Speed signal TATTLETALE

can be either a shutdown or an alarm. It is connected to a circuit
that monitors the magnetic pickup input at terminal 29. If the
input from the pickup fails, drops below the minimum voltage,
or is disconnected while the engine is running, the Loss of Speed
Signal TATTLETALE

circuit will activate and shutdown the

engine or operate the alarm circuit. It is recommended that the
shutdown circuit is used since this prevents the generator set
from running without overspeed protection. If this circuit is
used as an alarm, the auxiliary crank disconnect input MUST be
supplied to prevent malfunction of the shutdown circuits.
7-2.3.7 To reset the circuits, the Mode Selector switch must be
turned OFF and back on to either the AUTO or TEST position.
7-2.4 Normally, the engine will run until the start signal is
removed from terminal 28.
7-2.4.1 When the signal is removed, the following occurs to
stop the engine;

(a) The fuel relay drops out, removing battery voltage from

the run solenoid, terminal 6, and from the alternator
exciter, terminal 8. The circuits at terminal 5 and 7 will
close to reconnect the battery positive to these terminals.

(b) The run signal, terminal 19 turns off.
(c) The four engine function TATTLETALE

circuits are

locked out as soon as the fuel relay drops out. This
prevents any TATTLETALE

from tripping while the

engine coasts to a stop.

7-3 A900 SELECTRONIC

Control Alarm Circuit

Operation.

7-3.1 The A900 alarm circuits perform the following functions;

(a) Turns on individual alarm lights based on input switch

closures.

(b) Provides lamp test function for all alarm and shutdown lights.
(c) Activates either the local audible alarm relay, K-4, and

remote visual alarm relay, K-6, or the local audible alarm
relay, K-4, the remote audible alarm relay, K-5, and the
remote visual alarm relay, K-6, depending on the
requirements of NFPA-110 1988 paragraph 3-5.5.1 (d).

(d) Provides the audible alarm silence circuit.

7-3.2 A contact closure to ground, at the alarm circuit
input terminal, will cause the associated TATTLETALE

light to

turn on and an output transistor to close to ground and turn on a
remote indicating light.
7-3.2.1 The alarm circuits also operate the three relays for local
audible alarm, remote audible alarm and remote visual alarm as
required by NFPA-110. The normally open dry relay contacts,

DISCONTINUED

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