Hobart 6824-1, Operation And Maintenance Manual, Page: 29 / 214

i. Auxiliary underfrequency relay
The function of the auxiliary underfrequency relay (3) is to automatically open the
excitation-deenergization relay and disconnect the voltage regulator anytime generator
frequency drops to 380 Hz or below. This protects the voltage regulator
(Fig. 10) and ballast
resistors (2, Fig. 7) against overload which could be caused by very high voltage regulator
output in its attempt to maintain voltage when the generator is operating at a speed which
cannot produce normal voltage output.
NOTE: If the auxiliary underfrequency relay is tripped, it will be necessary to momentarily place engine
operating mode switch (5, Fig. 9) in BUILD-UP-VOLTAGE position to restore generator voltage.
f. Engine Control Panel (See Fig. 9)
This panel is mounted directly below the control box. In addition to engine controls and
instruments, generator output controls are mounted here. A plexiglass window in the canopy rear
door, slants outward at the bottom to form an opening for access to controls when the door is
closed.
(1)
Engine instruments
Engine operation is monitored by an ammeter
(4)
, a coolant temperature gage
(8)
, and an oil
pressure gage (10) . An hourmeter (6) records engine operating time.
a. Ammeter
The ammeter (4) indicates the direction and value of current flow in the 12-V DC electrical
system. Its graduated range is from -60 A through 0 A, to +60 A.
b. Temperature gage
The temperature gage
(8)
is a mechanical type of unit construction. It consists of a panel
mounted indicating mechanism which is connected by a capillary tube to a bulb mounted in
the engine cooling system. The gage indicates engine coolant temperature in the range of
100
o
F to 220
o
F.
c. Oil pressure gage and oil pressure switch
The oil pressure gage
(10)
is a bourdon tube type and indicates engine lubricating oil
pressure. It is graduated from 0 PSI to 75 PSI. An oil pressure switch is mounted in a tee
fitting directly behind the gage. The switch connects 12-V DC power to the engine control
system and to the generator 12-V DC control system when the engine is running.
d. Hourmeter
The hourmeter (6) is electrically driven from the 12-V DC battery system. The hourmeter
measures and records engine running time and will record up to 9999.9 hours on five
revolving drums. The hourmeter operates only when the engine is running and the oil
pressure switch is closed.
e. Fuel gage and blocking diodes
An electric fuel gage (2, Fig. 9) accurately indicates the quantity of fuel oil in the tank
provided for the generator set. A sending unit mounted in the trailer or truck mounted unit
fuel tank supplies the signal to the gage. Twelve-volt DC operating power is supplied to the
fuel gage from two sources. The gage functions when the engine is running, or when the
instrument lights are ON. Since two power sources are connected to the fuel gage system, a
blocking diode
(18)
is required in each power source circuit. One diode prevents energizing
the motor-generator control circuit when panel lights are ON. Another diode prevents
operation of the panel lights by the engine-generator circuit when the engine is running. To
check fuel quantity when engine is stopped, use switch
(3) to turn ON panel lights.
OM-499
May 31/94 Revised 1-1
Page 15