Hobart 6824-1, Operation And Maintenance Manual, Page: 79 / 214

(40)
Check underfrequency sensing relay, protective monitor, and indicating light. At some frequency
value (Hz, cycles-per-second) from 385 Hz down to 375 Hz, the underfrequency sensing relay
should function to signal the underfrequency circuit in the protective monitor module to OPEN
the load contactor holding circuit, thus OPENING the load contactor. To check the
underfrequency protective components, proceed as follows:
a. While the unit is operating normally under load, reduce generator output frequency by
reducing engine speed. Use the governed speed setting potentiometer (Ref. Fig. 9). Turn
adjusting screw
COUNTERCLOCKWISE gradually to reduce engine speed until frequency
meter indicates 386 Hz. Underfrequency protective relay should not function to open the
load contactor at this frequency. Reduce frequency in steps of 1 Hz.
b. If the protective system functions to open the load contactor and turn on the underfrequency
light after some frequency between 385 Hz and 37 Hz is reached, all components of the
system are functioning properly. If the load contactor is not opened within the above
frequency range, refer to Trouble Shooting Chart to determine which component is defective.
c. Return unit to normal operating condition.
(41) Check overfrequency sensing relay, protective monitor, and indicating light. This protective
circuit operates in exactly the same manner as the underfrequency circuit except its operating
range is from 415 Hz to 425 Hz. Its purpose also is to open the load contactor and turn on an
indicating light.
a. Check procedures are the same for overfrequency as for underfrequency (above) except
that engine speed is
INCREASED
to create a condition of overfrequency.
b. If the overfrequency system functions to open the load contactor and turn on the
overfrequency light after some frequency between 415 Hz and 425 Hz is reached, all
components of the system are functioning properly. If the load contactor is not opened within
the above frequency range, refer to Trouble Shooting Chart to determine which component
is defective.
(42) If the generator is operating under load at this point, place the contactor control switch (20, Fig.
2) in OFF position to open load contactor and disconnect load. There will be no further need for
the load bank in the following checks.
(43)
With the engine running at normal governed speed, check the entire unit for vibration and for
any parts which may have become loosened during the above checks. Tighten any loose
attaching hardware as required.
Electric shock, moving parts, and noise can kill or injure! If a metal sounding rod
is used to detect bearing noises, exercise extreme care to avoid injury.
(44)
Check 400-Hz generator bearings. Use a stethoscope or metal sounding rod to listen for
unusual noises. If using a metal rod, place one end on the generator housing and hold the other
end near the ear. Hold the rod with three fingers and use the index finger and thumb to form a
“sounding chamber” between the rod and the ear. Do NOT allow the rod to touch the ear. Listen
for “grinding” or pounding sounds which would indicate a defective bearing. An engine noise
may be “telegraphed” to the generator and misinterpreted as a generator noise. Send the unit to
overhaul if in doubt of bearing serviceability.
4. Generator Set Adjustment
a. Generator Adjustment
The 400-Hz generator is a brushless type requiring no adjustments of any kind.
b. Generator Control Adjustments
The following items may require adjustment at some time during the life of the equipment.
WARNING
OM-499
May 31/94 Revised 2-3
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