Hobart Brothers 6910, Operation And Maintenance Manual With Illustrated Parts List

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(a) While the unit is operating normally under load, reduce generator output frequency by reduc-
ing engine speed. Use the governed speed setting potentiomete. Turn adjusting screw COUN-
TERCLOCKWISE 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, re-
fer 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 overfre-
quency 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.
1) 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 hard-
ware as required.
(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 gener-
ator and misinterpreted as a generator noise. Send the unit to overhaul if in doubt of bearing service-
ability.
WARNING: 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.
OM-2028
June 20/89 Revised 2-3
Page 9