Hobart 6921 Series Operation And Maintenance Manual page 82

OM-2040 / Operation and Maintenance Manual
90D20 / Generator Sets
b. Return unit to normal operating conditions by adjusting voltage regulator rheostat and
pressing reset switch (9).
(39) Check operation of undervoltage sensing relay, indicating light, and time delay. At some value
between 103 V and 93 V, the undervoltage sensing relay (Ref. 1-1; 5, Fig. 9) should function to
activate the undervoltage time delay circuit. Five seconds after the time delay circuit is activated,
(if the undervoltage condition continues) it should function to open the protective monitor relay.
Which, in turn will open the load contactor to stop power delivery. As a result of the action, the
undervoltage indicating light (14) should be turned ON. A stopwatch or sweep-second hand
watch is required for this check. Proceed as follows:
a. With unit operating at normal load, use the voltage regulator rheostat (37, Fig. 1) to reduce
voltage to 104V. The load contactor should NOT open after a 5-second delay.
b. Reduce voltage in steps of 1-V, with a delay of at least 5 seconds between steps. Restart
stopwatch, or note position of sweep-second hand, each time voltage is reduced. At some
voltage value between 103 V and 93 V, and 4 to 12 seconds after a new voltage setting is
made, the load contactor should be step-by-step action of the undervoltage sensing relay,
time delay circuit, and protective monitor relay.
NOTE: The 4 to 12 second time delay is generally set at 5 seconds. It may be necessary to switch to
MANUAL control to obtain these low voltages.
c. If the load contactor is not opened at 103 V to 93 V, it will be necessary to refer to the
Troubleshooting Chart, Section 3-1, to determine which component of the undervoltage
protective circuit is defective. If the undervoltage circuit performs satisfactorily, return the unit
to normal operation by adjusting the voltage to 115 V, pressing the reset switch, and closing
the load contactor.
(40) Check underfrequency sensing relay, protective monitor, and indicating light. At some frequency
value from 385 Hz down to 375 Hz, (Hz=cycles-per-second) the underfrequency sensing relay
should function to signal the underfrequency circuit in the protective monitor module to OPEN
the load contactor holding circuit. To check the underfrequency protective components, proceed
as follows:
a. While the unit is operating normally under load, reduce generating or output frequency by
reducing engine speed. Use the governor speed setting potentiometer (Ref. Fig. 4).
Gradually turn adjusting screw COUNTERCLOCKWISE to reduce engine speed until
frequency meter indicates 386 Hz. Underfrequency protective relay should not function to
open the load contactor. Reduce frequency in steps of 1 Hz.
b. If protective system functions to open load contactor and turn on the underfrequency light
after some frequency between 385 Hz and 375 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 Troubleshooting 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 exactly as the underfrequency circuit except the operating range is from 415 Hz to 425
Hz. Its purpose 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 load contactor and turn on overfrequency light
after some frequency between 415 Hz and 425 Hz is reached, all components of the system
are functioning properly. If load contactor is not opened within the above frequency range,
refer to Troubleshooting Chart to determine which component is defective.
(42) If the generator is operating under load at this point, place the contactor control switch (29, 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 governor speed; check the entire unit for vibration and parts
which may have become loosened during the above checks. Tighten any loose attaching
hardware as required.
Chapter 2-3
Page 8
February 28/94 Revised