Hobart 6921 Series Operation And Maintenance Manual page 114

OM-2040 / Operation and Maintenance Manual
90D20 / Generator Sets
Protective Circuit
NOTE: The protective relays and protective monitor are not completely functional until the load contactor is
closed. Since it is not advisable to vary voltages for test purposes while delivering power to an
aircraft, the generator should be connected to a load bank for troubleshooting the protective circuits.
To avoid repetition, it will be assumed that the reset switch [(36), Fig. 1] has been pushed and the
load contactor has been closed before commencing each test.
Trouble, Symptom & Condition
1. Load contactor opens during
power delivery. Overvoltage
indicating light ON.
2. Load contactor opens during
power delivery. Undervoltage
indicating light ON.
Chapter 3-1
Page 22
Test or Inspection
a. The over voltage condition
may have been the result of
a sudden drop in the load,
or possible tampering with
voltage regulator
adjustment potentiometer
(37, Fig. 1 ), and may have
been a normal action.
b. Defective protective monitor
module (K14).
a. An undervoltage condition
caused the over-under-
voltage sensing module
(K26) to function normally.
b. Defective over-under-
voltage module (K26).
Corrective Action
Use voltage adjusting rheostat
(37, Fig. 1) to reduce voltage to
110-V AC. Observe voltmeter
and gradually increase voltage
with rheostat. If the
over-under-voltage sensing
module (K26) functions to open
the load contactor at any value
less than 130-V AC, it is
defective. Replace
over-under-voltage module or
complete protection relay tray.
If overvoltage light comes on at
110-V, unplug over-under
voltage module and close
contactor. If the Overvoltage
light comes on, the protective
monitor module (K14) is
defective.
NOTE: it is normal for the
undervoltage light to come on
during this test.
Observe generator voltage on
voltmeter and adjust to normal
115-V AC with voltage regulator
adjustment potentiometer (37,
Fig. 1). Resume normal
operation. If the load contactor
opens again and an under-
voltage condition is indicated by
light (DS38) proceed to step B.
Use voltage regulator
adjustment potentiometer(38,
Fig. 1) to reduce voltage to
104-V. Wait 12 seconds. The
undervoltage indicating light
should NOT be turned on. If the
light is turned on, the
undervoltage relay is defective.
Replace the module (K26) or
the protective relay tray.
February 28/94 Revised