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 TM 55-1905-223-24-3
NOTE
The insulation resistance of the main exciter rotor and main rotor are more difficult to measure and
are less significant. It is recommended that these resistances can be checked only when diagnosing a
problem.
(1)
Remove the AVR from the AVR box by removing the four cover captive screws, the cover, the four
threaded pillars, and then the AVR.
(2)
Disconnect all of the external leads from the AVR and separate the leads so terminals do not touch
each another or AVR box.
(3)
Remove both sides of the terminal box.
(4)
Remove the E0 and E2 leads from the overvoltage module. Separate the leads so they do not touch
anything.
(5)
Remove the two leads from the voltage adjust rheostat. Separate the leads so they do not touch
anything.
(6)
Remove the leads that connect the generator windings to the generator frame.
(7)
Open the generator main line circuit breaker (if there is one).
(8)
Disconnect any instrumentation and controls (voltmeter, wattmeter, load sensing governor,
synchronizer, etc.) as well as any capacitors that may be connected to the main le ds (U or U6, V or
a
V6, W or W6 and N).
(9)
Connect one lead of the megger to the ground connection point in the terminal box. Connect the other
lead to the terminal box.
(10)
Crank or operate the megger 2 or 3 turns. The megger should indicate zero ohms. A zero reading
shows that the ground is ok. If the megger does not read zero, move the clips to make better contact and
keep checking until a zero reading is obtained.
(11)
Remove the lead from the terminal box and connect it to generat r lead U or terminal U. Crank the
o
megger at approximately 120 rpm (2 turns per second) for 10 seconds and record the reading.
(12)
A reading of 1.5 megohms or above is satisfactory.
(13)
A reading of 0.5 to 1.5 (or 5.0 megohms for 4160 volt generators) indicates that the main insulation is
marginal. Dry the generator insulation and test again. Refer to step i. of this procedure.
NOTE
A reading of less than 0.5 megohms indicates an accidental grounding or a severe insulation problem.
Recheck to make sure no terminals are touching the frame.
4-13
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