Key to fig. 9-1.
If any defective diodes are replaced, repeat step (2)
or (3) above to determine if the generator defect has
(6) Tag and disconnect the two top leads at pins
15 and 16 of TB16 (located on the current transformer
Read the exciter field resistance at
pins 15 and 16 with a Wheatstone bridge. The exciter
field resistance should be between 2.90 and 3.55 ohms
at an ambient temperature of 65°F.
(7) Check insulation resistance with megger be-
tween pins 15 and 16 and the frame (para 9-2a). Re-
connect the two leads and remove tags. Before pro-
ceeding further with fault isolation, disassemble the
endbell from the generator.
(8) Disconnect generator rotor leads from the
rotating diode assembly and measure the field resis-
tance with a Wheatstone bridge. The resistance
value for a 50/60 Hz generator should be 0.38 to
0.46 ohms and 1.17 to 1.43 ohms for a 400 Hz gen-
erator at an ambient temperature of 65°F.
(9) Megger between field leads and frame (para
(10) Disconnect diodes and use a double Kelvin
bridge to read the resistance between leads on the
exciter rotor. On a 400 Hz set, this value is 0.042
to 0.051 ohm and on a 50/60 Hz set, this value is
0.019 to 0.023 ohm at a 65°F ambient temperature.
(11) Check insulation resistance with megger
between exciter armature !eads and frame (para 9-2a).
(12) Check resistance of surge protector (var-
istor) when disconnected from one side of generator
field. Value should be 1 megohm or more in each
direction. Check to be sure no physical damage is
evident on the surge protector.
(13) Tag and disconnect the twelve stator leads
at the voltage reconnection panel and measure each
of the six stator windings with a Kelvin double bridge.
On a 50/60 Hz generator, the resistance should be
0.0216 to 0.0264 ohm in each circuit. On a 400 Hz
set, the resistance value should be 0.0141 to 0.0173
ohm in each circuit, at ambient temperature of
(14) Connect all stator leads together and read
between the leads and the generator frame with a
megger (para 9-2a).
(15) Growler test
(a) Perform internal growler test on stator
by applying 110 Vac to the coil which is wound on the
cross bar of the H.
(b) Place the growler on the coil of the sta-
tor so that it is in direct contact with two adjacent
(c) A shorted coil produces a very heavy
current which is indicated by the rapid vibration of
a thin piece of metal, such as a hacksaw blade, held
over the other end of the coil.
(d) Perform a growler test on the rotor as
described in (a) above.
(e) Place the rotor on the growler and ener-
g i z e .
(f) Hold a thin piece of metal, such as a
hacksaw made, directly over the top slot of the rotor
and along the length of the slot. If the coil is shorted,
the blade will vibrate rapidly and cause a growling
(16) If the above test values are not within the
limits specified, proceed with the removal and re-
c. Defective Bearing Replacement. If a defective
bearing is indicated by vibration or noise at the bear-
ing housing on the endbell, the bearing can be re-
placed without removal of the generator from the set.