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2
1
1.
Exciter Rotor Lead
2.
Alternator Rotor Lead
Figure 4-4. A-C Generator (Rectifier Assembly)
1. Remove the bearing cap by removing the four
bolts shown in Figure 4-3 and both the bearing and
rectifier assembly will be visible.
2. Use a bearing puller to remove the bearing from
main shaft being careful to locate the puller on the inner
race of the bearing to avoid bearing damage. Once the
bearing is free, it is then necessary to disconnect the
alternator and exciter rotor leads as shown in Figure 4-4.
3. Remove the three hold-down cap screws which
secure the rectifier assembly to its adapter. Once this
procedure is complete the rectifier assembly is free for
removal.
Follow the testing procedures outlined in section
4.5.1.d.
After the rectifier assembly has been repaired or
replaced, reverse the procedure as stated above, being
careful that all lead connections are tight and that set
screws are locked with Loctite. (Loctite Corporation)
d. Testing Diodes With An Ohmmeter
A
A
RED LETTERING
Forward
Polarity
Diode
A (+) = Anode
Figure 4-5. Diode Testing
1. Remove the diode from the circuit. Do not press
body out of heat sink. Disconnect the lead. Do not use
excessive heat in unsoldering the lead.
2. A good diode should have a low resistance when
the + of an ohm meter is connected to the anode and the
--- to the cathode, and a high resistance when the polarity
of the ohmmeter is reversed. (See Figure 4-5)
If both resistances are high or both are low, then the
diode is defective and should be replaced.
2
1
C
C
BLACK LETTERING
"R" After Number
Reverse
Polarity
Diode
C (---) = Cathode
e. Rotor Damage
The damper bars of the generator prevent excessive
hunting when AC generators are operated in parallel.
Damper bars, because they must have a low electrical
resistance and are subjected to extreme centrifugal
forces, must be mechanically secure and permanent.
Consequently, they are welded to end plates completely
covering the field.
All rotors are static and dynamically balanced to a
high degree on precision machines to assure minimum
vibration. They will, therefore, remain dynamically stable
at speed well beyond the synchronous speed of the
generator. The rotors on generators are, however,
subjected to extreme centrifugal forces which can
increase beyond safe operating limits at overspeed.
Therefore, the prime mover should be adequately
governed to prevent overspeed.
Damage to the rotor can also occur due to
overheating which can be caused by the air flow being
restricted from dust or other foreign objects collecting in
the air passage.
If a rotor becomes defective, it should be returned to
the factory with full nameplate data, because the rotor
coils are enclosed in welded squirrel case winding. To
repair a rotor the special tooling and technique of the
factory is necessary and essential. Should a failure occur,
Carrier Transicold should be notified immediately and
steps will be taken to get the generator back into service
with the least expense; and more important, to determine
the cause of the failure and take steps to prevent a
recurrence.
f. Generator Windings (Drying)
High voltage (dielectric) testing must not be
performed to the machine without first
observing NEMA rules. The insulation of this
generator winding may be safely checked by
using a megger. A high megger reading indicates
low insulation leakage.
Generators that have been in transit or storage for
long periods may be subjected to extreme temperature
and moisture changes. This can cause excessive
condensation, and the generator windings should be
thoroughly dried out before bringing the generator up to
full nameplate voltage. If this precaution is not taken,
serious damage to the generator can result. The following
steps should be taken to effectively dry the generator
windings:
1. (a) Place generator in drying oven or hot room.
(b) Dry with warm air blower directed through
windings.
2. (a) If the generator has been operated and then
put into storage for any period of time, a P.D. George
#11127 type air-dry fungus resistant varnish should be
applied.
Experience has shown that it is necessary to take these
precautions in locations such as seaboard installations
and other highly humid areas. Some installations will be
4-5
WARNING
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