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•
Open the service doors at N-end shield of the machine.
•
Disconnect the wires connected to the diodes and exciter winding connection
cables.
•
Check the condition of the diodes by measuring the resistance over a diode in
both directions.
•
Replace the damaged diode(s).
•
Clean the contact surfaces, and apply electric joint compound.
•
Fasten the diode(s). Bind the connection leads of the diodes as on original
assembly.
•
Check fastening and locking of all rectifier bridge components.
•
Make sure that no tools etc. are inside the machine and close the service
covers.
After replacing the diodes, the condition of the diodes can be checked by
comparing no-load excitation current to commissioning values. A diode failure
results as a significant increase in excitation current.
7.8 Maintenance related to thermal performance and cooling
system
An increase in the machine's temperature is usually caused by:
•
A decline in the effect of the cooling system.
•
Excessive amounts of heat produced by the machine.
If the machine temperature exceeds normal values, determine which of these two
causes is responsible for the increase in the temperature. Excessive heat
production might be caused e.g. by a winding problem or by network unbalance
and in these cases corrective actions on the cooling system would be ineffective or
harmful.
7.8.1 Cooling System
The machine is cooled with a shaft mounted fan. The cooling air is taken in from
the surroundings air at the non-drive end and it is blown out from the top and the
sides of the machine at the drive end (IC 0A1). The cooling air should be as clean
as possible because dirt that drifts into the machine causes contamination and
reduces the efficiency of the cooling.
If winding or cooling air temperature detectors show an abnormal temperature, the
cooling system has to be checked. Either of the following two reasons could be
causing the problem in the cooling system:
•
Problems with air filters (if applicable):
•
Problems in air circulation inside the machine:
Other possible causes for poor cooling system performance include elevated
ambient temperature or high intake air temperature. In addition, bearing
malfunction might lead to high bearing temperature.
Check the condition of the air filters.
Ensure good air circulation inside the machine. The machine interior
should be cleaned and checked during overhauls or if problems arise..
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