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NOTE
When testing the diodes, observe the polarity
of the test terminals in relation to the diode
polarity.
The polarity of the diode is indicated by an
arrow on its housing.
The current conduction must occur
only in the anode-cathode direction,
i.e., in the condition of direct
polarization.
The diode is considered good when you have low ohmic
resistance (up to approximately 100 Ω) in its direct
polarization and high resistance (approx. 1 MΩ) in the
opposite direction. Defective diodes have ohmic
resistance of 0 Ω or greater than 1 MΩ in both directions.
In most cases, the test method which uses an ohmmeter
to the diodes is enough to identify faults in the diodes.
However, in some extreme cases it may be necessary to
apply the rated blocking voltage and/or current circulation
in order to detect a fault in the diodes. Due to all the work
required to perform these tests, if you are not sure of the
conditions of the diodes, it is recommended replace them.
6.6.3
Diode Replacement
In order to replace the diodes, proceed as follows:
Disconnect the six diodes from the exciter rotor winding;
Install three new diodes of the same polarity (AND or
CTD) in one of connecting bridges;
Install, on the other connecting bridge, three new diodes
with polarity opposite to that of the three diodes
previously installed;
Fix all the diodes, tightening them with a torque wrench,
observing the torques of Table 6-1;
Make the connections of the diodes with the exciter
rotor winding.
ATTENTION
It is vital that the tightening torques indicated be
observed so that the diodes will not be damaged
in the assembly.
Table 6-1 - Tightening torque of the diodes
Thread of the
Torque wrench
diode base
number (mm)
(mm)
M6
M8
M12
M16
6.6.4
Varistor test
The varistor is the device installed between the two diode
connecting bridges and is intended to protect the diodes
against overvoltage.
To test the operating conditions of the varistor, an
ohmmeter can be used. The resistance of a varistor must
be very high (± 20,000 ohm).
In case of damages to the varistor or if the resistance is
very low, it must be replaced.
30
l
Installation, operation and maintenance manual
Tightening
torque (mm)
11
2
17
4
24
10
32
30
6.6.5
Varistor replacement
In order to replace the varistor, WEG recommends that
you observe the following recommendations:
1. Replace the damaged varistor by a new varistor
identical to the original one;
2. In order to replace the varistor, loosen the screws that
fasten it to the diode connecting bridges ;
3. When removing the varistor, observe carefully how the
components were assembled so that the new varistor
will be installed the same way;
4. Before mounting the new varistor, make sure that all the
contact surfaces of the components are clean, leveled
and smooth so as to ensure a perfect contact between
them;
5. Fix the new varistor by tightening the screws that fasten
it to the connecting bridges just enough to make a
good electrical connection.
6.7 AIR FLOW
The air inputs and outputs of the alternator must be kept
clear, so that the heat exchange is efficient. If the heat
exchange is hindered, the alternator will overheat and the
winding may be damaged (burning of the alternator).
6.8 MAINTENANCE OF THE COOLING
SYSTEM
The tubes of the air-air heat exchanger (if applicable)
must be kept clean and clear to ensure a perfect heat
exchange. In order to remove the dirt accumulated in
the tubes, a rod with a round brush on the tip may be
used.
In case of air-water heat exchangers, periodic cleaning
in the radiator pipes is necessary in order to remove any
fouling.
NOTE
If the alternator is equipped with filters in the air
input and/or output, they must be cleaned with
compressed air.
If the dust is difficult to remove, wash the filter
with cold water and mild detergent and then dry
it in the horizontal position.
6.8.1
Maintenance of the radiators
If clean water is used, the radiator can remain in operation
for several years without the need for cleaning. With dirty
water, you need to clean it every 12 months.
The level of dirt in the radiator can be detected by the
increase in the air temperature in the output. When the
temperature of the cold air, under the same operating
conditions, exceeds the specified value, it can be
assumed that the pipes are dirty.
If corrosion is found, it is necessary to provide adequate
protection (i.e., zinc anodes, plastic cover with plastic,
epoxy or other similar protecting products) in order to
prevent greater damages to the parts already affected.
The external surface of all radiator parts must be always
kept in good condition.
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