TROUBLE SHOOTING
EN / M-SQ20 / 25 / November 2010
33
5.3.2
Residual voltage check / excitation
procedure (flashing)
When residual magnetism disappears there is no residual
voltage. Residual magnetism can disappear after the
generating set being out of service for a long period or a
short circuit. This can be solved by exciting the exciter field
coil directly, disconnecting the AVR first. Put 12 Volt
directly over the electronic regulator terminals taking
polarity into account.
Fig.25 Residual voltage check / excitation procedure.
When using the starter battery fuse it and limit the current
with a 30 Ohm resistor. Flashing can be done while the
engine is running and the wiring is connected. When a
variac (variable power supply) is available one should be
able to control the voltage manually.
CAUTION
If the starter set battery is used for exciting
one must take care. A short circuit can cause
heavy sparking, fire and injuries.
When external excitation does not bring back voltage, the
rotating rectifier diodes should be tested and a winding
resistant test should be executed.
5.3.3
Testing rotary rectifier diodes
The diodes in the rotor can be checked with a multimeter.
The diodes are integrated in a rectifier unit. When all
diodes are faulty the alternator will not generate any
voltage. When one or more diodes are faulty the alternator
will produce too low voltage.
The flexible leads connected to the diodes should be
disconnected at the terminal end, and the forward and
reverse resistance checked. A healthy diode will indicate a
very high resistance (infinity) in the reverse direction, and
a low resistance in the forward direction. A faulty diode will
give a full deflection reading in both directions with the test
meter on the 10,000 ohms scale, or an infinity reading in
both directions. In case of faulty diodes replace the
rectifier unit. One can get access to the diode unit by
taking off the backend of the alternator.
Fig. 26. Location rectifier diode block
5.3.4
Winding resistant values
If after establishing and correcting any fault on the AVR
and diodes, and output is still low when separately excited,
then the main rotor, stator auxiliary and exciter stator
winding resistance should be checked as the fault must be
in one of these windings. The respective leads must be
disconnected before taking the readings.
Resistance values should be within 10% of the values
given in the table below:
Resistance M-SQ20 / 25
•
Resistance main stator windings
0.33 Ohm
•
Resistance rotor windings
1.18 Ohm
•
Resistance auxiliary winding
3.85 Ohm
•
Resistance exciter stator winding
18 Ohm
•
Resistance exciter rotor winding
0.18 Ohm
