TROUBLE SHOOTING
EN / WHISPER 12 Ultra - Three phase for mobile applications / December 2008 / Copyright © 2008 Mastervolt
37
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 cover of the alternator.
Fig.27: Rotating rectifier diodes.
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:
Alternator models with 6 wires
•
Resistance main stator windings
0.39 Ohm
•
Resistance rotor windings
2.4 Ohm
•
Resistance auxiliary winding
7.5 Ohm
•
Resistance exciter stator winding
24 Ohm
•
Resistance exciter rotor winding
3x 0.32 Ohm
Alternator models with 12 wires
•
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
5.3.5 Meggering
One can try to measure resistance between the housing
and the windings with a multimeter which should read
infinity. When readings are infinity but a fault is suspected
one can do a high voltage resistance test (MEGGERING)
This procedure should be carried out by an expert. The
AVR should be disconnected during this test. A 500V
‘Megger’ or similar instrument should be used. The
insulation resistance to earth of all windings should be in
excess of 2 MOhm. Should the insulation resistance be
less than this value, drying out the generator windings is
essential. Drying out can be carried out by direct warm air
(60-80ºC) from a fan heater or similar apparatus into the
generator air inlets or outletst
