Testing Rotary Rectifier Diodes; Winding Resistant Values; Meggering - Whisper Power W-SQ-Pro User Manual

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.
EN / W-SQ-Pro 15/10.8 / W-SQ-Pro 18/15,5 – Three phase for mobile applications / February 2020
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
• Resistance rotor windings
• Resistance auxiliary winding
• Resistance exciter stator winding
• Resistance exciter rotor winding
Alternator models with 12 wires
• Resistance main stator windings
• Resistance rotor windings
• Resistance auxiliary winding
• Resistance exciter stator winding
• Resistance exciter rotor winding
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
outlets.
OPERATION
0.39 Ohm
2.4 Ohm
7.5 Ohm
24 Ohm
3x 0.32 Ohm
0.33 Ohm
1.18 Ohm
3.85 Ohm
18 Ohm
0.18 Ohm
39