TP-6137 5/0354 Section 9 Generator Troubleshooting
Procedure to test the rotor’s resistance.
1. To test the rotor resistance, use a Wheatstone
bridge. See Figure 9-5. See Figure 9-6 for
resistance values.
Figure 9-5 Rotor Resistance Test
Model Resistance
in Ohms at 20_ C(68_ F)
4EOZ (60 Hz) 2.57 ±10%
3.5EFOZ (50 Hz) 2.57 ±10%
Figure 9-6 Rotor Resistance Values
9.5 Field Diode Test
Check the rotor’s field diodes using a multimeter.
Disconnect the field diodes from the rotor and check the
forward and reverse resistance. A good diode
performance is indicated with a very high resistance in
the reverse direction and a low resistance in the forward
direction. See Figure 9-7. A failed diode gives a full
deflection reading in both directions with the ohmmeter
on 10,000 Ohms or an infinity reading in both directions.
Diode
Positive
Varistor
Figure 9-7 Field Diodes and Varistors Test
Failed Diodes Replacement
The rectifier assemblies are two sets of diodes and two
varistors, one for each rotor winding. Take care to
ensure that each diode is installed with the correct
polarity:
1. Check that the arrow on the diode surface direct
towards the fixing plate.
2. Check that the wire end with the eye terminal is
located between the diode and the fixing plate. The
other winding end must be welded at the upper part
of the diode.
3. While fitting the diodes at the plate, carefully
tighten the nuts to ensure good mechanical and
electrical contacts.
Surge Suppressor
The surge suppressor is a metal-oxide varistor shunted
to the field diodes to prevent high transient reverse
voltages in the field windings thus damaging the diodes.
The varistor is not polarized and shows, on a standard
ohmmeter, a virtually infinitive reading in both directions.
See Figure 9-7. If it is defective, normally a visible
inspection shows burning signs, it gives a full deflection
reading (short circuit).
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