Generator And Exciter Test; Diode Test - Hobart 6921 Series Operation And Maintenance Manual

OM-2040 / Operation and Maintenance Manual
90D20 / Generator Sets

4. Generator and Exciter Test

The generator fields may be tested with a "Kelvin" bridge. This is a double bridge type instrument
required for the very low resistances encountered in this test. It is understood that 0 resistance
indicates a SHORT CIRCUIT condition. An infinite resistance reading indicates an OPEN CIRCUITED
condition. See Fig. 9 below for resistance values.
NOTE: The equipment should be COLD or at normal ambient temperature when tests are made.
a. Disconnect generator stator leads at the power module panel.
b. Disconnect exciter leads from terminal board mounted on rear of engine control panel.
Positive lead is RED. Negative lead is YELLOW-BLACK.
c. Check resistance and compare to values given in Fig. 9.
TEST CONNECTION
GENERATOR STATOR
Phase A to N *
Phase B to N *
Phase C to N *
EXCITER A-B, B-C, A-C
Field (RED to YELLOW-BLACK)
ROTATING FIELD
Measure resistance at revolving field rectifier
(CR2) with exciter armature leads removed from
rectifier. (See Connection Diagram)

5. Diode Test

Test values for diodes are not given here because they could be misleading. Test values may vary
even between diodes of the same part number, rating, and manufacturer. General instructions for
testing diodes follows:
a. Disconnect diode lead(s).
CAUTION
NOTE: For units with 10 ohm, 100 watt ballast resistors (before Jan. 1976) voltage is 30-40 V DC.
b. Use a good quality ohmmeter. An instrument which indicates 50 ohms at the center of the
scale is preferable.
NOTE: The battery must be in good condition and the pointer adjusted to zero when the test lead points are
shorted together. Set the scale selector to RX1 and "zero" the ohmmeter before testing diodes.
February 28/94 Revised
Generator and Exciter Stator Test Values
o
(70 F Ambient Temperature)
Figure 9
The regulator output must never be grounded when the generator is turning.
NOMINAL RESISTANCE IN OHMS
0.00827
0.00835
0.00840
0.1
7.23
2.19
Chapter 2-3
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