Onan MJA Series Service Manual page 113

REVOLVING ARMATUHE
5.2 ADJUSTIVIENTS
The engine governor controls generator output frequency. At the same
time, engine speed determines generator output voltage, so voltage can
be adjusted over a small range adjusting the governor mechanism.
The voltage drop from no load to full load operation is determined
by
the engine governor sensitivity ..
To adjust output voltage slightly, within the range shown in Fig.
5.
D,
adjust the governor, Sect 3.4.2. This also changes the plant output fre­
quency' so be sure tlie the frequency stays within the maximum and mini
mum limits shown in Fig. 3. 4.B for revolvingarmature generators.
The voltage drop from no load to full load can be adjusted to within the
range shown in Fig. 5. D by adjusting the governor sensitivity.
GEN~1tATUt{ UL
Nominal
output
Voltage
Max - (at
no load)
Voltage
Min - (at
:full load)
Preferred
voltage spread
no load to full load
Preferred
drop
no load to full load
120
240
120/240
240
120/208
126.
252
126
252
228
no
22.0
110
220
198
4
8
4
8
8
118-114
236-228
118-114
236-228
214-206
FIG. 5.D VOLTAGE MAXIMUMS, MINIMUMS AND VOLT­
AGE DROPS
5.3 TESTING AND REPAIR
Most of the following tests can be performed without disassembling the
generator.
5.3.1 ARMATURE TESTING. - Before testing remove all brushes from
their holders.
1. Using a test lamp, Fig. 4.J, or ohmmeter, check the ac winding
for an open circuit between the slip rings.
If
an open circuit is
found, replace the armature.
2. Test both the slip rings and commutator for grouriding to the shaft.
3. Test the armature for an open circuit in the dc windings by check­
ingcontinuity between all adjacent bars of the commutator.
Touch the probes to 2 adjacent bars and check for continuity. Move
each probe over 1 bar and again check. Continue around the commu
tator. Adjacent bars that don't show continuity indicate an open arm­
ature winding. .