Onan NHC Series Operator's/Service Manual And Parts Catalog page 236

INSPECTION OF PARTS
1. Testing Armature for Short Circuits. Place the
armature in a- growler and hold a thin steel blade
parallel to the core and just above it while slowly
rotating the armature in the growler. A shorted
armature will cause the blade to vibrate and be
attracted to the core. Replace shorted armature.
See Figure 34.
FIGURE 34. ARMATURE SHORT CIRCUIT TEST
2. Testing Armature for Grounds. Touch armature
shaft or core and the end of each commutator bar
with a pair of ohmmeter leads. If the ohmmeter
reading is low, it indicates a grounded armature.
Replace grounded armature. See Figure 35.
F I G U R E 35. ARMATURE GROUND T E S T
3.
4.
Testing Armature for Open Circuit. The most
likely place for an open circuit to occur is at the
commutator riser bars. Inspect the points where
the conductors are joined to the commutator bars
for loose connections.
Testing Commutator Runout. Place armature in a
test bench and check runout with a dial indicator.
When commutator runout exceeds .004", commuta-
tor should be refaced (Figure 36).
FIGURE 36. CHECKING COMMUTATOR RUNOUT
5. Testing Armature Shaft Runout. The armature shaft
as well as the commutator may be checked. A bent
armature often may be straightened, but if the
shaft is worn, a new armature is required (Figure
37).
FIGURE 37. CHECKING ARMATURE SHAFT RUNOUT
6. Testing Field Coils for Grounds. Place one lead
on the connector and the other on a clean spot
on the frame after unsoldering shunt field coil
wire. If the ohmmeter reading is low, the fields
are grounded, either at the connector or in the
windings (Figure 38).
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