Onan NHC Series Operator's/Service Manual And Parts Catalog page 301

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 40.
3 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.
4 Testing Commutator Runout. Place armature in a
test bench and check runout with a dial indicator.
When commutator runout exceeds .004", com-
mutator should be refaced (Figure 42).
FIGURE 40. ARMATURE SHORT CIRCUIT TEST
FIGURE 42. CHECKING COMMUTATOR RUNOUT
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 41.
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 43).
FIGURE 4 1 . ARMATURE GROUND TEST
FIGURE 43. CHECKING ARMATURE SHAFT RUNOUT
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