5-15
1
CAUTION
Do not use sandpaper or emery cloth to remove burrs on
commutator. Otherwise, abrasive grit may remain on
commutator segments; this could lead to excessive
brush wear. Use only the recommended crocus cloth.
8. Check depth of mica on commutator. If undercut is less
than 0.008 in. (0.20 mm), use an undercutting machine
to undercut the mica to 1/32 in. (0.79 mm) deep. The
slots should then be cleaned to remove any dirt or cop-
per dust. See Figure 5-16. If an undercutting machine is
not available, undercutting can be done satisfactorily
using a thin hacksaw blade. After undercutting, lightly
sand the commutator with crocus cloth to remove any
burrs.
9. See Figure 5-17. Check for SHORTED ARMATURE with
a growler. Place armature on growler. Hold a thin steel
strip (hacksaw blade) against armature core and slowly
turn armature. A shorted armature will cause the steel
strip to vibrate and be attracted to the core. Replace
armatures if shorted.
10. See Figure 5-18. Check for a GROUNDED ARMATURE
with an ohmmeter or continuity tester. Touch one probe
to any commutator segment. Touch the other probe to
the armature core. There should be no continuity (infinite
ohms). If there is continuity, then the armature is
grounded. Replace grounded armatures.
11. See Figure 5-19. Check for OPEN ARMATURE with an
ohmmeter or continuity tester. Check for continuity
between all commutator segments. There should be con-
tinuity (0 ohms) at all test points. No continuity at any test
point indicates armature is open and must be replaced.
12. See Figure 5-20. Check for GROUNDED FIELD COIL
with an ohmmeter or continuity tester. Touch one probe
to the frame. Touch the other probe to each of the
brushes attached to the field coil. There should be no
continuity (infinite ohms). If there is any continuity at
either brush, then the field coil(s) are grounded and the
field frame must be replaced.
Figure 5-17. Shorted Armature Test Using Growler
1782a
Thin steel strip
Replace armatures which vibrate
Growler
Figure 5-18. Grounded Armature Test
Figure 5-19. Open Armature Test
Figure 5-20. Grounded Field Test
Commutator
1783a
Replace armatures showing continuity
Armature
core
1784a
Replace armature if no continuity at any test point
Commutator
Replace field frames if continuity is found
Brush on
field coil
3542a
Field frame
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