Quantcast Figure 4-14. Cutaway section of a commutator

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TM 5-683/NAVFAC MO-116/AFJMAN 32-1083
(3) Commutators. A cutaway section of a com-
the commutator. It results when copper particles
mutator is shown in figure 4-14. The primary
transfer to the face of the brush. These particles cut
source of unsatisfactory commutation is due to
through the commutator film, creating areas which
faulty operating conditions. Therefore, the principal
carry more than their share of current. They also
maintenance function is that of keeping the commu-
cause rapid wearing of the brushes and lead to com-
tator surface clean, concentric, smooth, and prop-
mutator resurfacing. Use of natural graphite
erly undercut. Surfaces of the commutator bars
brushes, which have cleaning action, cuts down on
must be even and free of ridges so that the commu-
the formation of film that hinders passing current.
tation is concentric, thus allowing any object held
Maintenance personnel should check for any evi-
against it to react as though it were held against a
dence of the lack of uniformity in brush action on
smooth surface cylinder. Inspection and mainte-
the commutator surface and film. Evidence of such
nance methods are as follows:
action should be corrected as soon as possible.
(a) Check for indication of brush chatter (fig
(c) Check for sparking. Sparking often re-
4-15). Brush chatter is most evident by chipped
sults from poor commutation. This condition may be
brushes. This condition results from either a poor
improved by repositioning the brushes so that they
commutator surface or high friction between the
are slightly against rotation. When repositioning
brush and commutator. High friction is normally
the brushes that do not provide better commuta-
caused by operating the motor under a light load,
tion, the commutating-pole air gap must be ad-
operating the motor under a load that exceeds the
commutator's rated capacity for prolonged periods
or, film build-up on the commutator. Remedies
(d) Check for flashing. Flashing normally re-
are: increase the load; reduce the load; and, clean
sults from conditions that cause a sudden change in
the commutator respectively.
the field strength of the motor's current or voltage.
(b) Check for threading or streaking (fig
Flashing can be prevented by frequent checks and
4-16). Threading or streaking of commutator sur-
elimination of conditions that contribute to short-
faces is characterized by fine lines inscribed around
Figure 4-14. Cutaway section of a commutator.


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