17
FIGURE 24. COMMUTATOR PROBLEMS
Copper wears rapidly at the edge of the commutator
bars.
Possible Causes:
Operation of the motor in dirty and abrasive
conditions.
Wrong type of brushes for this motor and operation.
6939
COPPER DRAG ON SURFACE
Under certain conditions copper is dragged over the
trailing edges of the commutator bars. It has the
appearance of small metal flakes or feathers. Copper
drag is caused by particles of copper, copper oxide and
contaminates that do not bond firmly to the surface of
the commutator. As the commutator rotates, the brushes
shove small flakes of this material toward or into the
commutator slots. This decreases the distance between
the bars. An arc formed at any bar can then be more
easily carried over to an adjacent bar resulting in a
flashover. A temporary solution of copper drag is to
clean the commutator occasionally with a light
brush–seater stone. If drag has progressed too far, the
slots must be raked and cleaned.
Possible causes of copper drag include:
a. Contaminated atmosphere.
b. Copper imbedded in brush face.
c. Hard spot in brush.
d. Wrong brush grade.
e. Excessive vibration.
f. Low current density.
OVERHEATING
Any condition which causes excessive heat to a small
area or the entire motor is detrimental.
Some of the motor’s final protests against neglect
and/or abuse are flashover between bars and discolored
insulation varnish.
CAUTION
Motor frame temperature above 121
°
C (250
°
F) can
damage internal components.
