formations of scum cannot be removed with canvas, the use of a fine commutating stone lightly applied
may be required; this will permit the operator to touch up the commutator lightly and maintain true
surface.
A commutator having a smooth glaze of chestnut color is in satisfactory condition; a slight
burning often occurs at the edges of every few bars due to the method of winding to get high electrical
efficiency. No maintenance is required for this type of defect. However, if such spots are deep enough
to cause a brush to leave the commutator if the commutator is dirty or smudgy and presents a raised
surface, it may be necessary to smooth its surface and bring it back to concentricity by grinding. When
the commutator is concentric, its radii are all the same length. Eccentricity may be discovered by
clamping a dial gage on the frame of the generator or motor and rotating the armature by hand, one
commutator bar distance at a time. The dial gage measures, in thousandths of an inch, variations in
commutator radius. An eccentricity or difference of 0.001 inch, within a distance less than the brush
spacing around the commutator, may indicate the necessity of stoning. For bad burns and extreme
eccentricity, it may be necessary to turn the commutator on a lathe.
Blackening of the commutator may be caused by oil from the bearings or by surges of current
from stuck brushes. Leads from the armature windings are soldered into the ends of the commutator
bars. If the armature has been overheated by overloading and solder has been thrown out, the motor
should be replaced. Flat spots on the commutator can be caused by an unbalanced armature,
commutator out of round, commutator bars too high or too low, locomotive driving wheels worn on one
side, or flat spots on the wheel. Burned spots on the commutator may result when locomotive power
and brakes are applied simultaneously. Burned edges of a commutator segment indicate a winding or
cross connection open in the armature.
2.24. MAIN POWER SWITCHES
Main power switches must be inspected as follows to determine the need for any maintenance or
repair. Insure clearance of all moving parts and correct excessive play. Check contacts for proper
alinement, pressure, and wiping action wear, adjust when necessary. If contact tips are burned or pitted,
clean or replace them. Tighten electrical connections and contactor mounting, if necessary. Item 3 in
figure 2.14 shows the contactor mounting bolt. Check interlock fingers and bars for worn, burned, or
pitted areas. Keep magnet in good working order.
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