TM 55-6115-491-40
lead on one commutator bar riser and then to one slip
3-15.
REPAIR AND REPLACEMENT.
ring. If the lamp lights, the rotor is grounded and must
3-16.
Rotor.
be replaced.
a. Test the rotor for grounds with a test lamp
by placing one lead on the rotor shaft and the other
Note
Use a 60-watt test lamp in series with 110 volts for test purposes. If a 110-volt source is not
available. 220 volts may be used with two 110-volt lamps connected in series.
Do not use a power source greater than 220 volts.
b. Test the exciter armature in a growler for
found, the rotor should be replaced after making sure
that the fault is definitely in the coil insulation.
short or open circuits. If any electrical faults are
Note
A shorted or open exciter armature coil may have intermittently burned commutator bars, in
which case the rotor must be replaced. If a whole series of consecutive bars is burned, the
trouble may be due to eccentricity, oil or grease on commutator, worn or sticky brushes, or
rough bearings; refinish the commutator in any of these instances. Particles of carbon dust
clogging the undercut mica spaces can cause armature short circuits; therefore, be sure the
spaces can cause armature short circuits; therefore, be sure the commutator has been blown
clean with compressed air prior to testing.
c. To check dielectric strength of rotor
c. To recondition commutator to within
insulation, apply 625 volts (RMS) for one second
tolerance limits, work must be performed on a good lathe
between each of the slip rings and ground, and between
accurately set up and adjusted. The commutator end of
the commutator and ground. There shall be no
rotor shaft should be supported on a bearing which can
indication of insulation breakdown.
be used as a master by lapping the inner race bore to
3-17. Refinishing Commutator.
obtain a slip fit on rotor shaft. Bearing should be
a. With bearings on the rotor shaft, install in a
supported in tail stock of lathe. Splined end of rotor
should be driven by a suitable collet or held on a center
lathe or between V-blocks on a surface plate to measure
in head stock and driven with a lathe dog.
eccentricity. Using a dial indicator, adjust gage to zero
d. Operate lathe at a speed of 500 to 600
after placing gage pointer in contact with the commutator
RPM and, using a suitable tool, take lightest cut possible
surface.
b. Rotate the rotor and check the dial indicator
with a fine feed. If the first operation fails to clean up the
entire commutator surface, take subsequent light cuts
for total reading through one full revolution. If
until the surface has been completely reconditioned.
eccentricity exceeds 0.001 inch, refinish the commutator
as described below. Bar-to-bar readings should not
differ more than 0.0002 inch.
Caution
The outside diameter of the commutator, when new, is 2.875 inches. When repeated
turning has reduced this diameter to less than 2.750 inches, rotor must be replaced.
During cutting operations, make certain that the previously installed commutator end
bearing is covered with masking tape to prevent foreign material from entering the
bearing.
e. After commutator has been turned, the
f.
After undercutting the mica, sand the
mica between the copper segments must be undercut
commutator to obtain a satin finish (25-40 microinches).
0.030 inch wide by 0.030 inch deep. Use a sharp
Use No. 5/0 sandpaper backed up with a block holder
undercutting tool and avoid any contact with the
having a radius slightly greater than the commutator and
commutator bars which would necessitate refinishing or
covering the full width of the surface. Then check for
reconditioning commutator because of scratches or
concentricity as described in steps a and b above.
gouges. Using a triangular scraping tool, remove all
burrs and sharp edges from the commutator bars.
3-13
