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 TM 5-683/NAVFAC MO-116/AFJMAN 32-1083
rotor bars. The symptoms of such conditions are
slowing down under load and reduced starting
torque. Repairs to cast aluminum rotors with open
bars are not feasible and such rotors have to be
replaced. Copper bar rotors can usually be repaired
by rebrazing the joints.
(2) Bearings. The bearings are the most critical
mechanical part of a motor. To assure maximum
life, bearings should be subjected to careful inspec-
tion at scheduled intervals. The frequency of inspec-
tion is best determined by a study of the particular
motor operating conditions. Bearings are subject to
metal fatigue and will eventually wear out even
though they are correctly applied, installed and
maintained. Fatigue failures are characterized by
flaking of the race surfaces along the ball or roller.
Fatigue is a gradual process, which is dependent
upon load and speed and usually is made apparent
in its early stages by an increase in the operating
temperature, vibration or noise level of the bearing.
Bearing failures not attributed to fatigue failure are
usually classified as premature. The majority of
these premature failures are caused by the fol-
Figure 4-5 Cleaning and drying motors in place.
lowing: incorrect bearing type; misalignment of the
motor or load; misalignment or improperly installed
winding. However, there is some hazard since the
bearing; rusting during storage; preloading or im-
heat generated in the inner parts is not readily
proper end-play adjustment; excessive thrust or ra-
dissipated. This method should be followed only un-
dial force; axial indentations; improper lubrication
der competent supervision. For synchronous mo-
and entrance of contaminants into bearing. Follow
tors, the "short circuit method" is sometimes used.
manufacturer's instructions and use lubricants as
This method is achieved by shorting the armature
specified. When using greases, store in clean con-
windings, driving the rotor and applying sufficient
tainers, handle with clean paddles or use a grease
field excitation to give somewhat less than full load
gun and keep containers covered. Do not overfill
armature current. Once the drying process has been
bearing housings. Overfilling contributes to heat
completed, insulation testing of the motor winding
is recommended to determine whether the insula-
build-up, damaged seals, leaks and collecting of
tion has been properly reconditioned. If a motor
dirt. Overheating is particularly true of bearings
running at high speeds. Machines that are normally
must continue to operate in a damp environment,
idle for long periods should be exercised on a sched-
then special enclosures are necessary to limit the
uled basis. Exercising will keep the oil circulated,
effects of a moist atmosphere.
reduce condensation within the housing and lessen
(c) Inspecting motor windings. Check wind-
ing tightness in the slots or on the pole pieces. One
the chances of flat spots developing on the bearing
condition which hastens winding failure is move-
races, balls or rollers. Inspect seals and vents regu-
ment of the coils due to vibration during operation.
larly. Periodic maintenance services of bearings in-
Check insulation surfaces for cracks, crazing, flak-
volves keeping a bearing dry, covered, clean and
ing, powdering, or other evidence of need to renew
lubricated as well as checking operating tempera-
insulation. Usually under these conditions, when
ture. A clean bearing is particularly critical because
the winding is still tight in the slots, a coat or two of
dirt means damage. It is therefore important to
air-drying varnish may restore the insulation to a
remember the following when cleaning bearings (fig
safe value. Check the winding mechanical supports
4-6):
for insulation quality and tightness, the binding
(a) Work with clean tools in clean surround-
ring on the stator windings, and the glass or wire-
ings. Do not use wooden mallets, dirty, chipped or
wound bands on rotating windings. E x a m i n e
brittle tools, or work on rough or dirty bench tops.
squirrel-cage rotors for excessive heating or discol-
(b) Remove all dirt from the housing before
ored rotor bars which may indicate open circuits or
exposing the bearings and take care to prevent loose
high resistance points between the end rings and
dirt from getting into the housing.
4-11
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