Quantcast Direct current (DC) motors

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TM 5-683/NAVFAC MO-116/AFJMAN 32-1083
or quantity of lubricant recommended by the man-
(c) Handle bearings with clean, dry hands in
ufacturer should be followed. In all cases, standard
conjunction with a clean, lint-free rag. This will
lubricating practices should be followed.
limit the chance of corrosion due to perspiration.
(4) Sleeve bearings. Sleeve bearings (fig 4-9)
(d) Handle a reusable bearing as carefully as
are most often used in fractional horsepower mo-
a new one.
tors. For older types of sleeve bearings, the oil
(e) Use approved solvents and oils for flush-
should be drained, the bearing flushed, and new oil
ing and cleaning. Apply fire-preventive precautions
if the solvent or oil is flammable.
added at least every year. Newer sealed type sleeve
(f) Lay bearings out on clean paper. Keep
bearings require very little attention since the oil
bearings wrapped in oil-proof paper when not in
level is frequently the only check needed for years of
(g) Protect disassembled bearings from dirt
(5) Insulation. Failure of insulation is another
and moisture.
major factor in motor breakdowns. Few types of
(h) Do not spin uncleaned bearings. Rotate
insulation failures can be readily repaired. Insula-
them slowly while washing. A bearing should not be
tion internal to the motor should be visually
judged good until inspected after cleaning.
checked and defects further investigated. Heat is
(i) Do not spin any bearing with compressed
one of the principal causes of insulation failure in a
motor. Make sure that the motor has adequate ven-
(j) Soak bearings thoroughly in plenty of sol-
tilation and that air openings are not obstructed.
vent. Then rinse them in a separate clean container
Also make sure that the motor is not overloaded
of clean solvent. Once cleaned, inspect the bearing
which increases operating temperatures. Most mo-
surfaces for nicks or scratches;. broken or cracked
tors are equipped with thermal overload devices
rings, separators, balls, or rollers; and discolored,
applied directly to the motor winding which meas-
overheated bearings. If the bearing is to be reused
ure increases in temperature. At a predetermined
in a short time, dip it in rust preventive, wrap in
temperature, the overload device will trip and dis-
grease-proof paper and store. For longer storage,
connect the motor from the circuit. When an over-
coat all bearing surfaces with a light protective
load device has tripped, the operator should deter-
grease, wrap in grease-proof paper and store.
mine the cause of overheating, correct it if possible,
(k) Clean the inside of the housing before
and reset the overload before restarting the motor
replacing bearings.
(para 5-4e). An indication of the condition of the
(l) Keep bearings in their original carton un-
insulation can be determined by performing an in-
til ready for use if they are new. Do not wash the oil
sulation resistance test (para 4-5).
or grease out of a new bearing. Do not disassemble
4-3. Direct current (DC) motors.
new bearings.
(m) Install bearings properly after cleaning.
On military installations, DC motors (fig 4-10) are
(3) Ball and roller bearings. External inspec-
used only if AC voltage is not available or where
tion of ball and roller bearings (fig 4-7) at the time
there is a wide range of speed control desired. The
of greasing will determine whether the bearings are
reason for using a DC motor is often solely to
operating quietly and without undue heating.
achieve speed control. DC motor speed can be varied
Equipped with a grease chamber, they can be very
intentionally by varying the field current on shunt
easily overgreased. Overgreasing may be prevented
wound motors or by varying the input voltage to
by opening the grease relief plug (fig 4-8) after
either series or shunt motors. DC motors are classi-
greasing has been completed and running the mo-
fied into different types based on the connection of
tor. When excess grease has drained through the
the various windings. Shunt and series are consid-
relief plug, secure the plug.
ered the two basic types of motors, as all others are
derivatives of the two.
Since ball bearings are often sealed, they require
a. Shunt motors. The most widely used type of
little maintenance but it is very important that the
DC motor is the shunt wound motor. As the name
grease be kept clean. This also applies to sealed
implies, these machines have the armature and
housings (with the exception of permanently sealed
field circuits connected in parallel (shunt) to a con-
bearings) which should be cleaned and regreased
stant source of voltage (fig 4-11a). While the term
every 2 years or as recommended by the manufact-
"shunt" is still used, relatively few motors are now
urer. The bearing housings may be opened to check
applied in this way. Shunt motors as now applied
the condition of the bearings and lubricant. If the
have their field circuits excited by a source of power
lubricant must be changed, the bearing and housing
that is separate from the armature source of volt-
parts should be thoroughly cleaned and new lubri-
age. The field excitation voltage level is usually the
cant added. Special instructions regarding the type


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