TM 5-683/NAVAC MO-116/AFJMAN 32-1083
laminated steel poles, furnish excitation for the mo-
tor. Inspect field winding insulation and determine
] [ F2]
if they are dirty and oil-soaked. Check for malfunc-
tioning controls which cause excessive field current
that can cause excessive heating and failure. Other
causes of winding heating are excessive voltage, in-
sufficient speed, off-neutral brushes, overloads and
partial short circuit in a field coil. Never run a DC
motor with the field circuit open. If the field wind-
ing is open-circuited, the motor will fail to start or
will operate at excessive speeds at light loads, and
excessive sparking will occur at the commutator.
Check the insulation resistance of the windings
(2) Brushes. Maintenance and inspection of
brushes should be performed regularly to ensure
the following (fig 4-13):
(a) Brushes are not loose in their holders.
R e p l a c e worn brushes. NOTE: Replacement
brushes are of several types and grades. Successful
motor operation depends upon proper selection of
the replacement brush best suited for the service
requirements of the motor.
(b) Brushes are not sticking in their holders.
Clean brushes and holder.
(c) The tools and/or heels of the brush face are
not chipped or cracked. Replace brush if damaged.
(d) Brush shunt leads are properly attached
to the brushes and their holders. Replace brush if
shunt lead is loose at the brush. Tighten if lead is
loose at the holder.
(e) Correct brush tension is maintained. Re-
adjust the brush spring pressure in accordance with
the manufacturers' instructions when adjustment is
provided. When adjustment is not provided, replace
(f) Brush holder studs are not loose. Tighten,
(g) Brushes are not discolored. Brushes
should have a highly glazed or very dull finish.
Figure 411. Main Types and Connections of DC Motors: a)
Clean the brushes when they become black or grey.
Shunt motor b) Series motor c) Compound motor
(h) Reset brushes at the correct angle.
(i) Reset brushes in the neutral plane.
d. Components of DC machines. The basic operat-
(j) Properly space brushes on the commuta-
ing and maintenance requirements for DC motors
tor to 1/32 inch.
are similar to those for AC motors. There are some
(k) Correctly stagger the brush holders.
special requirements due to the peculiar construc-
(l) Properly space brush holders from the
tion features of the DC motor. The recommenda-
commutator (usually l/16 to 3/16 inch.). If the
tions that follow, particularly those for the armat-
proper spacing is not maintained, the brushes will
ure (fig 412), also apply to AC synchronous
ride the surface of the commutator poorly. This is
motors, It is important that the armature be kept
especially applicable to the motor using an inclined
clean. Dust, grease, corrosive gases, moisture and
brush holder since the brush will be shifted to the
oil are particularly harmful.
neutral position, leading to poor commutation.
(m) Check to ensure that the correct grade of
(1) Field windings. The field is made up of a
brush, as recommended by the manufacturer, is be-
frame with field poles fastened to the frame's inner
circumference. The field windings, mounted on