test lamp and test points.
If the lamp
they should be replaced. Make sure the
lights when one test point is placed on
brush holders are clean and the brushes
the commutator with the other point on
are not binding in the holders. The full
the core or shaft,
brush surface should ride on the commu-
the armature is
tator to give proper performance. Check
grounded.
Grounds occur as a result of
by hand to insure that the brush springs
insulation failure which is often brought
are giving firm contact between the
about by overheating of the cranking mo-
brushes and commutator. If the springs
tor
produced by
excessively
long
are distorted or discolored they should
cranking periods or by accumulation of
be replaced.
brush dust between the commutator bars
and the steel commutator ring.
Armature Servicing
F i e l d Coil Checks
If the armature commutator is worn, dir-
ty, out of round, or has high insulation,
The internal circuit of the field coil is
the armature should be put in a lathe so
The field coils
the commutator can be turned down.
can be checked for grounds and opens
The insulation should then be undercut
by using a test lamp.
1/32 of an inch wide and 1/32 of an inch
deep, a n d t h e s l o t s c l e a n e d o u t t o r e -
move any -trace of dirt or copper dust.
As a final step in this procedure, the
commutator should be sanded lightly with
No. 00 sandpaper to remove any burrs
left as a result o f t h e u n d e r c u t t i n g p r o -
cedure.
The armature should be checked for
opens, short circuits and grounds as fol-
lows:
1. OPENS-Opens are usually caused by
excessively long cranking periods, The
most likely place for an open to occur is
at the commutator riser bars.
Inspect
t h e p o i n t s w h e r e t h e c o n d u c t o r s are
joined to the commutator bars for loose
connections. Poor connections cause arc-
ing and burning of the commutator bars
as the cranking motor is used.
Figure 10D-4. Internal Motor Circuit
If the
bars are not too badly burned, repair
can be effected by resoldering or weld-
GROUNDS-If the motor has one or more
ing the leads in the riser bars (using
coils normally connected to ground, the
rosin flux), and turning down the com-
ground connections must be disconnected
mutator in a lathe to remove the burned
d u r i n g t h i s c h e c k . Connect one lead of
material.
The insulation should then be
the, 110 volt test lamp to the field frame
undercut except as noted above.
and the other lead to the field connector.
If the lamp lights, at least one field coil
2. SHORT CIRCUITS-Short circuits in
is grounded, which must be repaired or
the armature are located by use of a
replaced.
This check cannot be made if
the ground connection cannot be discon-
growler. When the armature is revolved
in a growler with a steel strip such as a
nected.
hacksaw blade held above it, the blade
will vibrate above the area of the arma-
O P E N S - C o n n e c t t e s t Ia m p l e a d s t o e n d s
ture core in which the short circuit is
o f f i e l d c o i l s . I f lamp d o e s n o t l i g h t , t h e
located.
Shorts between bars are some-
field coils are open.
times produced by brush dust or copper
between the bars. These shorts can be
eliminated by cleaning out the slots.
F i e l d Coil Removal
Field coils can be removed from the field
3. GROUNDS-Grounds in the armature
can be detected by the use of a 110 volt
frame assembly by using a pole shoe
Electrical Components