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ELECTRICAL SYSTEMS
CRANKING SYSTEM
10-53
ammeter capable of reading several hundred
amperes. A voltmeter is connected between the
insulated motor terminal and ground on the
frame at a point free of grease and paint. A
speed indicator should also be used to measure
the armature revolutions per minute.
The torque test requires equipment such as
shown in figure 10-63. The cranking motor is
securely mounted and the brake arm hooked to
the drive pinion. With a one-foot brake arm as
shown, the torque will be indicated directly on
the scale in pounds feet when the specified volt-
age is applied to the motor. Specified voltage is
applied through the use of a high-current-
carrying variable resistance.
An armature growler and a test lamp with
pointed prods on the leads are also required
for checking internal condition of armature
and the field windings.
b. Interpreting Results of Tests
Test specifications are given under Electrical
Specifications (par. 10-3). The specifications
are given at low voltages so that torque and
ammeter readings obtained will be within the
range of testing equipment available in the field.
1.
Rated torque, current draw and no-load
speed indicates normal condition of cranking
motor.
2. Low free
speed
and high current draw
with low develop
ed torque
may result from:
(a)
Tight,
dirty, or worn bearings, bent
armature shaft or loose field pole screws which
would allow the armature to drag.
(b) Shorted armature. Check armature fur-
ther on growler (par. 10-28,
b).
(c) A grounded armature or field. Check by
raising the grounded brushes and insulating
them from the commutator with cardboard, and
then checking with a test lamp between the in-
sulated terminal and the frame.
If
lamp lights,
raise other brushes from commutator and check
fields and commutator separately to determine
whether it is the fields or armature that is
grounded.
3.
Failure
to
operate
with high current draw
may
result from:
(a)
A direct ground in the terminal or fields.
(b) Frozen shaft bearings which prevent the
armature from turning.
4. Failure to operate with no current draw
may result from:
(a) Open field circuit. Inspect internal con-
nections and trace circuit with test lamp.
(b) Open armature coils. Inspect the com-
mutator for badly burned bars.
(c) Broken or weakened brush springs, worn
brushes, high mica on the commutator, or other
causes which would prevent good contact be-
tween the brushes and commutator. Any of
these conditions will cause burned commutator
bars.
5. Low no-load
speed
with low torque and
low current draw indicates:
(a) An open field winding. Raise and in-
sulate ungrounded brushes from commutator
and check field with test lamp.
(b) High internal resistance due to poor
connections, defective leads, dirty commutator
and causes listed under item 4 (c). Running
free speed, an open armature will show exces-
sive arcing at the commutator bar which is
open.
.
6. High
free speed
with low
developed
torque and high current draw indicates shorted
fields. There is no easy way to detect shorted
fields, since the field resistance is already low.
If
shorted fields are suspected, replace the fields
and check for improvement in performance.
10-41 CRANKING MOTOR REPAIRS-
ON BENCH
a. Disassembly, Cleaning, and Inspection
When it is necessary to disassemble cranking
motor for any reason, make a complete clean
up and inspection to make sure all parts are in
satisfactory condition. See figure 10-64 for
identification of parts.
1. Disconnect plunger from shift lever by
removing adjusting screw link pin. Remove
solenoid switch.
2. Remove commutator cover band and dis-
connect brush leads from the field leads.
3. Unscrew the through bolts and separate
the commutator end frame, field frame, and
drive housing.
4. On Series 70 cranking motor, remove the
center bearing plate and thrust washer from
drive housing.
5. Remove
shift lever and spring, then re-
move drive assembly from drive housing.
6.
If
field coils are to be removed from field
frame, a pole shoe spreader (KMO 579) and
pole shoe screwdriver should be used to avoid
distortion of frame. See figure 10-65.
7. Clean all parts by wiping with clean
cloths. The armature, field coils, and drive as-
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