DRESSTA TD-25M EXTRA, Service Manual

Page: 322 / 595

SECTION 8
ELECTRICAL
Page 46
TD-25M EXTRA DRESSTA
CRANKING MOTOR 42 MT
j. Armature Servicing
If the armature commutator is worn, dirty, out of round, or has high insulation, the armature should
be put in a lathe so the commutator can be turned down. As a final step in this procedure, the
commutator should be sanded lightly with No. 00 sandpaper.
The armature should be checked for opens, short circuits and grounds as follows:
k. Checking
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 the points where the conductors
are joined to the commutator bars for loose connections. Poor connections cause arcing and
burning of the commutator bars as the cranking motor is used. If the bars are not too badly
burned, repair can often be effected by resoldering or welding the leads in the riser bars (using
rosin flux), and turning down the commutator in a lathe to remove the burned material.
2. Short Circuits - Short circuits in the armature are located by use of a growler. When the
armature is revolved in the growler with a steel strip such as hacksaw blade held above it, the
blade will vibrate above the of the armature core in which the short circuit is located. Shorts
between bars are sometimes produced by brush dust or copper between the bars. These shorts
can be eliminated by cleaning out the slots.
3. Grounds - Grounds in the armature can be detected by the use of a 110-Volt test lamp and test
points. If the lamp lights when one test point is placed on the commutator with the other point on
the core or shaft, the armature is grounded. Grounds occur as a result of insulation failure which
is often brought about by overheating of the cranking motor produced by excessively long
cranking periods or by accumulation of brush dust between the commutator bars and the steel
commutator ring.
l. Field Coil Checks
The various types of circuits used are shown in the wiring diagrams of Fig. 8.15. The field coils can
be checked for grounds and opens by using a test lamp.
Grounds-if the motor has one or more coils normally connected to ground, the ground connections
must be disconnected during this check. Connect one lead of the 110-Volt test lamp to the field
frame and the other lead to the field connector. If the lamp lights, at least one field coil is grounded
which must be repaired or replaced. This check cannot be made if the ground connection cannot
be disconnected.
Opens - Connect test lamp leads to ends of field coils. If lamp does not light, the field coils are
open.