To prevent centrifugal force from throwing the coils
Placing Coils in Slots
outward, wind a band of high-grade, steel piano wire on
a strip of Lest.kroid placed around the armature and
Before assembling the coils, insulate the armature
over the coils about 2 inches from the edge of the core.
core. This step is extremely important if the armature
Do this before the armature has been dipped and after
contacts the coils, you will have to do your work over.
prebaking. Banding wires should be placed on the
Clean the core slots and ends and true up the
armature windings while hot because then the wire is
laminations. Use polyamide paper, and let it extend
more flexible and can be pulled tighter. When the first
one-fourth inch beyond the slots to prevent the edges of
banding wire is wound on the armature, small tin clips,
the laminations from injuring the coils.
with insulation under them, are inserted under the wire.
The ground insulation consisting of flexible mica
When the required number of turns has been applied,
wrappers or layers of reinforced mica tape, is applied to
the ends of these clips are bent over and hold the wires
the coil sides that lie in the slots. Next, the formed coils
tightly side by side. The clips are then soldered with tin
are placed in the slots; the lower side first and then the
solder, and a thin coat of solder is run over the entire
upper side, until all the coils are inserted and the winding
band to secure the wires together.
is completed. Be certain that the coil pitch is correct. A
The end windings are secured, if necessary, by
strip of rigid laminate, type GME-MIL-P-15037, is
groups of wire wound on insulating hoods to protect the
placed in each slot between the lower and upper coil
coils. On the commutator end, strips of thin mica with
sides. A similar strip is placed at the back and front of
overlapping ends are usually placed on the commutator
the armature where top and bottom sides cross each
neck and held by a few turns of cord. On large
other. If the slots have straight sides, they are filled up
armatures, banding wires are sometimes placed over the
with a strip of rigid laminate, type GME-MIL-P-15037,
laminated portion of the armature. The laminations on
on the tops of the coils so they can be held down by the
these armatures have notches in which the banding wire
banding wires. In some armatures the slots are shaped
is placed
so that fiber wedges can be driven in each slot from one
end to hold the coils in place.
If you have to rebuild a large commutator, use
molded mica to insulate between the spider and the
Before soldering the coil ends to the commutator
commutator. Commutator mica is used as insulation
segments, test the winding for grounds, opens, and
between the segments. After the commutator is
shorts. When soldering, exercise care to prevent the
assembled, it is heated and tightened with a clamping
solder from falling or running down the back of the
ring.
commutator. This could result in a short circuit. Tip the
If shrink rings are provided, they are not put on until
armature so that the solder will not flow to the back of
the commutator has been tightened (while hot) and the
the commutator. Place the tip of the soldering iron on
banding wires tightly placed around it. If defective,
the commutator near the riser and wait until the iron
small commutators are usually completely replaced.
heats the riser sufficiently to melt the solder. Touch the
solder to the riser and allow it to flow around the lead
REWINDING FIELD COILS
and into the wire slot, and then remove the iron.
An ordinary soldering iron cannot supply sufficient
Remove the old field coil from the pole piece and,
heat fast enough to perform a satisfactory soldering job
if spare coils are available, install a new one. If a new
on a large armature. Therefore, apply a soft flame from
coil must be made, record all pertinent coil data as the
an acetylene torch to the outside end of the commutator
old coil is stripped.
segments to the riser ends where connections are made.
This data should include the following information:
Tin the coil ends that will be connected to the
commutator risers with a soldering iron. Next, tin the
1. The dimensions of the coil, both with the tape
slots in the commutator risers with heat from the torch.
on and with the tape removed
Then make the connections while applying the flame to
2. The weight of the coil without the tape
the outside end of the commutator segments. When
making the commutator connections, wrap the winding
3. The size of wire
with the proper tape for protection. Too much heat can
4. The type of insulation
damage the winding insulation. The completed
The two general classes of coils are shunt field coils
armature winding is checked electrically for continuity
and series and commutation field coils. Shunt field coils
and for shorted turns.
7-37
