Get a Starter Motor Runnin’ in Your Robot
field and armature and how the two brushes are welded
to the field. Note the complete lack of wear on this 1989
starter motor that was removed from a junk car.
The modification requires that the field be isolated
from the armature so its polarity can be reversed. Photos 6
and 7 show a close-up of the spot welds connecting the
field to the brushes and how the welds can be pulled apart.
The welds are made by wrapping the stranded wires with
copper foil and applying heat and pressure to finish it.
The welds are not very strong and the foil can be
unwrapped as shown.
The next step is to attach wires to the field so that
both ends of the field windings are outside the case. Since
there is very little room in the case, the connections have to
be very compact and neat. Also, solder connections will not
hold up to the heat and vibration of the motor. Therefore,
only a spot weld or crimp connection will work. For the
crimp connection, brass rings made for 3/16” compression
fittings can be purchased at any plumbing store.
While in use, the motor will get very hot and any
plastic tape or plastic wire insulation will fail. High
temperature automotive wire with rubber insulation and
cloth electrical tape are needed to make the modification.
Since the tape has little adhesion, it needs to be secured.
Cotton thread works well.
Photo 8 shows the wires spliced to the field and routed
out of the case. At the top center is the finished connection
while the bottom center shows an exposed connection.
The 3/16” compression sleeve accommodates the field wire
and 8 gauge wire.
The next step is to splice wires to the brushes so they
can be connected to the H-bridge. Photos 9 and 10 are 8
gauge wires spot-welded to the brushes and the brush
holder with all four brushes. The connections to the brushes
can be either spot welds or crimps with the 3/16” compression sleeves. In Photo 9, the red wire had plastic insulation
which melted during welding, while the black wire was
rubber insulated. Once again, the wires are insulated with
cloth electrical tape and held in place with thread.
Photo 11 demonstrates how the field and brush holder
go together and the challenge of fitting it into the tiny
space. The end cap for the motor is modified with several
deep slots so the new wires can be routed out of the case
in the least amount of space as shown in Photo 12.
After these electrical modifications are completed, the
one-way clutch from the Bendix needs to be disabled.
Photo 13 is the Bendix unit and Photo 14 is the Bendix unit
disassembled. The one-way clutch has five rollers that lock
in one direction and release in the other direction. The
modification is to replace the five rollers with five much
larger rollers and then grind five slots in the inner shaft to
accept the larger rollers.
Photos 15, 16, and 17 show the ring gear with three of
the five small rollers; the ring gear and shaft with five slots
ground into it; and the ring gear with the five large rollers
and inner shaft all assembled. (The third photo was taken
from the top so the shaft is not very clear.)
The completed motor in Photo 18 has two leads from
the field windings and one lead from the armature. The
case ground is the other connection to the armature. A few
unneeded parts have been sawed off the case to make the
motor more compact.
SERVO 10.2008 47