Servo - October 2008

Get a Starter Motor Runnin’ in Your Robot

PHOTO 9

PHOTO 10

PHOTO 11

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

PHOTO 12

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.