Get a Starter Motor Runnin’ in Your Robot
It is normal to find similar starters used on different brands
of autos. Photo 1 shows a few typical starter motors. The
two motors in the upper right side are from late 1980’s
Hondas. The larger motor was used in cars like the Accord,
while the smaller one came from a Civic. The large motor
on the bottom is a Denso starter used on many Toyota
models. The two starters on the left are newer style Denso
starters from a Toyota and a Corvette.
Auto salvage yards can be a good source for used
starters. They typically have a core pile waiting to be sold
to a buyer for remanufacturing and are usually worth only
a few dollars. In my experience, few starters ever wear
out. Most often the solenoid goes bad and the relay
inside it quits working. For robot use, starter cores are the
All starters have the same basic parts: a motor, gear
reduction, starter solenoid, and something that is generally
called a Bendix. Most of these parts have to be modified or
discarded before the starter can be used in a robot.
One necessary modification is to make the SW motor
reversible. It cannot be reversed by changing the polarity of
the input voltage. Instead, the polarity of the field winding
has to be inverted relative to the armature winding. This
modification will be covered later.
Since the Bendix is an over-running clutch that only
drives in one direction, it has to be modified to drive in
both directions. The final challenge is to attach a drive
sprocket of some sort to the starter. For some uses, it may
be easier to use the gear that came with the starter and
find a flywheel to match it. There are only a few standard
gear pitches used for most autos making it easier to find
a flywheel or ring gear to match the starter. These
modifications will also be addressed later.
A starter motor can be modified in a few hours with
great results. Driving a robot with PM motors is okay but a
robot with SW motors is a totally different experience. The
robot will have an awesome hole shot and will feel like it
can accelerate forever.
I use a simple relay control system and have never
investigated using an electronic speed controller (ESC) to
control a starter motor. One problem with using an ESC is
that only the field winding is controlled by the H-bridge.
The simplest solution would be to use relays for the H-bridge and use the ESC to control the current to the motor.
The relay control circuit is shown in Figure 2 with a possible
Rewiring the Motor to Make it
Since most starters are similar, these modifications
will apply to almost all of them. Photo 2 shows the
disassembled Honda starter motor and Photo 3 shows the
parts that have to be modified. Starters are four-pole
motors equivalent to having two motors in parallel. There
are two sets of field windings and two sets of brushes.
The power input goes to the two sets of field windings.
The fields are connected to two brushes which power the
armature. Finally, there are two more brushes connected to
ground to complete the circuit. Photos 4 and 5 show the
46 SERVO 10.2008