engine. Figure 3 is a motor that I saw
advertised on eBay. It is a Curtis &
Crocker bipolar motor from the late
1880s that I believe is a fan motor. It
was being sold as a collectable. Many
electric motors of that era had open
structures that exposed the internal
wiring to errant fingers. Today’s
liability lawyers would salivate over
seeing such a motor on today’s market.
Motors for Robots
Steam engines, hydraulics, pneumatics, spring-wound motors, shape-memory actuators (Muscle wires),
solenoids, and even twisted rubber
bands have powered experimental,
industrial, service, and toy robots for
years, but we really rely on the good old
electric motor for robot power. And
that ‘good old motor’ was usually a
DC permanent magnet (PM) motor. I
won’t go into detail how each type of
electric motor works, as I am sure the
vast majority of SERVO’s readers are
quite familiar with the repulsion and
attraction of the motor’s armature
poles and the field poles. We all know
how the commutator at the end of the
armature of a DC motor allows the
brushes to continually change the
magnetic fields on the armature to keep
it spinning. There are numerous links
on the Internet that will go into depth
on all types of DC and AC motors.
Back in the ‘70s, robot builders
relied on surplus businesses, junk yards,
car parts, and toys for their motors.
Motors have always fascinated me and
I had amassed over 500 electric motors
and gearmotors in my collection
several years ago; most were from
surplus places. I got to thinking, “why
does anybody need so many motors?
Am I ever going to build a hundred
different types of robots?” Since I
couldn’t come up with any good
answers and with my wife’s urging, I
managed to cull my collection down
to about 50 of the best ones and then
gave a bunch away at Seattle Robotics
Society meetings, sold a bunch, and
donated some to schools.
from a favorite surplus place of mine
in Pasadena, CA that was about a 40
mile drive north from my home at the
time in Long Beach, CA. C&H Sales
had thousands of military surplus
motors and the staff knew me well
enough to let me wander in the back
of the store, plowing through bins of
complex military mechanisms to pull
out the best motors. I could test them
on-site for speed, torque, operating
voltage range, current at various
loads, and mounting methods. I
almost always chose gearhead motors
as these didn’t require a separate
attached gearbox or belt reduction.
Some of the best gearmotors were
planetary drives made by TRW and other
suppliers to the government. Something
that I found out rather quickly is a
person cannot go by the listed current
draw or voltage to determine the actual
torque and output speed of a particular
motor. As an example, a particular
gearmotor may draw one amp at 28
VDC at full load and 20 RPM with 100
oz-in of torque, and another only 500
mA at 28 VDC for the same output.
Look at a surplus motor catalog and
you’ll see what I mean. Choose wisely.
Quite a few of these motors —
especially the military variety — were
24 or 28 VDC, but would run nicely at
a reduced voltage such as 12 VDC.
Pittman made (and makes) a great
series of gearmotors that operate
nicely at 12 VDC, and I used many of
these for robot appendages, grippers,
and head motion applications. C&H
has shrunk quite a bit in the past
few years and now sells most of their
stock on eBay. In the ‘80s, they had a
great monthly catalog that went to
quarterly several years later and was a
great reference for their motors.
Considering that these motors were
surplus, they probably
had been sitting on
shelves or dark
military warehouses
for years and were
not constructed using
the latest technology.
AlNiCo and ferrite
magnets seemed to
be the technology for
PM motors in those
days. Universal motors
FIGURE 3. Curtis & Crocker bipolar motor.
were one of the most popular military
surplus styles, though these motors
were not particularly good for robot
applications. They used an electromagnet winding for the field poles
instead of permanent magnets and could
operate on AC, as well as DC current.
Vacuum cleaners still use this type of
motor. Don’t use these for robots.
Another popular type of DC motor
was the pancake motor or printed circuit
armature motor. These motors have
the advantage of compactness that is
especially nice when coupled to a
geartrain. They have low weight, are
fairly energy efficient, and start and
stop fast due to low inertia of the
armature. I was given a box of PMI
pancake motors that I used for all sorts
of projects. I did find that they could
be damaged when they were overloaded as the armature did not have
an iron core to soak up and dissipate
excess heat. I found this out the hard
way when a pancake motor I was
using had the copper-wound armature
so cooked that it was warped.
Figure 4 from the University of New
FIGURE 4. Pancake motor.
FIGURE 5. Pancake
motor armature.
Surplus Store Motors
The best of these motors came
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