CHAPTER 17 ■ DC MOTORS
The example of a soda can connected one centimeter away isn’t totally accurate because the can itself
is longer than one centimeter. In reality, you need to be careful to determine exactly the force from a large or
odd-shaped object. At the very least, measure from the center of the mass to help determine the distance the
mass is “connected” as far as the torque is concerned.
Calculating Torque Needed for a Robot
The proper calculations involved in determining the minimum torque needed for a robot-driving motor are
complex. It depends on where the mass is located, if the mass shifts, how steep of an angle the robot must
climb, and the other forces being generated against the robot (such as an attacking opponent).
Luckily, torque is not a major concern for motors that drive the wheels of lunchbox-size robots. If the
speed (RPM) and other factors (availability, price, dimensions, weight, voltage) of the motor fit the design,
torque is likely to be acceptable.
You can always begin an experiment with one kind of motor and then swap in a lower-or higher-rated
torque motor until you’re happy with the outcome. Compare advertised motor torque specifications using
the torque conversion tables provided in this chapter.
Extra motor torque isn’t a problem as long as the other factors of the motor meet your robot’s needs.
In fact, it’s best to leave a safety margin by providing motors rated at a higher torque. If the motor can
provide triple or more of the necessary torque continuously, not only will the robot run cooler and more
efficiently without damaging the motors, but also the robot will have strength to spare if you decide to add
new parts or loads.
Be aware that other parts on your robot are likely to break (wheels, gears, treads, body structure) if they
are not strong enough to support the forces acting on the robot. That is, even with properly rated motors, the
physical energy must be transmittable through wheels, treads, or legs.
Voltage Characteristic of DC Motors
In datasheets and advertisements, motor specifications are disclosed at “nominal voltage.” That’s the voltage
the manufacturer expects the motor to run at. Common voltages are 3 V, 6 V, 12 V, 18 V, and 24 V. Larger
motors can support even higher voltages.
Most DC brush motors can be run between 50% and 125% of their nominal voltage. For example, a
12 V motor could be run at any voltage from 6 V to 15 V. Below 50% of the nominal voltage the motor may
not turn. Above a certain voltage the motor may overheat or breakdown.
That being said, many scientific or high-precision motors run at even 10% of their nominal voltage.
At the other extreme, many motors in combat robots are run at 200% of their nominal voltage.
Try to pick a motor that has a voltage that most closely matches your batteries. Unless you’re a
particularly crafty expert, don’t design a robot with a 9 V battery and 3 V motors. Likewise, don’t build a
robot with a 3 V battery and 24 V motors.
Understanding the Relationship Between Voltage and Speed
The greater the voltage, the greater the speed, up to the maximum permitted by the motor. For example, if
you have a 12 V motor and it is running a little too slow, try 14 V. If it is running too fast, try 9 V.
Speed changes in direct proportion to the change in voltage. Figure 17-19 shows a graph of a test
performed on an escap-brand motor (made by Portescap). As voltage is increased by 1.5 volts at a time, the
speed increases by about 530 RPM. Of course, the amount of speed change in other motors will be different.