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MOTOR SPECS 221

Figure 21- 3 Servo motors for radio
control (R/C) model airplanes and cars
are DC motors, but with added control
electronics and built- in gear reduction.

Compare the R/C servo motor in Figure 21- 3 with the DC motors in Figure 21- 1. The first
thing you’ll notice is that R/C servo motors come in a neat little rectangular box. This is one
of their most alluring traits. Sizes of R/C servos are standardized (more or less), and they even
have standardized mounting flanges, allowing you to easily add them to your robot creations.
R/C servos can be— and often are— used for the same jobs as stepper motors. Because
they’re cheaper, easier to find, and simpler to use than steppers, R/C motors have almost
completely replaced the stepper motor in amateur robot designs.
For this reason, we devote a separate chapter just to them. See Chapter 23, “Using Servo
Motors,” for more information on using R/C servo motors— not only to drive your robot
creations across the floor but to operate robot legs, arms, hands, heads, and just about any
other appendage.

Motor Specs

Motors come with extensive specifications. The meaning and purpose of some of the specifi-
cations are obvious, but for others, they aren’t. Of all the specifications for motors, only a
small handful are truly meaningful to the amateur robot builder, so I’ll just concentrate on
those. These specs are operating voltage, current draw, speed, and torque.

OPERATING VOLTAGE
All motors are rated by their operating voltage. With some small DC “hobby” motors, the
rating is typically a range, something like 4.5 to 6 volts. For others, an exact voltage is speci-
fied. Either way, most DC motors will run at voltages higher or lower than those specified for
it. A 6- volt motor is likely to run at 3 volts, but it won’t be as powerful. It will also run slow.
Many motors will refuse to run, or will not run well, at voltages under about 40 or 50 per-
cent of the specified rating. Similarly, a 6- volt motor is likely to run at 12 volts. As you may
expect, as the speed of the motor increases, the motor will exhibit greater power.

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