CHAPTER 17 ■ DC MOTORSMost DC motors spin at 3000 RPM to 8000 RPM. At the high end, that means the motor can rotate its
shaft around 8000 times before a clock’s second hand completes a single rotation. That’s really fast!
Initially it may seem that faster is better, but that’s not the case. For a line-following robot with
slow-reacting sensors, fast speed could drive the robot off the line before the brain has time to react. On the
other hand, too little speed could put your audience to sleep. Sandwich runs its motors at around 137 RPM.
Even though DC motors naturally spin at thousands of RPM, there are some good ways to alter the final
output speed. In fact, designers are rarely forced into accepting the base speed of a motor. Information on an
effective speed manipulation technique, gear reduction, appears at the end of this chapter.
Common robot wheel-driving speeds are between 40 RPM (precision movement) and 250 RPM
(very fast). Cooling fans usually rotate at between 3000 RPM and 6500 RPM. Robot arms or directional
sensors are often rotated well below 60 RPM.
Measuring RPM
By attaching a piece of tape or half-colored disc to a motor shaft (see Figure 17-16), you can measure the
number of times the shaft rotates in a minute. Depending on your brain-eye coordination and propensity
towards distraction, you are unlikely to be able to count speeds faster than 120 RPM by eye. Even toy motors
running at 3 V can quickly make the target a blur.
Figure 17-15. The second hand on a clock rotates at 1 RPM
Figure 17-16. Black and white segmented discs and a masking tape flag can help in measuring RPM