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232 USING DC MOTORS

usually indicate whether it’s bidirectional; or at least, if it’s not, the description will specify that
the shaft turns CW (clockwise) or CCW (counterclockwise) only.
See the sections later in this chapter under “Controlling a DC Motor” for various ways DC
motors can be reversed.

Reviewing DC Motor Ratings

Motor ratings, such as voltage and current, were introduced in Chapter 21. Here’s a quick
recap of the main points of interest when selecting and using a DC motor for your robot:

• DC motors can often be operated at voltages above and below their specified rating. If the
motor is rated for 12 volts and you run it at 6 volts, the odds are the motor will still turn
but at reduced speed and torque. Conversely, if the motor is run at 18 volts, the motor will
turn faster and will have increased torque.
• But this does not mean that you should intentionally underdrive or overdrive the motors
you use. Significantly over driv ing a motor may cause it to wear out much faster than nor-
mal. However, it’s usually fairly safe to run a 10- volt motor at 12 volts or a 6- volt motor at
4 or 5 volts.
• DC motors draw the most current when they are stalled. Stalling occurs if the motor is
supplied current but the shaft does not rotate. Any battery, control electronics, or drive
circuitry you use with the motor must be able to deliver the current at stall, or major prob-
lems could result.
• The rotational speed of a DC motor is usually too fast to be directly applied in a robot. Gear
reduction of some type is necessary to slow down the speed of the motor shaft. Gearing
down the output speed has the positive side effect of increasing torque.

Controlling a DC Motor

As I’ve noted, it’s pretty easy to change the rotational direction of a DC motor. Simply switch
the power lead connections to the battery, and the motor turns in reverse. And when you
want the motor to stop, merely remove the power leads to it.
That’s fine for when you’re playing around on your workbench, but what are the options
when the motor is part of a robot? You have several, actually, and each has its place. The ones
you’ll read about in this book are:

• Switch
• Relay
• Bipolar transistors
• MOSFET transistors
• Motor bridge modules

Motor Control by Switch

You can manually operate your robot using switches. This is a good way to learn about robot
control and experiment with different types of robot base designs. The switches attach to your
robot by wires. You can control both the operation and the direction of the motors.

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