Page 253 - Robot Builder's Bonanza

222 CHOOSING THE RIGHT MOTOR

I don’t recommend that you run a motor continuously at more than 50 to 80 percent its rated
voltage, at least not for long. The electrical windings inside the motor may overheat, which can
cause permanent damage to the motor. Motors not designed for high- speed operation may turn
faster than their construction allows, which literally could cause them to burn up.

If you don’t know the voltage rating of a motor, you can take a wild guess at it by trying
various voltages and seeing which one provides the greatest power with the least amount of
heat and mechanical noise. Let the motor run for several minutes, then feel the heat on the
outside of the motor case. Listen to the motor; it should not seem as if it is straining under the
stress of high speeds.

CURRENT DRAW
Current draw is the amount of current, in milliamps or amps, that the motor requires from
the power supply. Current draw is more important when the specification describes motor
loading, that is, when the motor is turning something or doing work. The current draw of a
free- running (no- load) motor can be quite low. But have that same motor spin a wheel propel-
ling a robot across the floor, and the current draw might increase several hundred percent.
Most DC motors use a permanent magnet inside. In these motors, which are the most
common, current draw increases with load. You can see this visually in Figure 21- 4. The more
the motor has to work to turn the shaft, the more current is required. The load used by the
manufacturer when testing the motor doesn’t follow any kind of standard, so in your applica-
tion the current draw may be more or less than that specified.
A point is reached when the motor does all the work it can do and no more current will
flow through it. The shaft stops rotating; the motor has stalled. This is considered the
worst- case condition. The motor will never draw more than this current unless it is shorted
out. If your robot is designed to handle the stall current, then it can handle anything.
When adding a motor to your robot you should always know the approximate current draw
under load. All multimeters can be used to test current drawn by a motor. Learn how in the
section “Testing Current Draw of a Motor,” later in this chapter.

SPEED
The rotational speed of a motor is given in revolutions per minute, or RPM. Many continuous
DC motors have a normal operating speed of 4000 to 7000 RPM. Certain special- purpose

Motor stalls (stops)
from too much load
6
5
Figure 21- 4 All motors draw current;
4
the amount of current depends on the
Current
draw 3 design of the motor and how much load is
As load increases, placed on the motor. As the load increases,
2 so does current draw
so does the current. At some point, the load
1 is too much for the motor and it stops
0 turning, but it still consumes current. This
Load on motor shaft is called the stall current.

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