Page 257 - Robot Builder's Bonanza

226 CHOOSING THE RIGHT MOTOR

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Input
voltage Motor
Figure 21- 7 Test the current draw
of a motor by interrupting the power
line to the motor. Place the + and –
leads of the multimeter between this
Meter set to
DC amps power line, and take the reading.
(be sure to use
high amperage input!) The meter must be dialed to read
DC current.
wires, the kind with heavy- duty alligator clips on the ends. You need two jumpers, one each
for the red and black test leads. Then,
1. Dial the meter to the 10A setting.
2. Apply power to the motor.
3. Observe the reading on the meter. It will be in amps. A reading of 0.30, for example,
means 30 mA, or 0.3 of an amp. A reading of 1.75 means 1.75 amps.

This gives you the free- running torque of the motor. For other torque tests, you need to
load down the motor.

Small hobby motors without a gearbox can be loaded down just by squeezing the shaft
between your fingers. As you slow the motor down, watch the current increase. If you
can stop the shaft from turning completely, the motor will be stalled and the current
shown in the stall current. That’s about as high as the current will ever get from the
motor.
Small hobby motors with a gearbox can be loaded down by applying pressure to the
output shaft of the motor or one of the gears in the gearbox nearest the shaft. This, of
course, assumes the gears in the gearbox are accessible.
G With some gearbox motors you can temporarily remove the motor from the gearbox. Test it
alone, then put the motor back into the gearbox.

Larger motors are harder to test just by stopping the shaft with your fingers. It’s better to
attach a wheel to the motor (the larger the diameter, the better), then try to apply load
to the wheel. Don’t use a pair of pliers or another tool to clamp down on the motor
shaft or gears, as this may cause damage. At the very least, you can take the free- running
measurement.

INDIRECT MOTOR CURRENT MEASUREMENT

You can still determine current draw from your motors if your multimeter lacks a high current
input or if the motor you’re testing draws over 10 amps. The solution is to insert a low- value,
high- wattage resistor inline between the + battery terminal and the motor.

21-chapter-21.indd 226 4/21/11 11:50 AM