Page 258 - Robot Builder's Bonanza

TESTING CURRENT DRAW OF A MOTOR 227

Power resistor
1-10 ohm, 10 watts
(see text)

2.86

Input Figure 21- 8 An alternative
voltage Motor
method for testing current draw is to
place a low- ohm (1 to 10 ) high-
wattage resistor inline between the
Meter set to power source and the motor. Change
DC volts
the meter to read voltage. See the
text for how to correlate the voltage
reading to current.
The value of the resistor can be anything between 1 and 10 ohms, but it needs to be high
wattage—10 or 20 watts, even more for motors that draw in excess of 10 amps.

1. Plug the red (+, positive) lead into the standard volts/ohm/current jack.
2. Use jumper clips to insert the resistor into the motor circuit, as shown in Figure 21- 8.
3. Dial your multimeter to DC volts. If your meter is not autoranging, select a voltage at
or just higher than the voltage you will apply to the motor.
4. Connect the red (+, positive) lead of your multimeter to the side of the resistor closest
to the voltage source.
5. Connect the black (–, negative) lead of your multimeter to the other side of the resistor.
6. Apply power to the motor.
What you’ll see is the voltage developed across the resistor. You then use one of the simple
formulas of Ohm’s law to determine the current flowing through the resistor. The formula is

I = E/R

where I is current, E is voltage, and R is resistance.
You are solving for I, or current, because you know the voltage developed across the resis-
tor and you know the value of the resistor, in ohms. Let’s suppose:

• The measured voltage is 2.86, and
• The resistor is rated at 10 ohms.

I = 2.86/10
or

I = 0.286, or 286 milliamps

Watch the voltage go up as you apply load to the motor shaft. See the previous section on
how to test for currents under load and when stalled, using nongeared and geared motors.

21-chapter-21.indd 227 4/21/11 11:50 AM