Evaluating DC Motors

1
Evaluating DC Motors
Determining Motor Stall Current
2
Measuring Armature Coil Resistance
The Multi-Meter selector is set to read
resistance in units called ohms ? (The Blue
portion of the selection dial) The selector is
set to the lowest range (0-200? ) The Value
measure is 0.9 ohms.
3
Measuring Armature Coil Resistance
Set the meter to read ohms. Use the lowest range.
Short the meter leads and record the value. This
is the meter error value. Attach your meter to
the two motor leads. Slowly turn the armature
shaft and record values at 45 intervals. Average
these values.
Caution Turning the armature shaft quickly,
induces a current in the armature windings. This
Will damage the meter.
4
Calculate the Stall Current
E
Ohms Law The basic Algebra of electronics
I
R
I Current in Amperes
E Voltage
R Resistance in Ohms, ?
5
Calculate the Motor Stall Current
The sample motor yields an average measured
resistance value of 0.6 ?. The intended
BattleBot power supply will provide 12 Volts emf
(electro motive force). The meter error 0.3 ?
E
12V
I
40 amps
R
0.6 ? - 0.3 ?
Predicted Motor Stall Current 40 Amperes
6
Directly Measuring Motor Stall current is a
Learning Experience.
Voltmeter
12V Battery
Sample Motor
Ammeter
NC MO Switch