Another indication of a short circuit will be a higher
than normal current draw by the motor. A higher than
normal current draw can also indicate other problems
or needed adjustments and does not always indicate
a short circuit in a motor winding.
1. COMMUTATOR BARS
2. TEST FOR SHORT CIRCUIT BETWEEN WIND-
INGS
3. RESISTANCE SHOULD BE BETWEEN 1 OHM
AND INFINITY
Figure 15. Test for an Armature Winding Short
Circuit
TEST FOR SHORT CIRCUIT TO
ARMATURE SHAFT
NOTE: Clean the dirt and brush dust from the com-
mutator area before making a test for a short circuit.
An ohmmeter (R × 10,000 scale) can be used to test
for a short circuit between an armature winding and
the armature shaft. Put one probe on the armature
shaft and the other probe on a commutator bar. See
Figure 16. A resistance of less than 1 megohm indi-
cates a problem between a winding and the armature
shaft.
1. ARMATURE SHAFT
2. COMMUTATOR
3. PROBES TO OHMMETER
4. RESISTANCE SHOULD BE GREATER THAN
1 MEGOHM
Figure 16. Test for Short Circuit to Armature Shaft
TEST FOR OPEN CIRCUIT IN FIELD COIL
The field windings in large electric motors normally
have less than 1 ohm of resistance. If an ohmmeter
(R × 1 scale) is used to check the resistance between
the two terminals of the winding, a resistance greater
than 1 ohm indicates a problem in that winding or cor-
rosion in the terminal connection. See Figure 17.
Tests for Damaged Field and Armature 0620 YRM 0294
26
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