1. FIELD COIL TERMINAL
2. RESISTANCE SHOULD BE LESS THAN 1 OHM
Figure 17. Test for Open Field Circuit
TEST FOR SHORT CIRCUIT IN FIELD
COIL
A short circuit in a motor field winding is difficult to test
because of the normal low resistance (less than
1 ohm) of a good field. Special equipment is neces-
sary to check for a short circuit in a motor winding. A
motor with a short circuit in a field winding will have a
different sound when it begins to operate, but a serv-
ice person must have experience to hear and under-
stand the difference in sound. A winding with a short
circuit will also run hotter than a good winding and
can have indications of heat damage.
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.
TEST FOR SHORT CIRCUIT BETWEEN
FIELD AND MOTOR CASE
Make sure that carbon dust has been cleaned from
the motor before making this test. An ohmmeter (R ×
10,000 scale) can be used to test for a short circuit
between the field and the motor case. Put one probe
on the motor case and the other probe on a field ter-
minal. Check the resistance between the field terminal
and the motor case. A resistance of less than 1 meg-
ohm indicates a problem between the field terminal
and the motor case.
BRUSH HOLDER TEST
Make sure the carbon dust has been removed from
the brush holders. Use an ohmmeter (R × 10,000
scale) to measure the resistance between the brush
holder and the motor case. The correct resistance is
an indication of infinity (∞).
0620 YRM 0294 Tests for Damaged Field and Armature
27
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