following the manufacturer's instructions in using the
ohmmeter, field coil tests can be made more quickly
and accurately than by using a test lamp circuit.
ARMATURE TEST.There are two practical
tests for locating shorts, opens, and grounds in
armaturesthe growler test and the bar-to-bar test.
To test for short circuits, place the armature on the
V-block of the growler and turn on the current. With a
thin metal strip (hacksaw blade is good) held over the
core, as shown in figure 2-17, rotate the armature
slowly through a complete revolution. If a short is
present, the steel strip will become magnetized and
vibrate. To find out whether the armature coils of the
commutator are short-circuited, clean between the
commutator segments and repeat the test. Should the
thin metal strip still vibrate, the armature is short-
circuited internally and must be replaced.
Not all armatures can be tested for short circuits by
the method just described. These armatures can be
identified by excessive vibration of the saw blade all
around the armature during the test. With these
armatures, test for short circuits by using the
milliampere contacts on an ac millimeter, as shown in
figure 2-18. In doing so, keep the armature stationary
in the V-block and move the contacts around the
commutator until the highest reading is obtained. Then
turn the armature to bring each pair of segments under
Figure 2-17.Using an armature growler.
the contacts and read the milliammeter at the same
time. The readings should be nearly the same for each
pair of adjacent bars. If a coil is short-circuited, the
milliammeter reading will drop to almost zero.
Test the armature for grounds by using the test
light circuit, which is a part of most modern factory-
built growlers (fig. 2-19). Place the armature on the
V-block and touch one of the test probes to the
Figure 2-18.Testing an armature for a short circuit with a
Figure 2-19.Testing an armature for grounds.