the wire to reduce such exposure since heat is a principal enemy of insulation.
2.21. COILS
The most common method of testing for short circuits in field coils is the growler test, and many
repair shops are equipped with a transformer or growler for this purpose. A sketch of a growler is given
in figure 2.13. The growler test is a very simple and effective method, since no accurate electrical
measurements are required. Apply pressure to the field coil when it is being tested, to approximate
actual conditions when it is clamped in the frame. If pressure is not applied, the coil may show no
defect on the test but may give trouble from short-circuited turns when clamped between the pole tips
and the inside surface of the frame. With the test in operation, after adjusting the knife switches for the
desired number of turns on the transformer, position the removable portion of the core without the field
coil. Note the deflection of the ammeter, which indicates the value of primary current. Open the line
switch and place the coil in position around the transformer core; close the switch to reapply voltage to
the transformer. If the ammeter shows the same deflection as before, the coil is free of short-circuited
turns; if there is a short circuit, a much heavier current will flow in the primary coil of the transformer,
indicated by a greater deflection of the ammeter.
2.22. DIPPING AND BAKING
Coils which have been in operation 4 or 5 years should be dipped and baked as a preventive
maintenance measure. Some coils, such as field coils, are dipped before mounting on their core. With
armatures, the entire assembly is suspended with the axis vertical and the commutator on the upper end,
and lowered into the compound up to, but not including, the commutator. The varnish is hot, and the
coils or armature are cleaned and preheated in an oven before dipping. Dip for 5 to 10 minutes, drain for
5 minutes, and then place in oven for about 6 hours at about 300F. When an armature is rewound, it is
baked before banding and again after banding.
2.23. COMMUTATOR
Whenever the locomotive comes in for maintenance, the commutator should be touched up with
a canvas-covered block to remove excessive formation of oil scum. On switching locomotives, this
work is done at the time of monthly inspection. If excessive
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