and burn out the winding. You can detect grounded coils
in idle machines by measuring insulation resistance (see
table 7-5). You can connect a Megger, or similar
insulation measuring device, to the commutator and to
the shaft or frame of the machine to properly measure
the resistance of the insulation of the coils.
You can make emergency repairs by cutting out a
short-circuited or open-circuited armature coil. This will
permit restoration of the machine to service until
permanent repairs can be made. However, permanent
repairs should be made as soon as possible. Cutout the
coil by disconnecting both ends of the coil and installing
a jumper between the two risers from which the coil was
disconnected. The coil itself is then cut at both the front
and rear of the armature to prevent overheating of the
damaged coil. A continuity test from one end to the back
of the coil will locate the turns of the faulty coil. If a pin
or needle is used to puncture the insulation for this test,
use insulating varnish to fill the tiny hole if the wrong
coil is pierced Insulate all conducting surfaces exposed
by the change in connection.s and tie all loose ends
securely to prevent vibration
Figure 7-30.--Classfifcation or armature windings.
You must be able to identify various types of
armature windings to interpret trouble indications and
segment leaves the brushes, the circuit is broken,
to make the necessary repairs.
causing a spark to jump the gap. The open will
LAP AND WAVE WINDINGS.-- Armature
eventually scar the commutator segments to which the
windings, irrespective of how the elements are placed
ends of the open coil are connected.
on the armature core, are generally classified as LAP or
When a ground occurs in an armature coil of a
WAVE windings. The classification designates the
running machine, it will cause the ground test lamps on
method of connecting the ends of the elements, or coils,
the main switchboard to flicker on and off as the
to the commutator. If the ends of the coil are connected
grounded coil segment passes from brush to brush
to adjacent commutator segments or to segments that
during rotation of the armature. Two grounded coils will
are close together, the coil is designated as a
act the same as a short circuit across a group of coils.
lap-connected coil, and the winding is a lap winding
(figs. 7-30, view A, and 7-31, view A).
Overheating will occur in all of the coils in the group
Figure 7-31.--Full-pitch coils.
7-25
