create a path for current between insulated parts of the
motor and lead to a fire.
Frequent checks of the condition of the banding
wire that holds down the windings are necessary to
The insulation of the field coils should be tested
determine if the wires are tight, undamagd, and have
periodically with a resistance-measuring device. If a
not shifted. Also check the clips securing the wires to
ground is detected in the field circuits (shunt, series, and
see that the solder has not loosened.
interpole) of a dc machine, you should disconnect all
You can detect some armature troubles while
circuits from each other and test separately to locate the
making inspections of running machines. Heat and the
grounded circuit. The coils in the circuit must be opened
odor of burning insulation may indicate a short-circuited
and tested separately to locate the grounded coil. It can
armature coil. In a coil that has some turns shorted, the
then be repaired or replaced as necessary.
resistance of one turn of the coil will be very low, and
the voltage generated in that turn will cause a
When an open circuit develops in the field windings
high-current flow. This results in excessive heating that
of an ac or dc generator that is carrying a load, it will
could cause the insulation to burn. If the armature is
be indicated by the immediate loss of load and voltage.
readily accessible, you can detect the short-circuited coil
An open in the shunt field winding of an operating dc
immediately after stopping the machine because the
motor may be indicated by an increase in motor speed,
shorted coil will be much hotter than the others. In idle
excessive armature current, heavy sparking, or stalling
machines, you can identify a short-circuited coil by the
of the rotor. When an open occurs in the field circuit of
presence of charred insulation.
a machine, you must secure it immediately and examine
the circuit to locate the fault. An open circuit will usually
An open armature coil in a running machine is
occur at the connections between the coils and can be
indicated by a bright spark, which appears to pass
detected by visual inspection. The opening of an
completely around the commutator. When the segment
energized field circuit induces a high voltage that can
to which the coil is connected passes under the brushes,
the brushes momentarily y complete the circuit; when the
damage the field insulation causing a safety hazard.
Table 7-5.--Satisfactory Insulation Resistance for dc Generators and Motors (except propulsion and auxlliary generators for