Piper COMANCHE PA-24-180 Service Manual page 539

PIPER COMANCHE SERVICE MANUAL
tator, brushes and intemal connections. Sticking brushes, a dirty or gummy commutator (Refer to
paragraph 9-18), or poor connections may prevent the generator from producing any output. Thrown
solder on the cover band indicates that the generator has been overloaded (allowed to produce
excessive output) so it has overheated and melted the solder at the commutator riser bars. The
solder has been thrown out: often, this leads to an open circuit and burned commutator bars. If the
brushes are satisfactorily seated and are making good contact with the commutator, and the cause
of trouble is not apparent, use a set of test points and a test lamp as follows to locate the trouble
(leads must be disconnected from generator terminals'.
b. Raise the grounded brush from the commutator and insulate with a piece of cardboard.
Check for grounds with test points from the generator main brush to the generator frame. If the
lamp lights, it indicates that the generator is internally grounded. Location of the ground can be
found by raising and insulating all brushes from the commutator and checking the brush holders,
armature, commutator and field separately. Repair or replace defective parts as required (Refer
to paragraph 9.16).
NOTE
If a grounded field is found, check the regulator contact points.
since a grounded field may have permitted an excessive field cur-
rent which will have burned the regulator contact points. Burned
regulator points should be cleaned or replaced as required.
c. If the generator is not grounded, check the field for an open circuit with a test lamp. The
lamp should light when one test point is placed on the field terminal or grounded field lead and
the other is placed on the brush holder to which the field is connected. If it does not light, the
circuit is open. If the open is due to a broken lead or bad connection, it can be repaired. but if
the open is inside one of the field coils, it must be replaced.
d. If the field is not open, check for a short circuit m the field by connecting a battery of the
specified voltage and an ammeter in series with the field circuit. Proceed with care, since a
shorted field may draw excessive current which might damage the ammeter. If the field is not
within specification, new field coils will be required (Refer to paragraph 9-16).
NOTE
If a shorted field is found, check the regulator contact points, since
a shorted field may have permitted excessive field current which
would have caused the regulator contact points to bum. Clean or
replace points as required.
e. If the trouble has not yet been located, check the armature for open and short circuits.
Open circuits in the armature are usually obvious, since the open circuited commutator bars will
arc every time they pass under the generator brushes so that they will soon become burned. If the
bars are not too badly burned and the open circuit can be repaired. the armature can usually be
saved. In addition to repairing the armature, generator output must be brought down to specifi-
catons to prevent overloading by readjustment of the regulator.
f. Short circuits in the armature are located by use of a growler. The armature is placed in
the growler and slowly rotated (while a thin strp of steel such as a hacksaw blade is held above
ELECTRICAL SYSTEM
ISSUED: 8/18/72
3 A17