Common faults on armatures described
The most common fault with armatures is worn commutators, caused by the friction of carbon brushes against the commutator surface. Over time the insulation between the commutator segments becomes proud as the copper of the commutator surface wears away. This is especially so with older commutators as the insulation that was used to separate the commutator segments was mica, which is harder than the copper. When the commutator has worn down to this point it will also quickly wear down the brushes.
This fault can usually be repaired without rewinding the armature by turning the commutator brush surface down below the damage and then undercutting the insulation between the commutator segments.
Another common fault is for the armature to burn out. This can be caused by an number of different problems such as overloading, poor airflow, regulator failure, stalling, insulation breakdown, earthing etc. If the armature has burned out then it will need to be rewound.
Before the invention of resin-covered wire, copper wire was insulated by the method double-cotton covered, a thin strand of cotton wrapped around the wire. Overtime the expansion and contraction of the copper wire through constant heating and cooling, combined with centripetal force, can cause the turns within the coils to short-out. The shorted turns get hot as the voltage generated in them has only the resistance of that one turn so the current it draws can be high and that high current generates heat. This heating causes further shorts and more heating until eventually the winding itself burns out.
If too much current is demanded from the dynamo, either through a fault on the voltage regulator, too many high-power light bulbs, a fault on the battery or a low resistance introduced to the system, then the armature can get very hot. As the armature heats up, the internal resistance of the armature goes up causing more power to be dissipated across the armature itself, generating even more heat. This can cause the solder connecting the armature coils to the commutator to melt. As the armature is spinning the solder is thrown from the armature, causing the connections between the armature coils and the commutator to be broken.
Earthing (grounding) is a common fault which occurs when part of the winding becomes connected to the metal core of the armature. The armature windings are insulated from the metal core and the metal core is often connected to the negative side of the battery, through the body of the dynamo (unless it is an insulated return in which the negative side of the dynamo is connected to the battery with a wire). Earthing usually occurs when insulation brakes down, usually through overheating or through fatigue on the edge of a slot through constant heating, cooling and spinning.