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Dynamos: Common faults

Explaining the common faults on dynamos

Virtually all the failures on these systems are mechanical in origin.


The brushes that make contact with the commutator wear down over time, especially on older units. The commutator has segments of copper separated from one another by a material called mica. When the armature is new the commutator is undercut to make the mica lower than the surface of the commutator. As the copper wears down, the mica is revealed. The mica is much harder then the copper so it begins to wear down the brushes quickly - to the point that either the brush is too small to allow the spring to apply sufficient pressure to the brush to keep it in contact with the commutator, or until the brush is worn away completely.

Voltage Regulator

The voltage regulator is a common failure on a dynamo. The regulator is switching at a high rate and the contacts either wear or the capacitor fails. Capacitor failure causes excessive load on the regulator contacts putting high resistance in the circuit thereby reducing the current being fed through the field coil such that its field strength may not be sufficient to allow the armature to develop enough voltage to overcome the battery terminal voltage.

The contacts can also weld shut allowing the dynamo's output voltage to continue to rise. This can cause the battery to boil or the armature to burn 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 it 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.

For more information about the causes of armature failures please read our section about common faults on armatures.

Field coil

For more information about the causes of field coil failures please read our section about common faults on field coils.


Bearing often fail through lack of greasing or simply due to the number of operations, especially the sleeve bearing used on C40 and C35's. As the bearing wear they can also damage the armature shaft, and in extreme circumstances the armature core can end up grinding against the pole shoe.