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Alternators: Common faults

Explaining the common faults on alternators

Water Ingress

Water can get into the alternator because they are open ended to allow air to pass through them during operation to keep them cool. In the winter the salt that is put onto the roads can get into them and start to attack the resin and paint on the stator. This can cause the laminations to expand due to rusting so that the laminations come into contact with the stator coils causing the winding to short circuit to earth (ground).

Carbon Brushes

The carbon brushes may wear away over time, breaking the circuit through the rotor coil so disabling the magnetic excitation field.

Slip Rings

Because the slip rings are in constant contact with the carbon brushes the metal rings can be completely worn away with similar effects as the carbon brushes wearing away.


Diodes can fail usually from overheating but also from corrosion. This causes the rectifier to stop allowing current to flow, resulting in the failure to charge the battery.

Voltage Regulator

The voltage regulators can fail and this is usually caused by a fault elsewhere in the alternator that has created a voltage spike; damaging its components. This can stop the voltage regulator from sensing when to switch on and off.

Bearing Failure

A very common problem is bearing failure which can, in extreme cases, cause the rotor and stator irons to clash. The mechanical stress may displace the stator laminations and destroy the integrity of the winding therein.

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