The following illustration represents the voltage progression of an intact alternator. The level of the individual waves depends on the current load of the alternator. The length of the waves depends on the engine speed. The higher the engine speed the shorter the waves.
At idle speed with consumer units switched on, an error-free alternator should provide approximately this picture.
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Examples of faulty alternators are shown below.
The current as a function of engine speed also applies here.
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Alternator with defective diode
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Alternator with broken winding