ALTERNATOR
The automotive alternator is used in modern cars, trucks and most automobiles where power needs to be generated to power lights and other electrical parts of a motor vehicle. The alternator is required to charge the battery and to power the electrical system when its engine is running. Until the 1960s, automobiles used DC dynamo generators. With the availability of affordable silicon diode rectifiers, alternators became the modern choice of power. Alternators have several advantages over direct-current generators in that they are lighter and cheaper. They use slip rings which greatly extended brush life. The brushes in an alternator carry only excitation current, a small fraction of the current carried by the brushes of a DC generator, which carry the generator's entire output. A set of rectifiers is required to convert AC to DC power.
Automotive alternators are usually belt driven at 2-3 times crankshaft speed. The alternator runs at various RPM which varies the frequency and since it is driven by the engine the alternating current is rectified to direct current.
Automotive alternators require a voltage regulator which operates by modulating the small field of current in order to produce a constant voltage at the battery terminals. Early designs used a discrete device mounted elsewhere in the vehicle between 1960 and 1970. Intermediate designs were incorporated the voltage regulator into the alternator housing between 1970 and 1990. Modern vehicles today mostly regulate voltage via the electronic or engine control unit or abbreviated the ECU. In recent years, alternator regulators are linked directly to the vehicle's computer system and various factors including air temperature obtained from the intake air temperature sensor, battery temperature sensor and engine load are evaluated in adjusting the voltage supplied by the alternator.
The field windings are initially supplied power from the battery via the ignition switch and "charge" warning red light indicator on the dashboard instrument cluster. Once the engine is running and the alternator is generating power, a diode feeds the field current from the alternator main output equalizing the voltage across the red light warning indicator which goes then goes off on the instrument cluster. Some warning indicator circuits are equipped with a resistor in parallel with the lamp that permit excitation current to flow if the warning light burns out. The driver should check that the warning indicator is on when the engine is not running or before starting the engine otherwise, there might not be any indication of a failure of the belt which may also drive water pump.
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