Alternators are used in automobiles to charge the battery and to power all the car's electric systems when its engine is running. Alternators have the great advantage over direct-current generators of not using a commutator, which makes them simpler, lighter, and more rugged than a DC generator. The stronger construction of alternators allows them to turn at higher speed, allowing an automotive alternator to turn at twice engine speed, improving output when the engine is idling. The availability of low-cost solid-state diodes from about 1960 allowed auto manufacturers to substitute alternators for generators. Automotive alternators use a set of rectifiers (diode bridge) to convert AC to DC. To provide direct current with low ripple, automotive alternators have a three-phase winding.
Modern car alternators also have a voltage regulator built into them. Typical car alternators generate the field using a DC current through slip rings. The field current is much smaller than the output current taken from the fixed stator windings, and so heavy duty slip rings are not required. For example, in an alternator rated to produce 70 amperes of DC, the field current will be less than 2 amperes. The voltage regulator operates by modulating the small field current in order to produce a constant voltage at the stator output. In many older designs of car, the field windings are initially supplied via the ignition switch and charge warning light, which is why the light glows when the ignition is on but the engine is not running. Once the engine runs and the alternator is generating, a diode feeds the field current from the alternator main output, thus equalizing the voltage across the warning light which goes out.
This system is simple and avoids the need for a heavy duty switch in the main alternator output circuit, which can carry very high currents—up to 100 amperes (though typical cars have 40–60 ampere alternators). One drawback of this arrangement is that if the warning light fails, no priming current reaches the alternator field windings and so it cannot bootstrap itself. The engine will still run from the battery for a while, and the lack of warning light will fail to alert the driver that anything is wrong. Modern systems have more complex electronic monitoring and do not suffer from this drawback.
Very large automotive alternators used on heavy equipment or emergency vehicles may produce 150 amperes. Very old automobiles with minimal lighting and electronic devices may have only a 30 ampere alternator. Hybrid automobiles replace the separate alternator and starter motor with a combined motor/generator that performs both functions, cranking the internal combustion engine when starting, providing additional mechanical power for accelerating, and charging a large storage battery when the vehicle is running at constant speed. These rotating machines have considerably more power electronic devices for their control than the simple automotive alternator described above.
Basic Components
The alternator consists of:
A voltage regulator monitors battery voltage and varies current flow through the rotor field circuit, and thus controls the strength of the rotating magnetic field. This keeps system voltage to a safe level.
Source: CDX Global & Wikipedia - en.wikipedia.org
