Alternator

From Academic Kids

An alternator is a generator that produces alternating current by converting mechanical energy to electrical energy. The first practical alternator was invented by Nikola Tesla in 1891, Template:US patent. In principle any AC generator can be called an alternator, but usually the word refers to small rotating machines driven by automotive and other internal combustion engines.

Contents

Theory of operation

Alternators generate electricity by the same principle as DC generators. When magnetic field lines cut across a conductor, a current is induced in the conductor. In general, an alternator has a stationary part (stator) and a rotating part (rotor). The stator contains windings of conductors and the rotor contains a moving magnetic field. The field cuts across the conductors, generating an electrical current, as the mechanical input causes the rotor to turn.

The rotor magnetic field may be produced by induction (in a "brushless" generator), by permanent magnets, or by a rotor winding energized with direct current through slip rings and brushes. Automotive alternators invariably use brushes and slip rings, which allows control of the alternator generated voltage by varying the current in the rotor field winding. Permanent magnet machines avoid the loss due to magnetizing current in the rotor but are restricted in size owing to the cost of the magnet material. Brushless AC generators are usually larger machines than those used in automotive applications.

Automotive alternators

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. Car 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 alterator 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 amps (though typical cars have 40–60 amp 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 amp 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.

See also

The Alexanderson alternator produced alternating current up to 100,000 Hz, and was used for radio communications.

External links

it:Alternatore pl:Alternator fi:Generaattori

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