Science Fair Project Encyclopedia
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, US patent no 447921
Alternators are generally simpler, more reliable, and can be more efficient than direct current generators because they do not need to use brushes and commutator/slip rings, though in practice many designs do.
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 field may be induced (as is in induction motors) or in the form of permanent magnets. Permanent magnet alternators have the potential to be the most efficient since no mechanical energy is used to create the field. In practice many alternators create the field by using a DC feed through slip rings to the rotor windings.
Alternators are used in automobiles to charge the battery and to power all the car's electric systems when its engine is running. Car alternators use a set of built in rectifiers (diode bridge) to convert AC to DC. 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. In fact 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 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). This would need a very heavy duty switch instead of the light duty one used for the ignition switch in most cars. 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.
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- Tesla, Nikola, "US447921 Alternating Electric Current Generator". USPTO.
- Tesla, Nikola, "The Ewing High-Frequency Alternator and Parson's Steam Engine". 12-17-1892. (Pepe?s Tesla Pages, DOC)
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