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An alternator is a type of electric generator used in modern automobiles to charge the battery and to power the electrical system when its engine is running.
Until the 1960s, automobiles used DC dynamo generators with commutators. As silicon-diode rectifiers became widely available and affordable, the alternator gradually replaced the dynamo. This was encouraged by the increasing electrical power required for cars in this period, with increasing loads from larger headlamps, electric wipers, heated rear windows, and other accessories.
The modern type of vehicle alternators were first used in military applications during World War II, to power radio equipment on specialist vehicles.[i] After the war, other vehicles with high electrical demands — such as ambulances and radio taxis — could also be fitted with optional alternators.[1]
Alternators were first introduced as standard equipment on a production car by the Chrysler Corporation on the Valiant in 1960, several years ahead of Ford and General Motors.[1][2]
Some early automobiles, like the Ford Model T, used a different sort of charging system: an engine-driven magneto which generated low-voltage alternating current that was supplied to trembler coils, which provided the high voltage needed to generate ignition sparks. (This was different from a true ignition magneto, which generates high voltage directly.) Since such a magneto system only depended on the engine''s motion to generate current, it could even be used when starting a manually cranked engine, provided the crank was pulled sharply, so that the magneto would produce enough current for the coils to make good sparks.
The Model T incorporated its magneto into the engine flywheel. The first Model Ts used the magneto solely for the trembler coil ignition. Beginning with the 1915 model year, Ford added electric headlights, also powered by the magneto.[3][4] The magneto circuit was strictly AC, with no battery included. (There was a switch on the ignition coils to use a battery instead, which could be helpful when starting in cold weather, but Ford neither provided a battery nor did it encourage the use of one before it introduced an electric starter in 1919. The owner would have to install the battery themselves and charge it externally.)
Starting in the 1919 model year, Ford upgraded the Model T to include an electric starter, which was standard for some models and optional for others. This starter installation also included a battery, charged by a conventional dynamo, and the lights were now powered by the battery. However, the flywheel magneto still powered the ignition, and since models without the starter had no battery, they continued to use magneto-powered lights.[5][6]
Alternators have several advantages over direct-current generators (dynamos). Alternators are:
A set of rectifiers (diode bridge) is required to convert AC to DC. To provide direct current with low ripple, a polyphase winding is used and the pole-pieces of the rotor are shaped (claw-pole). Automotive alternators are usually belt-driven at 2–3 times crankshaft speed, speeds that could cause a commutator to fly apart in a generator. The alternator runs at various RPM (which varies the frequency) since it is driven by the engine. This is not a problem because the alternating current is rectified to direct current.
Alternator regulators are also simpler than those for generators. Generator regulators require a cutout relay to isolate the output coils (the armature) from the battery at low speed; that isolation is provided by the alternator rectifier diodes. Also, most generator regulators include a current limiter; alternators are inherently current-limited.
The claw pole design produces an AC waveform that is more efficiently rectified than a sine wave.
Despite their names, both ''DC generators'' (or ''dynamos'') and ''alternators'' initially produce alternating current. In a so-called ''DC generator'', this AC current is generated in the rotating armature, and then converted to DC by the commutator and brushes. In an ''alternator'', the AC current is generated in the stationary stator, and then is converted to DC by the rectifiers (diodes).
Typical passenger vehicle and light truck alternators use Lundahl or ''claw-pole'' field construction. This uses a shaped iron core on the rotor to produce a multi-pole field from a single coil winding. The poles of the rotor look like fingers of two hands interlocked with each other. The coil is mounted axially inside this and field current is supplied by slip rings and carbon brushes. These alternators have their field and stator windings cooled by axial airflow, produced by an external fan attached to the drive belt pulley.[7]
Alternators can also be water-cooled in cars.
Larger vehicles may have field coil alternators similar to larger machines.[9]
The windings of a 3 phase alternator may be connected using either the delta or star (wye) connection regime set-up.[10]
Brushless versions of these type alternators are also common in larger machinery such as highway trucks and earthmoving machinery. With two oversized shaft bearings as the only wearing parts, these can provide extremely long and reliable service, even exceeding the engine overhaul intervals.
In recent years,[when?] alternator regulators are linked 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.
Older automobiles with minimal lighting may have had an alternator capable of producing only 30 amperes. Typical passenger car and light truck alternators are rated around 50–70 A,[citation needed] though higher ratings are becoming more common, especially as there is more load on the vehicle''s electrical system with air conditioning, electric power steering and other electrical systems. Very large alternators used on buses, heavy equipment or emergency vehicles may produce 300 A. Semi-trucks usually have alternators which output 140 A. Very large alternators may be water-cooled or oil-cooled.
Hybrid electric vehicles replace the separate alternator and starter motor with one or more combined motor/generator(s) that start the internal combustion engine, provide some or all of the mechanical power to the wheels, and charge a large storage battery. When more than one motor/generator is present, as in the Hybrid Synergy Drive used in the Toyota Prius and others, one may operate as a generator and feed the other as a motor, providing an electromechanical path for some of the engine power to flow to the wheels. These motor/generators have considerably more powerful electronic devices for their control than the automotive alternator described above.
You hop in your car and crank it up. The engine turns over grudgingly but eventually starts. After warming up the engine for a minute, you begin your journey home. Just as the heater begins to remove the frosty conditions inside the car, the radio begins to cut out. Soon after, the dash lights start to dim ever so slightly, but you keep driving.
Eventually, the radio and the heater quit altogether, and the comfortable warmth is slowly replaced by the outside chill. Your headlights are the next thing to start flickering. Now you''re getting worried. Only 15 more minutes and you''re home.
But the headlights dim to the point of being dangerous and the unthinkable happens: the engine starts to miss. Less than 5 miles (8 kilometers) from home, your engine dies, along with everything else in the car. You coast to a stop on the side of the road and pull out your cellphone. It''s dead, so you plug it in to call for help. Guess what? No power. It''s past midnight, and you''re stranded on the side of the road.
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