Alternators are used in modern automobiles to charge the battery and to power the electrical system when its engine is running. Until the 1970s, automobiles used DC dynamo generators with commutators. With the availability of affordable silicon diode rectifiers, alternators were used instead. Alternators have several advantages over direct-current generators. They are lighter, cheaper and more rugged. They use slip rings providing greatly extended brush life over a commutator. 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 (diode bridge) is required to convert AC to DC. To offer direct current with low ripple, a three-phase winding is used and the pole-pieces of the rotor are shaped (claw-pole) to produce a waveform similar to a square wave instead of a sinusoidal. Automotive alternators are usually belt driven at 2-3 times crankshaft speed.
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.
Typical passenger vehicle and light truck alternators use Lundell or claw-pole field construction, where the field north and south poles are all energized by a single winding, with the poles looking like fingers of two hands interlocked with each other. Larger vehicles may have salient-pole alternators similar to larger machines.
Automotive alternators need a voltage regulator which operates by modulating the small field current to produce a constant voltage at the battery terminals. Early designs (c.1960s-1970s) used a discrete device mounted elsewhere in the vehicle. Intermediate designs (c.1970s-1990s) incorporated the voltage regulator into the alternator housing. Modern designs do away with the voltage regulator altogether; voltage regulation is now a function of the electronic control unit (ECU). The field current is smaller than the output current of the alternator; such as, a 70 A alternator may need only 7 A of field current. The field current is supplied to the rotor windings by slip rings. The low current and relatively smooth slip rings ensure greater reliability and longer life than that obtained by a DC generator with its commutator and higher current being passed through its brushes.
The field windings are initially supplied power from the battery via the ignition switch and “charge” warning indicator (which is why the indicator is on when the ignition is on but the engine is not running). 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 warning indicator which goes off. The wire supplying the field current is often called the “exciter” wire. The drawback of this arrangement is that if the warning lamp burns out or the “exciter” wire is disconnected, no current reaches the field windings and the alternator will not generate power. Some warning indicator circuits are equipped with a resistor in parallel with the lamp that let excitation current to flow if the warning lamp burns out. The driver should check that the warning indicator is on when the engine is stopped; otherwise, there might not be any sign of a failure of the belt which may also drive the cooling water pump. Some alternators will self-excite when the engine reaches at a certain speed.
Older automobiles with minimal lighting may have had an alternator capable of producing only 30A. Typical passenger car and light truck alternators are rated around 50-70A, 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 amperes. Semi-trucks usually have alternators which output 140A. Very large alternators may be water-cooled or oil-cooled.
In recent years, 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.
Efficiency of automotive alternators is limited by fan cooling loss, bearing loss, iron loss, copper loss, and the voltage drop in the diode bridges. At partial load efficiency is between 50-62% depending on the size of alternator and varies with alternator speed. This is like very small high-performance permanent magnet alternators, such as those used for bicycle lighting systems, which make an efficiency around 60%. Larger permanent magnet alternators can make higher efficiencies. Large AC generators used in power stations run at carefully controlled speeds and have no constraints on size or weight. They have higher efficiencies, as high as 98%.
Hybrid automobiles replace the separate alternator and starter motor with one or more combined motor/generator(s) (M/Gs) that start the internal combustion engine, offer some or all the mechanical power to the wheels, and charge a large storage battery. When more than one M/G is present, as in the Hybrid Synergy Drive used in the Toyota Prius and others, one may run 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.