Alternator (automotive) - Wikipedia Page 1 of 5

Alternator (automotive)
From Wikipedia, the free encyclopedia

Alternators are 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. With the availability of affordable silicon diode
rectifiers, alternators were used instead. 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.
Alternator (silver) mounted on a V8
engine

Contents
◾ 1 History
◾ 1.1 Ford Model T
◾ 2 Advantages over dynamos
◾ 3 Operation
◾ 4 Field regulation
◾ 5 Hybrid vehicles
◾ 6 References

History
Vehicle alternators were first used by the military from WWII, to power radio equipment on specialist
vehicles.[i] Post-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]

Ford Model T

The first car to use an alternator was an unusual system fitted to early Model T Fords. This entirely AC
system was first used solely to power the trembler coil ignition system when the engine was running.[3]
When starting, a battery was used instead – cranking the engine was entirely manual. This system was
sometimes used to also provide electric lighting. Being an AC system, there was no battery in this
circuit. The starting battery was removed from the car for charging, a rare event as it was only needed
when starting. The generator was usually described as a magneto, although this was not an ignition
magneto (even though it was used to power the ignition) as it did not provide sparks itself.

https://en.wikipedia.org/wiki/Alternator_(automotive) 12/29/2016
Alternator (automotive) - Wikipedia Page 2 of 5

When the Model T was upgraded with electric lighting from the factory, a conventional dynamo was
installed instead. This then permitted battery charging as well.

Advantages over dynamos

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 provide 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
sinusoid. 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.

Operation
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 Lundell

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. Cut-away of an alternator, showing
The poles of the rotor look like fingers of two hands interlocked the claw-pole construction; two of the
with each other. The coil is mounted axially inside this and field wedge-shaped field poles, alternating
current is supplied by slip rings and carbon brushes. These N and S, are visible in the centre and
alternators have their field and stator windings cooled by axial the stationary armature winding is
airflow, produced by an external fan attached to the drive belt visible at the top and bottom of the
opening. The belt and pulley at the
pulley.[4]
right hand end drives the alternator.
Modern vehicles now use the compact alternator layout. This is
electrically and magnetically similar, but has improved air cooling. Better cooling permits more power
from a smaller machine. The casing has distinctive radial vent slots at each end and now encloses the
fan. Two fans are used, one at each end, and the airflow is semi-radial, entering axially and leaving
radially outwards.[5] The stator windings now consist of a dense central band where the iron core and
copper windings are tightly packed, and end bands where the windings are more exposed for better heat
transfer. The closer core spacing from the rotor improves magnetic efficiency. The smaller, enclosed
fans produce less noise, particularly at higher machine speeds.[5]

Larger vehicles may have salient pole alternators similar to larger machines.[6]

https://en.wikipedia.org/wiki/Alternator_(automotive) 12/29/2016
Alternator (automotive) - Wikipedia Page 3 of 5

The windings of a 3 phase alternator may be connected using either the

Delta or Star (Wye) connection regime.[7] set-up.

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.

Field regulation
Automotive alternators require a voltage regulator which operates by
modulating the small field current to produce a constant voltage at the Compact alternator
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 much smaller than the output current of the alternator; for example, 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 supplied with power from the battery via the ignition switch and regulator. A
parallel circuit supplies the "charge" warning indicator and is earthed via the regulator (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
referred to as 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 permit 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 indication of a
failure of the belt which may also drive the cooling water pump. Some alternators will self-excite when
the engine reaches a certain speed.

Older automobiles with minimal lighting may have had an alternator capable of producing only 30 A.
Typical passenger car and light truck alternators are rated around 50-70 A, 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.

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.

https://en.wikipedia.org/wiki/Alternator_(automotive) 12/29/2016
Alternator (automotive) - Wikipedia Page 4 of 5

Efficiency of automotive alternators is limited by fan cooling loss, bearing loss, iron loss, copper loss,
and the voltage drop in the diode bridges. Efficiency reduces dramatically at high speeds mainly due to
fan resistance. At medium speeds efficiency of today's alternators is 70-80%.[8] This betters very small
high-performance permanent magnet alternators, such as those used for bicycle lighting systems, which
achieve an efficiency around 60%. Larger permanent magnet electric machines (that can operate as
motors or alternators) can achieve today much higher efficiencies. Pellegrino et al.,[9] for instance,
propose not particularly expensive designs that show ample regions in which efficiency is above 96%.
Large AC generators used in power stations run at carefully controlled speeds and have no constraints
on size or weight. They have very high efficiencies as high as 98%.

Hybrid vehicles
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, provide some or all of 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 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.

References
i. See Fitted For Wireless.
Wikimedia Commons has
media related to Automobile
1. "Alternators and Generators". Allpar.
alternators.
2. "Valiant by Chrysler" (PDF). Valiant promotional brochure.
Chrysler Corporation (Australia). 1962. "Another Chrysler
"first" ... the amazing new alternator"
3. "Model T Ford Electrical System".
4. "Electrical System and Power Supply". Automotive Handbook
(3rd ed.). Bosch. 1993. pp. 770–771. ISBN 0-8376-0330-7.
5. Bosch & 3rd, p. 771
6. Bosch & 3rd, pp. 771–772
7. "Understanding 3 phase alternators....". windstuffnow.com.
Retrieved 2012-07-24.
8. Horst Bauer (ed.) Automotive Handbook 8th Edition, Robert
Bosch GmbH, Stuttgart, 2011, ISBN 978-0-8376-1686-5, page
993
9. G. Pellegrino, A. Vagati, P. Guglielmi, "Performance
Comparison Between Surface-Mounted and Interior PM Motor
Drives for Electric Vehicle Application" IEEE Transactions on
industrial electronics, vol 59, No 2, February 2012 pag. 809

Retrieved from "https://en.wikipedia.org/w/index.php?title=Alternator_(automotive)

&oldid=747551306"

Categories: Alternators Electrical generators Automotive charging circuits

https://en.wikipedia.org/wiki/Alternator_(automotive) 12/29/2016
Alternator (automotive) - Wikipedia Page 5 of 5

◾ This page was last modified on 3 November 2016, at 00:42.

◾ Text is available under the Creative Commons Attribution-ShareAlike License; additional terms
may apply. By using this site, you agree to the Terms of Use and Privacy Policy. Wikipedia® is a
registered trademark of the Wikimedia Foundation, Inc., a non-profit organization.

https://en.wikipedia.org/wiki/Alternator_(automotive) 12/29/2016