Alternator
Alternator is a device
electromechanical auto
parts that converts mechanical energy to alternating current
electrical energy. In General, any AC
electrical generator can be called an alternator, but usually the word
refers to small rotating machines driven by automotive and other
internal combustion engines. Most of the alternators use a rotating
magnetic field but linear alternators are occasionally used. In UK, large alternators in power stations
which are driven by steam turbines are called turbo-alternators.
Automotive Alternators
Alternators are used in modern
automobiles to charge the battery and to power a car's electric system
when its engine is running. The stronger construction of automotive
alternators allows them to use a smaller pulley so as to turn twice as
fast as the engine, improving output when the engines are idling. The
availability of low-cost solid-state diodes from about 1960 onward
allowed car manufacturers to substitute alternators for DC generators.
Automotive 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.
General passenger vehicle and light
truck alternators use claw-pole field construction, where the field
north and south poles are all energized by a single winding, with the
poles looking rather like fingers of two hands interlocked with each
other. Larger vehicles have salient-pole alternators similar to larger
machines. The automotive alternator is usually belt driven at 2-3 times
the engine crankshaft speed.
Modern automotive alternators have
a voltage regulator built into them. The voltage regulator operates by
modulating the small field current in order to produce a constant
voltage at the stator output. The field current is much smaller than the
output current of the alternator; for example, a 70-amp alternator may
need only 2 amps of field current. The field current is supplied to the
rotor windings by slip rings and brushes. The low current and relatively
smooth slip rings ensure greater reliability and longer life than that
obtained by a DC generator with its commentator and higher current being
passed through its brushes.
In comparison, very small
high-performance permanent magnet alternators, such as those used for
bicycle lighting systems, achieve an efficiency of around only 60%.
Larger permanent magnet alternators can achieve much higher efficiency.
Efficiency of automotive alternators are limited by fan cooling loss,
bearing loss, iron loss, copper loss, and the voltage drop in the diode
bridges; at part load, efficiency is between 50-62% depending on the
size of alternator, and varies with alternator speed.
A typical automotive alternator
mounted in a spacious pickup truck engine bay.
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 is starts 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. The wire supplying
the field current is often referred to as the "exciter" wire. The
drawback of this arrangement is that if the warning light fails or the
"exciter" wire is disconnected, no excitation current reaches the
alternator field windings and so the alternator, due to low residual
magnetism in the rotor will not generate any power. The driver may check
for a faulty exciter-circuit by ensuring that the warning light is
glowing with the engine stopped.
Very large automotive like truck
and other alternators used on buses, heavy equipment or emergency
vehicles may produce 300 amperes. Very large automotive alternators may
be water-cooled or oil-cooled. Very old automobiles with minimal
lighting and electronic devices may have only a 30 ampere alternator.
Typical passenger car and light truck alternators are rated around 50-70
amperes, though higher ratings are becoming more common.
Many alternator voltage regulators
are today linked to the vehicle's on board computer system, and in
recent years other factors including air temperature (gained from the
mass air flow sensor in many cases) and engine load are considered in
adjusting the battery charging voltage supplied by the alternator.
Brushless Alternators
Terminology
The stationary part of a motors or
alternators is called the stator and the rotating parts are called the
rotors. The coils of wire that are used to produce a magnetic field are
called the field and the coils that produce the power are called the
armature. These coils are commonly called the “windings”.
Construction
The brushless alternator is
composed of two alternators built end-to-end on one shaft. Smaller
brushless alternators may look like one unit but the two parts are
readily identifiable on the large versions. The exciter has stationary
field coils and a rotating armature (power coils). The main alternator
uses the opposite configuration with a rotating field and stationary
armature. A bridge rectifier, called the Rotating Rectifier Assembly, is
mounted on a plate attached to the rotor.
Main Alternator
The main alternator has a rotating
field as described above and a stationary armature (power generation
windings).
Automatic Voltage Regulator (AVR)
The AVR regulates the alternator's
output voltage by varying the amount of current in the stationary
exciter field coils.
Hybrid automobiles
Now a day new generation is a
Hybrid automobile, which replace the separate alternator and starter
motor with a combined motor/generator that performs 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 powerful electronic devices for their control
than the simple automotive alternator described above.
