Description: The alternator consists of a spinning set of electrical windings
called a rotor, a stationary set of windings called a stator, a rectifier
assembly, a set of brushes to maintain electrical contact with the rotor, and
a pulley. All of these parts except the pulley are contained in an aluminum
housing. Today?s alternators use compact, electronic voltage regulators
that may be housed inside the alternator or the voltage regulator function
may be handled by the vehicle?s powertrain control module (PCM).
Purpose: The alternator generates direct current for recharging the battery
and for powering vehicle electrical loads.
Maintenance Tips/Suggestions: Have the alternator?s drive belt tension
checked at every oil change. A loose belt can reduce alternator output and
run down your car?s battery. Each spring, prior to travel season, it?s wise
to have your car?s charging system tested as part of a comprehensive
starting, charging and battery test. This test will determine whether your
car?s alternator is putting out the proper amount of current and voltage.
Your car?s alternator is designed to recharge the battery after slight
discharging such as engine starting; the alternator is not designed for
charging heavily discharged (?dead?) batteries. Relying on the alternator to
charge a heavily discharged battery can overload the alternator and cause
damage. In such cases, use a battery charger instead. An alternator
problem can cause a discharged battery, poor accessory and light
operation, frequent bulb replacement, repeat voltage regulator failures,
erratic engine operation, or a dashboard warning light to illuminate. To
pinpoint the cause, have your car?s charging system checked out by a
qualified service technician. Life expectancy of the average alternator is
75-100 thousand miles.
Standard Automobile Alternator
A key component of an automobile's charging system, which provides
current to recharge the battery and develops electricity to power all other
electrical components when the engine is running, is the alternator. The
other component of the charging system is the voltage regulator. The basic
function of the alternator is to generate the electricity required to start and
run the automobile, while the regulator is designed to control the amount of
voltage that circulates through the system. This discussion will focus entirely
upon the alternator, specifically, the principle of operation by which it works
and its main components. An alternator (Fig. 1) consists of rotor assembly,
a stator assembly, and a rectifier mounted in a housing.
Figure 1. Parts of an Alternator
Alternator Housing. The housing is usually made up of two pieces of die-cast
aluminum. Aluminum is used because it is a nonmagnetic, lightweight
material that provides good heat dissipation. Bearings supporting the rotor
assembly are mounted in the front and rear housing. The front bearing is
usually pressed into the front housing or onto the rotor shaft. It is usually a
factory-lubricated ball bearing. The rear bearing is usually installed with a
light press fit in the rear housing.
Stator Assembly. The stator is clamped between the front and the rear
housing. A number of steel stampings are riveted together to form its frame.
Three windings around the stator frame are arranged in layers in each of the
slots on the frame. At the other end, they are connected into the rectification
Rotor Assembly. The rotor assembly consists of a rotor shaft, a winding
around an iron core, two pole pieces, and slip rings. The rotor is pressed
into the core. Six-fingered, malleable, iron pole pieces are pressed onto the
shaft against each end of the winding core. They are placed so that the
fingers mesh but do not touch. When direct current is passed through the
field coil winding, the fingers become alternately north and south poles. A
slip ring assembly is pressed on to the rear end of the rotor shaft and
connected to the two ends of the field winding.
Two brushes are held against the slip rings by springs, usually mounted in
plastic brush holders that support the brushes and prevent brush sticking.
Each brush is connected into the circuit by a flexible copper lead wire. The
brushes ride on the slip rings and are connected through a switch to the
battery. When the switch is closed, current from the battery passes through
one brush, through the slip ring, and then through the field winding. After
leaving the field winding, current flows through the other slip ring and brush
before returning to the battery through the ground return path. The flow of
electrical energy through the field winding, called field current, creates the
magnetic field for the rotor.
Rectifier Assembly. The rectifier assembly consists of six diodes mounted
either in the rear housing or in a separate small housing called a rectifier
bridge. Three of the diodes are connected to ground, and three are mounted
in an insulator. Since the mounting assembly carries off heat caused by the
operation of the diode, it is often called a heat sink.
A fan and pulley assembly is either pressed onto the rotor shaft or held with
a nut. The pulley drives the rotor through an engine accessory drive belt.
The fan behind the alternator pulley pulls air in through vents at the rear of
the alternator to cool the diodes.
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