This arrangement has many advantages over a dc
Automotive generators are disassembled only
generator armature. All the current generated in a dc
when major repairs are to be made. Other than cleaning
armature must be transferred to the system through
commutators and replacing worn brushes during
brushes. Where the output is high, the brushes have to
periodic maintenance checks, automotive generators
be very large to handle the power. In the alternator, this
need few major repairs during normal service life.
problem is eliminated because the stator windings are
However, if neglected, generators will develop troubles
stationary. Only low voltage field current passes
that cannot be remedied in the field. In this case, the
through the alternator's brushes. Since the stator
generator must be removed form the vehicle and taken
windings are stationary, they are not affected by
to the shop for repairs.
centrifugal force as are the loops in the armature of a dc
ALTERNATING CURRENT (AC) GENERATOR
generator.
The rotor consists of four parts--a coil, a core, and
two rotor halves or spiders. The rotor is assembled by
The output requirements of automotive electrical
placing the coil on the core and then fitting the rotor
generators have increased considerably in recent years
halves around them. These are assembled on a shaft,
because of the growing popularity of power-consuming
and the coil leads are connected to slip rings.
electrical accessories, such as heavy-duty heaters and
Because of the rugged construction of the rotor
air-conditioners.
halves, this unit can be rotated at very high speeds
A conventional dc generator that would produce
without damage. The limiting factor is the rpm rating of
the required amount of electricity at both a high- and
the bearings that hold the shaft in the housing.
low-speed range would be so large that its practical
A basic alternator (fig. 6-17) consists of one
application would be limited. An ac generator
winding or loop in the stator and a single pair of poles in
(alternator), on the other hand, can produce enough
the rotor. When the rotor of this machine is turned
power to fulfill almost any need over a speed range
through 360 degrees, it induces a single cycle of
varying from idle to top engine speed.
alternating current. If a rotor with six pairs of poles
Because of the small size of the alternator, it can be
were put in the basic alternator (fig. 6-18), six cycles of
adapted to almost any piece of equipment. It is
alternating current would be generated every time the
constructed to withstand vibrations and high speeds
rotor made one complete revolution.
that are encountered in normal service.
If two more windings were added to the stator (fig.
The alternator and the conventional dc generator
6-19), six cycles of alternating current would be
operate on the same theory. The field produces lines of
generated in each loop for every revolution of the rotor.
force that are cut by the loops of the armature winding,
This type of machine is called a three-phase alternator.
inducing alternating current. Basically, the two
The alternator is self-limiting in respect to the
generators are composed of the same functional
output current, which is about 130 percent of its rating.
parts--a rotor, a stator, and a device for extracting
It is limited due to the impedance (caused by inductive
direct current from alternating current.
In the dc generator, the stator is the field, the rotor is
ROTOR
the armature, and the commutator and brush assemblies
LOOP
are the devices for extracting direct current from
alternating current.
The components of the alternator are the rotor, the
stator, the brush assembly, and the rectifier. The rotor is
a rotating magnetic field serving the same function as
the fixed field in a dc generator, and the rotor current is
provided through brushes and a slip ring assembly. The
rotating field induces a current flow in the stator, which
has three fixed coils of wire, physically located 120
O
180O
360O
STATOR
0
degrees apart and connected to produce a three-phase
output. The output of the stator is wired directly to the
rectifier, which consists of six diodes mounted inside
the alternator. The rectifier is full-wave, converting the
ASf06017
ac output of the stator to direct current.
Figure 6-17.--A simple alternator.
6-18
