Automotive Electrical and Electronic Systems Classroom Manual Fifth Edition Update by John F. Kershaw

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Magnetism 63
Induced current travels from the conductor
through the commutator and out through the
brushes (Figure 4-24A). At the instant the looped
conductor is turned so that the induced current
changes direction (Figure 4-24B), the commutator
also rotates under the brushes, so that the
brushes now contact the opposite commutator
segments. Current now flows out of the other half
of the commutator, but the same brush is there to
receive it. This design is called a brush-rectified,
or commutator-rectified, generator, and the output
is called pulsating direct current. Actual DC
generators have many armature windings and
commutator segments. The DC voltages overlap
to create an almost continuous DC output.
AC Generator (Alternator)
Principles
Since 1960, virtually all automobiles have used an
AC generator (alternator), in which the movement
of magnetic lines through a stationary conductor
(Figure 4-25) generates voltage. A magnet
called a rotor is turned inside a stationary looped
conductor called a stator (Figure 4-25). The
induced current, like that of a DC generator, is
constantly changing its direction. The rotation of
the magnetic field causes the stator to be cut by
flux lines, first in one direction and then the
other. The AC must be rectified to match the battery
DC by using diodes, which conduct current
in only one direction. This design is called a
diode-rectified alternator. We will study diodes
in Chapter 10 and alternators in Chapter 8 of
this book.
See the section on “Diagnostic Strategies” in
Chapter 4 of the Shop Manual.
Self-Induction
Up to this point, our examples have depended upon
mechanical energy to physically move either the
conductor or the magnetic field. Another form of
relative motion occurs when a magnetic field is
forming or collapsing. When current begins to flow
in a coil, the flux lines expand as the magnetic
Figure 4-24. The commutator and brushes conduct
pulsating direct current from the looped conductor.
Figure 4-25.
A simplified alternator.