Figure 13-21. Typical Alternator.
quickly due to arcing caused by the large current carried
(2) The ac generator does not retain residual
magnetism in its field. Because of this, an ac generator-
equipped vehicle cannot be push started with a
(2) The rotating field in the ac generator is much lighter
completely dead battery.
and less susceptible to centrifugal force. Because the
c. Comparison of Output Characteristics. It can be
brushes must only transmit field current, they can be
much smaller and will last longer due to less arcing.
seen from figure 13-23 that the dc generator has a much
narrower speed-producing range than the ac generator.
(3) The ac generator uses smooth sliprings on its rotor
The initial startup is at a much higher rpm, which is
that produce very little arcing during use. This compares
undesirable for vehicles that operate mostly in low-speed
with the segmented commutator of the dc generator,
rang-s. As high speeds are reached, the dc generator
which causes large efficiency losses due to drag and
output will fall below its rated output largely due to the
brushes bouncing on the commutator segments, causing
(4) The ac generator is much smaller and lighter than
its dc counterpart.
13-23. The Automotive Alternator.
a. The Basic Alternator. A basic alternator would
consist of one winding or loop in the stator and a single
(1) The ac generator requires electronic rectification
pair of poles in the rotor (fig. 13-24). When the rotor of
through the use of silicon diodes. Modern rectifier
this machine is turned through 360 degrees, it will induce
bridges, though extremely durable under normal
a single cycle of ac just as the simple generator armature
conditions, are extremely sensitive to accidental polarity
reversal. This can result from jump starting, battery
charging, and battery installation.
b. Rotor Design (Fig. 13-25). The rotor is designed
with two pole pieces that sandwich the