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

Charging System Operation 159
Figure 8-27. A typical current path during rectification
in a delta-type stator. (DaimlerChrysler Corporation)
Figure 8-29. The three-phase voltage from one revolution
of the rotor. (Reprinted by permission of Robert
Bosch GmbH)
Figure 8-30. An AC generator (alternator) with six
pairs of N-S poles would produce the solid line voltage
trace. The dashed line represents an alternator
with one pair. (Reprinted by permission of Robert Bosch
GmbH)
Figure 8-28. A typical current path during rectification
in a Y-type stator. (DaimlerChrysler Corporation)
Rectification with
Multiple-Pole Rotors
The three-phase voltage output used in the examples
(Figure 8-29) is the voltage, which would
result if the rotor had only one N and one S pole.
Actual AC generator rotors have many N and S
poles. Each of these N-S pairs produces one complete
voltage sine wave per rotor revolution, across
each of the three windings. One complete sine
wave begins at zero volts, climbs to a positive,
peak, drops past zero to a negative peak, then
returns to zero, or baseline voltage. In Figure 8-30,
the sine wave voltage caused by a single pole is
shown as a dashed line. The actual voltage trace
from one winding of a 12-pole AC generator is
shown as a solid line. The entire stator output is
three of these waves, evenly spaced and overlapping
(Figure 8-31). The maximum voltage value
from these waves pushes current through the
diodes (Figure 8-32).
After rectification, AC generator (alternator)
output is DC voltage, which is slightly lower than
the maximum voltage peaks of the stator output.
The positive portion of the AC sine wave greater
than the DC output voltage is viewable on an
oscilloscope in what is called an AC generator
(alternator) ripple pattern (Figure 8-33).
Excitation Field Circuit
Field current through the rotor windings creates
the magnetic field of the rotor. Field current is
drawn from the AC generator output circuit once