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

160 Chapter Eight
Figure 8-31. This is the total output of a three-winding,
multiple-pole AC generator.
The diodes receive the maximum volt-
Figure 8-32.
age values.
Figure 8-33. AC generator (alternator) ripple is the AC
voltage exceeding DC output voltage.
Figure 8-34. Some AC generators (alternators) have
additional diodes to rectify field current. (Reprinted by
permission of Robert Bosch GmbH)
the AC generator has begun to produce current.
However, there is not enough residual magnetism
in the rotor poles to induce voltage during start
up. An AC generator cannot start operation independently.
Field current must be drawn from
another source in order to magnetize the rotor and
begin AC generator output.
The other source is the vehicle battery connected
to the rotor winding through the excitation,
or field, circuit. Battery voltage “excites”
the rotor magnetic field and begins output.
When the engine is off, the battery must be disconnected
from the excitation circuit. If not, it
could discharge through the rotor windings to
ground. Some AC generators use a relay to control
this circuit. Other systems use the voltage
regulator or a part of the ignition switch to control
the excitation circuit.
Once the AC generator has started to produce
current, field current is drawn from the AC generator
output. The current may be drawn after it
has been rectified by the output diodes. Some AC
generators draw field current from unrectified
AC output, which is then rectified by three additional
diodes to provide DC to the rotor winding
(Figure 8-34). These additional diodes are called
the exciter diodes or the field diodes.
Circuit Types
AC generators (alternators) are designed with different
types of field circuits. The two most common
types are A-circuits and B-circuits. Circuit
types are determined by where the voltage regulator
is connected and from where the field current
is drawn.