Mercury MerCruiser MCM 454 Mag MPI Service Manual page 359

CHARGING SYSTEM
Description
The alternator has a rotor, which is supported in two end frames by ball bearings, and is
driven by a pulley at approximately twice engine speed. The rotor contains a field winding
that is enclosed between two multiple-finger pole pieces. The ends of the field winding are
connected to two brushes (mounted in the rear end frame) which make continuous sliding
(or slipping) contact with the slip rings. The current (flowing through the field winding)
creates a magnetic field that causes the adjacent fingers of the pole pieces to become
alternate north and south magnetic poles.
A 3-phase stator is mounted directly over the rotor pole pieces and between two end frames.
It consists of three windings wound 120 degrees electrically out-of-phase on the inside of
a laminated core. The windings are all connected together on one end, while the other ends
are connected to a full-wave rectifier bridge.
The rectifier bridge contains six diodes which are arranged so that current flows from
ground, through the stator and to the output terminal, but not in the opposite direction.
When current is supplied to the rotor field winding, and the rotor is turned, the movement
of the magnetic fields created induces an alternating current into the stator windings. The
rectifier bridge then changes this alternating current to direct current which appears at the
output terminal. A diode trio also is connected to the stator windings to supply current to the
regulator and the rotor field during operation.
Voltage output of the alternator is controlled by regulating the current supplied to the rotor
field. This is accomplished by a transistorized voltage regulator that senses the voltage at
the battery and regulates the field current to maintain alternator voltage within prescribed
limits for properly charging the battery. Current output of the alternator does not require
regulation, as maximum current output is self-limited by the design of the alternator. As long
as the voltage is regulated within the prescribed limits, the alternator cannot produce
excessive current. A cut-out relay in the voltage regulator also is not required, as the rectifier
diodes (which allow current to flow in one direction only) prevent the battery from
discharging back through the stator.
Due to the lack of residual magnetism in the rotor pole pieces, a small amount of current
must be supplied to the rotor field to initially start the alternator charging. This is accom-
plished by means of an excitation circuit in the regulator which is connected to the ignition
switch. Once the alternator begins to produce output, field current is supplied solely by the
diode trio, as explained, preceding.
The alternator also is equipped with a fan (mounted on the rotor shaft) which induces air flow
through the alternator to remove the heat created by the rectifiers and stator. A capacitor
protects the rectifier system from high voltages and suppresses radio noise.
Index
Page 4C-4
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SERVICE MANUAL NUMBER 23
90-861326--1 MARCH 1999