Brushless Alternator (Typical Example)
(1) Regulator. (2) Roller bearing. (3) Stator winding. (4) Ball
bearing. (5) Rectifier bridge. (6) Field winding. (7) Rotor
assembly (8) Fan.
Brushless alternators have no need for slip rings or brushes,
and the only part that has movement is the rotor assembly. All
conductors that carry current are stationary. The conductors
are: the field winding, stator windings, rectifying diodes and
the regulator circuit components.
The rotor assembly has many magnetic poles like fingers with
air space between each opposite pole. The poles have
residual magnetism (like permanent magnets) that produce a
small amount of magnet-like lines of force (magnetic field)
between the poles. As the rotor assembly begins to turn
between the field winding and the stator windings, a small
amount of alternating current (AC) is produced in the stator
windings from the small magnetic lines of force made by the
residual magnetism of the poles. This AC current is changed
to direct current (DC) when it passes through the diodes of the
rectifier bridge. Most of this current goes to charge the battery
and to supply the low amperage circuit, and the remainder is
sent on to the field windings. The DC current flow through the
field windings (wires around an iron core) now increases the
strength of the magnetic lines of force. These stronger lines of
force now increase the amount of AC current produced in the
stator windings. The increased speed of the rotor assembly
also increases the current and voltage output of the alternator.
Alternator (Brush Type)
Brush Type Alternator (Typical Example)
(1) Stator. (2) Field coil (rotor). (3) Regulator.
In brush type alternators, stator (1) is stationary but the field
coil (rotor) (2) rotates. Slip rings and brushes are used for
supplying excitation current to the rotating field coil.
Brush type alternators can be battery energized or self
energized. Self energized alternators depend on residual
magnetism like the brushless alternators. Battery energized
alternators, with less residual magnetism, use current from the
batteries for field current to produce the magnetic lines of
force. When the alternator's voltage exceeds the battery
voltage, the charging system will be self contained and battery
voltage will not be needed.
The voltage regulator is a solid state (transistor, stationary
parts) electronic switch. It feels the voltage in the system and
switches on and off many times a second to control the field
current (DC current to the field windings) for the alternator to
make the needed voltage output.