An alternator differs from a conventional DC shunt generator in that the armature is stationary, and is called the stator, while the field rotates and is called the rotor. The higher current values in the alternator's stator are conducted to the external circuit through fixed leads and connections, rather than through a rotating commutator and brushes as in a DC generator. This eliminates a major point of maintenance.
The alternator uses a 3-phase stator winding. The rotor consists of a field coil encased between 6-poled, interleaved sections, producing a 12-pole magnetic field with alternating north and south poles. By rotating the rotor inside the stator, an alternating current is induced in the stator windings. This alternating current is changed to direct current by diodes and is routed out of the alternator through the output terminal. Diode rectifiers act as one way electrical valves. Half of the diodes have a negative polarity and are grounded. The other half of the diodes have a positive polarity and are connected to the output terminal.
Since the diodes have a high resistance to the flow of current in one direction, and a low resistance in the opposite direction, they are connected in a manner which allows current to flow from the alternator to the battery in the low resistance direction.
The high resistance in the other direction prevents the flow of current from the battery to the alternator. Because of this feature, there is no need for a circuit breaker between the alternator and the battery.
Residual magnetism in the rotor field poles is minimal. The starting field current must, therefore, be supplied by the battery. It is connected to the field winding through the ignition switch and the charge indicator lamp or ammeter.
As in the DC shunt generator, the alternator voltage is regulated by varying the field current. This is accomplished electronically in the transistorized voltage regulator. No current regulator is required because all alternators have self limiting current characteristics.
An alternator is better than a conventional DC shunt generator because it is lighter and more compact, because it is designed to supply the battery and accessory circuits through a wide range of engine speeds, and because it eliminates the necessary maintenance of replacing brushes and servicing commutators.
The transistorized voltage regulator is an electronic switching device that is located on the alternator housing. It senses the voltage at the auxiliary terminal of the alternator and supplies the necessary field current for maintaining the system voltage at the output terminal. The output current is determined by the battery electrical load, such as operating headlights or heater blower. The regulator is a sealed unit on which no adjustments are possible.