Alternator: Description and Operation

Alternators are composed of the same functional parts as the conventional D.C. generator but they operate differently: The field is called a rotor and is the turning portion of the unit. A generating part, called a stator, is the stationary member, comparable to the armature in a D.C. generator. The regulator, similar to those used in a D.C. system, regulates the output of the alternator-rectifier system.
The power source of the system is the alternator. Current is transmitted from the field terminal of the regulator through a slip ring to the field coil and back to ground through another slip ring. The strength of the field regulates the output of the alternating current. This alternating current is then transmitted from the alternator to the rectifier where it is converted to direct current.
These alternators employ a three-phase stator winding in which the phase windings are electrically 120° apart. The rotor consists of a field coil encased between interleaved sections producing a magnetic field with alternate north and south poles. By rotating the rotor inside the stator the alternating current is induced in the stator windings. This alternating current is rectified (changed to D.C.) by silicon diodes and brought out to the output terminal of the alternator.

Fig. 1 Bosch 90 Amp Alternator:

This alternator is conventional design, Fig. 1, with rotor magnetic field spinning within the stationary stator windings. The rotor is supported in the housing on ball bearings which are pressed on the shaft. Field current is supplied through the brush set that rides against the rotor slip rings. Because voltage is induced in ``Wye'' wound stator which produces alternating current, a rectifier assembly is installed between stator and alternator output terminal. The rectifier assembly contains six positive and six negative silicon diodes that allow only the positive side of alternating current to reach the output terminal.