[97Silverado]

Here is a good explanation of how an alternator works:

Alternator Construction and Operation

• An alternator or A.C. generator creates voltage by rotating a magnetic field inside a looped conductor.

Alternator Components
Rotor

• A multi-pole electromagnet that is rotated by a belt & pulley.

• Usually 4 to 7 pole pairs.

Rotor Components

• Winding & Slip Rings

• Magnetic Pole Pieces

• Shaft With Iron Core

Stator

• A circular laminated iron frame that holds the looped conductor.

• An automotive alternator stator usually has three separate looped conductors.

• Each conductor has as many loops as the rotor has pole pairs.

• The current output is 3-phase (3 over-lapping A.C. currents).

Stator Winding Types

• Wye (Y) - 3 looped conductors connected in series. Higher voltage output at low speeds.

• Delta - 3 looped conductors connected in parallel. Higher current output at low speeds.

Rectifier Bridge

• A set of 6 or more diodes used to rectify the A.C. current in the stator into D.C. current before it leaves the alternator.

• The diodes are mounted in a heat sink.

The Charging System consists of two circuits: Output and Field
-Output Circuit:

• The output circuit consists of the stator windings, rectifier bridge, battery and all electrical loads.

• The stator produces current in two conductors during the same time period.

• As the rotor rotates, the stator windings that produces current change every 120 degrees. This produces 3-phase output.

• As the magnetic poles alternate past the loops they produce A.C. voltage.

• The rectifier bridge consists of positive diodes connected to (B+) and negative diodes connected to (B-) in pairs.

• The stator output goes into the bridge as A.C. and leaves as D.C.

-Field Circuit:

• The field circuit may consist of the rotor winding, slip rings & brushes, voltage regulator, indicator bulb and wiring.

• The field circuit controls the current flow through the rotor winding.

• This current is called the “excitation” current.

• Excitation current is provided by the battery before the stator begins producing current.

• The stator provides excitation current after the alternator starts producing current.

Voltage Regulator

• The voltage regulator is part of the field circuit.

• It limits alternator output voltage by controlling the strength of the rotor field.

• The regulator senses battery voltage and controls system voltage to a safe level by pulsing the rotor current.

• Long pulses result in a stronger field and higher voltage output and vice versa.

• Maximum alternator current output is limited by stator CEMF.

• Output voltage will exceed 16V if not regulated.

Field Circuit Types:
-“A”

• The regulator supplies the ground for the field circuit. The other side of the field is connected directly to battery B(+).

• Both brushes are insulated.

• “A” for after rotor!

-“B”

• The regulator supplies B(+) for the field circuit.

• One brush is insulated, the other is grounded.

• “B” for before rotor!

-Isolated

• The ignition switch supplies B(+) to the field.

• The regulator supplies the ground. Both brushes are insulated.

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Alternator Output

• Measure current output at the alternator B(+) wire while loaded to 12V at 2000rpm.

• Should be within 10% of rated output.

• Example: Alternator rating; 120 amps Tested output should be 0.9 x 120 amps = 108 amps minimum