There are three principal circuits used in an AC generator:
- The charging circuit: Consists of the stator windings and rectifier circuits.
- The excitation circuit: Consists of the rotor field coil and the electrical connections to the coil.
- The preexcitation circuit: Supplies the initial current for the field coil that starts the buildup of the magnetic field.
For the AC generator to produce current, the field coil must develop a magnetic field. Hie AC generator creates its own field current in addition to its output current.
For excitation of the field to occur, the voltage induced in the stator rises to a point that it overcomes the forward voltage drop of at least two of the rectifier diodes. Before the diode trio can supply field current, the anode side of the diode must be at least 0.6 volt more positive than the cathode side. When the ignition switch is turned on, the warning lamp current acts as a small magnetizing current through the field. This current preexcites the field, reducing the speed required to start its own supply of field current.
Note: If the battery is completely discharged, the vehicle cannot be push started because there is no excitation of the field coil.
FIGURE. The diode trio connects the phase windings to the field. To conduct, there must be 0.6 V more positive on the anode side of the diodes.
FIGURE. Schematic of a charging system.