Delco 27SI – Systems Operation

Normal Operation

SMCS – 1405

The field winding uses DC voltage to create a magnetic field. The magnetic field magnetizes the rotor. The rotor rotates the magnetic field on the inside of the stator. The stator generates AC voltage. The rectifier changes the AC voltage into DC voltage. Part of the DC voltage returns to the field winding in order to maintain the magnetic field. The remainder of the DC voltage is supplied to the battery and to the electrical systems through battery terminals.

Alternator Operation Schematic

 Illustration 1 g00901286
Electrical Schematic of the 27-SI Series Alternator

(1) Field Winding (Exciter)

(3) Stator Windings

(4) Regulator

(5) Rectifier

(6) Capacitor

(7) Diode Assembly

The illustrations above show the electrical schematics for the alternators.

The alternator has two circuits: the charging circuit and the excitation circuit. The charging circuit functions during normal operation. The excitation circuit functions during normal operation and during start-up.

Charging Circuit

The charging circuit supplies current to the battery and to the electrical systems. The stator windings generate three-phase AC voltage. The positive diodes and the negative diodes change the AC voltage into DC voltage. The DC voltage allows current to flow to the battery terminals.

Excitation Circuit (Normal)

The excitation circuit supplies current to the field winding (exciter) during normal operation. The alternator is self-excited. The rotor contains a core that acts as a rotating magnet. The rotating magnetic field induces voltages in the stator windings. The stator windings generate three-phase AC voltage. The exciter diodes and the negative diodes change the AC voltage into DC voltage. The DC voltage allows current to flow in the field winding. The current induces a stationary magnetic field in the field winding. The stationary magnetic field keeps the rotor core magnetized. The process continues as the rotor operates normally.

Excitation Circuit (Start-up)

The excitation circuit also supplies current to the field winding (exciter) during start-up. The alternator depends on the residual magnetism in the rotor core in order to achieve normal operation. Once the rotor begins to move, the residual magnetism induces weak voltages in the stator windings. The voltages cause a weak current to flow through the excitation circuit. The current produces a stationary magnetic field in the field winding. The stationary magnetic field strengthens the magnetism in the rotor core. The voltage in the stator windings increases. The effect is cumulative. The voltage continues to rise until the regulator controls the output voltage. The regulator controls the output voltage by varying the current in the field winding (exciter).

The alternator functions normally when the excitation circuit produces the breakdown voltage of two diodes in series. The diodes are one exciter diode and one negative diode. The rotor generates the breakdown voltage at the turn-on speed (2000 rpm). When the rotor reaches 2000 rpm, the alternator generates an output.

Regulator Operation

The alternator charges the battery. The alternator also supplies power to the electrical systems. In order to prevent overcharging the battery or damaging the systems, the voltage regulator keeps the output voltage at a constant level. For 24 volt systems, the voltage is regulated to 28 ± 1 volts. For 12 volt systems, the voltage is regulated to 14 ± .5 volts. The regulator maintains the voltage regardless of variations in load or variations in rotor speed.

The alternator output voltage is directly related to exciting current. For example, increasing the exciting current increases the output voltage. By controlling the exciting current, the regulator compensates for variations in load. As a result, the output voltage remains constant up to the maximum current output.

The output voltage is regulated to 28 ± 1 volts (14 ± .5 volts) by periodically increasing and decreasing the exciting current. If the output of the alternator is below 29 volts (14.5 volts), the exciting current rises and the voltage rises. If the voltage exceeds 29 volts (14.5 volts), the regulator turns off the exciting current. The drop in current reduces the output voltage. When the voltage drops below 27 volts (13.5 volts), the regulator turns on the exciting current. The rise in current increases output voltage to 29 volts (14.5 volts). The cycle is repeated. The cycles occur quickly, so that the output voltage remains constant.

The alternator output voltage is directly related to rotor speed for a given electrical load. For example, increasing the rotor speed increases the output voltage. By controlling the exciting current, the regulator compensates for variations in rotor speed. At low speeds, a higher average exciting current results. At high speeds, a lower average exciting current results.

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