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How DC Alternator Works?

ALTERNATOR


Alternator Components:
(1) Brush holder
(2) Rear frame
(3) Rotor
(4) Stator
(5) Drive end frame
(6) Fan assembly
(7) Slip rings
(8) Rectifier

-The alternator has three-phase, full-wave, rectified output. The alternator uses brushes to generate electricity. The alternator is an electrical component and a mechanical component that is driven by a belt from engine rotation. The alternator is used to charge the storage battery during engine operation. The alternator is cooled by a fan that is a part of the alternator. The fan pulls air through holes in the back of the alternator. The air exits the front of the alternator and the air cools the alternator in the process.

-The alternator converts mechanical energy and magnetic energy into alternating current (AC) and voltage. This process is done by rotating an electromagnetic field (rotor) that is direct current (DC) inside a threephase stator. The alternating current and the voltage that is generated by the stator are changed to direct current. This change is accomplished by a system that uses three-phase, full-wave, rectified outputs. The three-phase, full-wave, rectified outputs have been converted by six rectifier diodes that are made of silicon. The alternator also has a diode trio. A diode trio is an assembly that is made up of three exciter diodes. The diode trio rectifies field current that is needed to start the charging process. Direct current flows to the alternator output terminal.

-A solid-state regulator is installed in the back of the alternator. Two brushes conduct the current through two slip rings to the field coil on the rotor. Also, a capacitor is mounted in the back of the alternator. The capacitor protects the rectifier from high voltages. The capacitor also suppresses radio noise sources.

-The voltage regulator is a solid-state electronic switch that controls the alternator output. The voltage
regulator limits the alternator voltage to a preset value by controlling the field current. The voltage regulator feels the voltage in the system. The voltage regulator switches ON and OFF many times per second in order to control the field current for the alternator. The alternator uses the field current in order to generate the required voltage output.

Alternator Operation Schematic

Electrical Schematic (Delco-Remy 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).

NOTE: 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.


Read More:
How DC Starter Motor Works?
DC Starter Motor Components
DC Alternator Components







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