How a car generator works

When an external circuit energizes the field winding via brushes, a magnetic field is generated, magnetizing the claw poles into north and south poles. As the rotor rotates, the magnetic flux alternates in the stator windings. According to the principle of electromagnetic induction, alternating induced electromotive force (EMF) is generated in the three-phase stator windings. This is the principle of power generation in an AC generator.

The prime mover (i.e., the engine) drives the DC-excited synchronous generator rotor, rotating at a speed n (rpm). This induces an AC voltage in the three-phase stator windings. When the stator windings are connected to an electrical load, the motor outputs AC power. The rectifier bridge inside the generator converts the AC power into DC power, which is then output at the output terminals.

An AC generator consists of two parts: the stator winding and the rotor winding. The three-phase stator windings are arranged on the housing at an electrical angle of 120 degrees. The rotor winding consists of two pole claws. When DC power is applied to the rotor windings, they are excited, forming north and south poles. Magnetic lines of force originate from the north pole, pass through the air gap, enter the stator core, and then return to the adjacent south pole. As the rotor rotates, the rotor winding cuts the magnetic lines of force, generating sinusoidal electromotive forces in the stator windings with a mutual angle difference of 120 degrees, i.e., three-phase alternating current. This is then converted to direct current by the diode-based rectifier.

When the switch is closed, the battery supplies current. The circuit is as follows:

Battery positive terminal → charging indicator → regulator contact → field winding → ground → battery negative terminal. At this point, the charging indicator lights up because current is flowing.

However, after the engine starts, as the generator speed increases, the generator terminal voltage continues to rise. When the generator output voltage equals the battery voltage, the potentials at the generator terminals "B" and "D" are equal. At this point, the charging indicator turns off because the potential difference between the terminals is zero. This indicates that the generator is operating normally and the excitation current is being supplied by the generator itself. The three-phase AC electromotive force generated by the three-phase winding in the generator is rectified by diodes and outputs DC power to supply power to the load and charge the battery.