When the external circuit through the brush to energize the field winding, it generates a magnetic field, the claw pole is magnetized as N and S pole. When the rotor rotates, the magnetic flux changes alternately in the stator winding. According to the principle of electromagnetic induction, it can be known that an alternating induced electromotive force is generated in the three-phase winding of the stator. This is the principle of alternator power generation.

The synchronous generator rotor with DC exciter is driven by the prime mover (ie, the engine) to rotate at n (rpm) and the three-phase stator windings sense the AC potential. If the stator winding resistance access to electrical load, the motor has AC power output, after the rectifier bridge within the generator to convert AC into DC output from the output terminal.

The alternator is divided into two parts: the stator winding and the rotor winding. The three-phase stator windings are distributed on the housing in the electrical angle of 120 degrees with each other, and the rotor winding is composed of two pole claws. When the rotor winding connected to DC when excited, the two poles form N pole and S pole. Magnetic lines start from the N pole, through the air gap into the stator core and then back to the adjacent S pole. Once the rotor is rotated, the rotor winding will cut the magnetic flux lines and produce sinusoidal emf in the stator windings with different 120 degree electric degrees, that is, the three-phase alternating current, and then the rectifying element made up of diodes becomes the direct current output.

When the switch is closed, the battery is supplied with current first. Circuit is:

Battery positive → charge indicator → regulator contact → excitation winding → ground → negative battery. At this moment, the charging indicator light will be on due to the current passing through.

However, after starting the engine, as the generator speed increases, the terminal voltage of the generator also increases continuously. When the output voltage of the generator is equal to the battery voltage, the potentials of the "B" terminal and the "D" terminal of the generator are equal. At this time, the charge indicator is extinguished because the potential difference between both ends of the generator is zero. Indicates that the generator is operating normally and the excitation current is supplied by the generator itself. The three-phase AC EMF generated by the three-phase winding in the generator is rectified by a diode, and then the DC power is output to supply the load and charge the battery.