An automatic voltage regulator (AVR) allows the alternator to maintain a stable voltage from no-load to full-load operation. The stator of the main machine supplies power to the excitation field through the AVR, and can automatically adjust the current of the excitation field. The AVR responds to the voltage induction signal from the stator winding of the host, and controls the rectified output power of the excitation armature by controlling the low-power excitation field to achieve the purpose of controlling the host magnetic field. The three-phase four-wire AC voltage output by the main stator is proportional to the current of the main rotor winding.

AVR has a characteristic that voltage / frequency is proportional. When the speed of the unit decreases, the AVR can adjust the output voltage of the alternator accordingly. When the load is suddenly increased, this proportional characteristic effectively protects the diesel generating set.