AS540 AVR Brushless Generator Automatic Voltage Regulator AS 540 OEM AS540 AVR provides closed loop control by sensing the alternator output voltage at the main stator windings and adjusting the exciter the stator field strength. Voltage included in the exciter rotor rectified by the rotating diodes, magnetises the rotating main field which includes voltage in the mani stator windings. A seff-excited AVR receives power from the alternator output terminals or a special auxiliary winding in the main stator winding.