When the rotor being driven by the prime mover, a small magnetic field was set up and due to this, a small voltage is induced. The AVR will sense this low voltage and compare it with the required voltage level. The AVR will find initially the sensed voltage is considerably low than the required voltage. The AVR will provide such power from the main stator winding to establish the exciter field. The exciter also have a small amount of residual magnetism…
The power from the main output winding (rectified by going through the AVR) will add to this residual level to produce a greater magnetic field strength. With the exciter magnetic field strength increased, the a.c. output voltage from the exciter rotor will also increase. This voltage is rectified by the rotating diodes. This extra excitation adds to the residual level of the main field and produces an increase in output voltage from the main stator.
The AVR senses this increase and will further increase the excitation field excitation. The main stator voltage is building up until the sensed voltage is the same as the required voltage.
A change in output due to load current is automatically compensated for by the AVR. This will adjust the excitation under all circumstances in order to achieve minimum error between the sensed output voltage and the required voltage.