Automatic Voltage Regulator Principle

To better understand the principle of the 3 phase automatic voltage regulator, we will first briefly understand the generator excitation system. Here, I will use a static excitation system as an example. As is well known, in a static excitation system, the generator output is fed to a thyristor bridge rectifier. This thyristor bridge rectifier converts the AC current into DC current. Note that the DC current output of the thyristor bridge can be controlled by controlling the firing angle of the thyristors. The DC current output from the thyristor bridge is then fed into the excitation winding of the generator.

Static Excitation System:
Let's assume the field current at any time is If. Then, the air gap flux of the generator can be written as Φ = KIf, where K is some constant. However, we are interested in maintaining the generator terminal voltage Vt given by Vt = 1.414nfNΦ, where the symbols have their usual meanings. From the above, it can be seen that changing If will change the terminal voltage Vt.
Therefore, voltage regulation can be achieved by controlling the excitation current. The three phase automatic voltage regulator performs this by changing its firing angle.