Automatic voltage regulator (AVR)

                                                AVR

R1-Sets volts value
R2-Trimming resistor (Powerfactorcorrection)
R3-Trimmer
Carbonpile- Control resistance for AVR
Operating coil-Along with carbon pile form the controlling elements
CCT and PT-Are the detecting elements, the CCT acts as a feed forward device indicating future voltage changes by detecting variation in current flow
Stabilising element-  Is the capacitor across the Exciter (may be replaced by a resistor)  

                   The A.C. voltage is applied to the operating coil through a full wave rectifier. This A.C. voltage supply induced in the potential transformer and the circulating current transformer may vary under varying load conditions such as direct on line starting of relatively large motors. The capacitor connected across the coil smoothes the D.C. output from the rectifier.  

            If the A.C. applied voltage falls, the field of the solenoid weakens, and the resistance of the carbon pile decreases. With less exciter circuit resistance the current in the exciter field increases thus increasing the output voltage of the A.C. generator.

            The automatic voltage regulator voltage output may be adjusted with the hand regulator R1 in the exciter field. Before synchronising the alternator the open circuit voltage is adjusted with the hand regulator R1. 

           After synchronising, and after the kW loading has been adjusted on the prime mover governor, the field excitation under steady load conditions may be adjusted using the Trimming resistor R2. Using the trimming resistor the power factor of the incoming machine will be equalised with the machines already in use. 

 

             If the load power factor now changes then the terminal voltage will regulate badly, e.g. a rise from 0.8 to Unity Power factor will cause a rise in terminal voltage of about 20 %. So a small Voltage Trimmer R3 is provided across each current transformer to adjust terminal voltage when there is a change in overall power factor