AVR's

R1-Sets volts value

R2-Trimming resistor (Power factor correction)
R3-Trimmer

Carbon pile-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
Stabilizing 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

Modern A.V.R. (Zenor Bridge)

Voltage across the Zenor diodes remains almost constant independent of current variations. Smoothed D.C. output is applied to the voltage reference bridge. This bridge is balanced at the correct generator voltage output with no potential difference between 'A' and 'B'.

If the generator voltage fails, current through the bridge arms falls and current flows from 'A' to 'B' through the amplifier.

If the generator voltage falls, current through the bridge arms falls and current flows from 'B' to 'A' through the amplifier.

If the generator voltage rises, Current through the bridge arms rises with current flow from 'A' to 'B' through the amplifier.

The signal from the amplifier will automatically vary the field excitation current, usually through a silicon controlled rectifier ( Thyristor) control element.

The Silicon Controlled Rectifier (Thyristor) is a four layer, three terminal, solid state device with the ability to block the flow of current, even when forward biased, until the gate signal is applied. This gate signal could come from a Zener diode Voltage reference bridge. The gate signal will switch on the forward biased S.C.R. and current flows through the exciter field. When reverse biased the S.C.R. will again block current flow. Due to inductance of the field winding the S.C.R. would continue to pass current for a part of the negative cycle. By fitting a 'free wheeling' diode the current though the Thyristor falls quickly at the end of the positive cycle. In some circuits the excitation current is designed to be excess of requirements, so that the gate signal reduces flow.