This is a Controller for automatic operation of OLTC(on 
load tap changer) for a Power Transformer as per pre 
settings of Higher & Lower Voltage Limits.
This Controller switches OF & ON the OLTC for keeping 
output voltage of transformer as per set limit and works 
contineously.

to regulate the excitation voltage in generator AVR is used.
it compares the voltage, current and frequency of the 
generator.

Basically this is a automatic controller or mean to say 
that a automatic controlling switch mainly used for control 
the voltage of any source, load or any electrical network.
Its main uses are automatic operation of OLTC (on load tap 
changer) for a Power Transformer, to regulate the 
excitation voltage in generator, temp control of electrical 
furnace etc.

what is the History of automatic voltage regulator?

AS teh name itself explains it regulates to the ref. 
voltage,
a) In case of generator the Terminal Voltage is ref. & its 
o/p is excitation voltage thereby regulating TV
b) In case of transformer the PT input is the ref. & Tap 
changer is continuosly operated until the PT input is 
regularised. In Both these cases AVR are used,

AVR is an abbreviation for Automatic Voltage Regulator.It 
is important part in Synchronous Generators, it controls 
the output voltage of the generator by controlling its 
excitation current. Thus it can control the output Reactive 
Power of the Generator

AVR increases or decreases exciter current for a more 
linear voltage and frequency.

AVR increase or decrease excitation voltage for linear 
voltage.Frequency depends on primover speed.

the fullform of avr is Autometic voltage regulator, 
basically avr is used in transformer and genrators.

in transformer it works with PT and  a tap changer motor 
which autometically change the tap to control the voltage.

in genrators avr sense the voltage with the help of PTs and 
control the exciting current in exciter to canrol the 
voltage.