Automatic Voltage Regulator (AVR) is a control system. As its name suggests, It is not possible to operate manually.
It stabilises the output voltage of generator.
It divides the reactive load between the generators in parallel operation.
It maintains the excitation of the system under fault conditions.
Why we need AVR
It is important that the voltage of a supply system be maintained constant otherwise the performance of motors, lights etc. will be affected. Unfortunately, the voltage of an unregulated A.C generator can vary 30% or more from no-load to full-load. Close control of generator voltage can only be achieved by regulating the voltage automatically by means of an AVR unit.
Performance of AVR
If load is connected to a generator then the voltage will suddenly fall to a low value then slowly recover.
Two stages occur:
- A transient stage followed by recovery
- Steady state conditions
The real need for an AVR is to deal with the voltage dip following the sudden connection of extra load and to restore the voltage quickly without undue overshooting or regeneration (voltage hunting).
Effect of internal drop in generator windings is called voltage dip.
Components of AVR
An AVR is like a control system and it consists of certain basic units. It works on the Error Detection principle.
Sensing: The generator voltage is sensed by direct connection to the generator output terminals.
Measurement: The sensed voltage is fed to the measurement unit where it is converted to a low value direct voltage proportional to the generator voltage
Comparator: This produces a fixed reference direct voltage with which the measured value is compared. The difference between the two is called the Error voltage. A voltage trimmer enables the desired value of generator voltage to be set as required.
Amplifier: This amplifies the error to increase the accuracy and sensitivity of the AVR.
Control: This is actuated by the amplified error signal and adjusts the field current to a value necessary to maintain the generator voltage at the rated value. This unit is also called correcting unit.
A practical AVR requires some additional units for satisfactory operation. Complete AVR circuit is complex and it includes a few pre-set variable resistors for the control of sensitivity, off-set error and stability.
Types of AVR
Carbon pile regulator
Vibrating contact regulator