Automatic Voltage Regulator(AVR)
Automatic voltage regulator is the full form of AVR. In many industrial installations, fluctuations of load are heavy, a typical instance being the operation of electrical cranes. Due to rapid variations in the load from instant to instant, the voltage also fluctuates considerably, because of varying voltage drop in the armature circuit.
To overcome this unsatisfactory feature, automatic voltage regulators are usually provided to maintain the generator voltage reasonably constant in spite of the fluctuating load, i.e. to increase the voltage when the load is high and reduce it when the load comes down.
There are several types of automatic voltage regulators in use, but all of them are based on the principle of short-circuiting of a part of the generator or exciter field resistance, thereby raising the excitation of the generator, to compensate for the drop in voltage due to increase load. The rise and fall in voltage itself is utilized to do this automatically, so as to maintain the voltage nearly constant whatever may be the variation in load.
(a) Brown Bowery regulator:
Here a rotating arm moves; depend upon the terminal voltage, across a set of contacts, short circuiting some part or more of the resistance inserted in the exciter field circuit.
(b)Carbon Pile regulator:
Here a set of carbon plates is assembled as a compact unit and connected as a resistance in series with the field circuit. The value of resistance of the carbon pile gets reduced as the pressure applied on it is increased. A solenoid coil connected across the supply controls the pressure applied and thereby the voltage developed by the generator.
(c)Terrill voltage regulator:
Here a pair of vibrating contacts short circuits a portion of the resistance in the field circuit intermittently. The ratio of the time the contacts are closed to that they are open, changes with the load and determines the average field excitation and therefore the voltage.
In all these cases a time lag device, such as a dashpot, is provided to ensure that fluctuation of very short duration is rendered ineffective.
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