I already posted few article about the generator, its types and some operation principle with stapes. In this post I will discuss about the AVR (Automatic Voltage Regulator) system and try to learn how it works.
Some small type generators have an inherent ability to produce reasonably constant voltage if the load varies sometime automatic voltage control is required in the usual form of generator. Automatic Voltage Regulator (AVR) use to refer to the preceding sections is based on a closed-loop control system principle. The basis of this closed loop control system is shown in figure.
By this figure, the output voltage is converted usually through a transformer or resistor network to a low voltage dc signal and this feedback signal is subtracted from a fixed reference voltage to produce an error signal. Now the error signal use to process by a compensator before being amplified to drive the rotor excitation current. This change in rotor excitation current produces a variation in output voltage, closing the control loop. If the gain of the control loop system is large enough then only a small error signal is required to produce the necessary change in excitation current but a high amount of gain can lead to instability in the circuit system with oscillations in the output voltage. The main purpose of the compensating circuit is to enable small errors signal to be handled with stable way. Now the most common form of compensator is a PID circuit system in which the error is amplified Proportionately (P), Integrated (I) and Differentiated (D) in three parallel circuits system before being added together. Many AVRs have own adjustment Potentiometers which allow the gains of each channel to be varied in order to achieve the best performance level. If you don’t know the Potentiometers then please do Google for it. Now the integral term enables compensator output to achieve at zero error and this produces the minimum error in output voltage.
Now a day’s many AVRs are offered with digital circuitry. But the principle of the feedback loop remains the same and the feedback signal is converted to digital signal from analog by using an analogue to digital converter. All calculations are performed digitally in a microprocessor and the output is on or off. Using pulse width modulation (PWM) to vary the average level of dc supplied to the rotor excitation winding. Alternatively the phase angle of a thyristor bridge can be used to vary the output level. This type of control system is known as phase-angle control. Now the continuous improvement of power electronic controls system and processor power is providing the further advances in voltage and speed control with more flexible protection of the generator with its connected circuits. This consists of a Permanent Magnet Generator driven by a variable speed engine and feeding a power electronic frequency-changer circuit system which delivers an output at constant frequency. Microprocessor is used to control the switching of the output devices and regulate the engine speed depending on the load applied to the generator. If needed low power demand then engine speed is reduced to minimize noise increase efficiency and extend life of generator.
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