What is an Automatic Voltage Regulator (AVR)?
While some small generators have an inherent ability to
produce a reasonably constant voltage as the load varies, it is clear from the
previous explanations that some form of automatic voltage control is required
in the usual form of generator. The Automatic Voltage Regulator (AVR) already
referred to in the preceding sections is based on a closed-loop control
principle.
The basis of this closed loop control is shown in Fig. 5.14.
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.
Fig. 5.14 Closed-loop voltage control
The error signal is processed by a compensator before being
amplified to drive the rotor excitation current. The change in rotor excitation
current produces a variation in output voltage, closing the control loop.
If the gain of the control loop is large enough then only a
small error is required to produce the necessary change in excitation current,
but a high gain can lead to instability in the circuit, with oscillations in
the output voltage. The purpose of the compensating circuit is to enable small
errors to be handled in a stable way.
The most common form of compensator is a PID circuit in
which the error is amplified proportionately (P), integrated (I) and
differentiated (D) in three parallel circuits before being added together. Many
AVRs have adjustment potentiometers which allow the gains of each channel to be
varied in order to achieve the best performance.
The principle of the feedback loop remains the same, but the
feedback signal is converted to digital form using an analogue-to-digital
converter. The 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 is known as phase-angle control. The
continuous improvement in power electronic controls and processor power is
bringing further advances in voltage and speed control, with more flexible
protection of the generator and its connected circuits.
This consists of a permanent-magnet generator driven by a
variable-speed engine and feeding a power electronic frequency-changer circuit,
which delivers output at constant frequency.
Thanks for this basic tutorial on how the automatic voltage regulator works.
ReplyDeleteThis is very nice blog. This blog are provide such a useful information . It is also helpful us.
ReplyDeleteBest Automatic Voltage Controllers Transformer manufacturers and Supplier in India