2018.01 / a
4
Electric Power Generation
Installation and maintenance
R121
AVRs
5657 en -
1 - GENERAL DESCRIPTION
The R121 automatic voltage regulator
(AVR) is a compact, high-performance
encapsulated unit. It incorporates the latest
technology and efficient components to
achieve a high degree of miniaturisation
when used with 3-phase and 1-phase AC
brushless generators within its input and
output limits. The unit offers excellent
reliability.
The AVR provides DC excitation to the
exciter field of a brushless generator to keep
the voltage within the approximate operating
limits from NO-LOAD to FULL LOAD.
The typical recovery time in the event of
sudden loading is around 0.5 sec. to recover
97.5% of the rated voltage. Transient
performance such as voltage dip and
recovery time is mainly determined by the
generator and exciter design parameters.
Optimum AVR performance can be obtained
by keeping full-load excitation to around
60 VDC.
The generator uses a true average sensing
circuit, dV/dt snubber and special filter
circuits to manage NON-LINEAR loads
such as battery chargers, DC motors, etc.
Voltage regulation is only guaranteed for
linear loads. Severely distorting NON-
LINEAR loads can cause regulation
problems.
Each AVR is tested prior to dispatch as part
of a quality plan, for standard voltage and
frequency.
A soft-start circuit is included which provides
smooth control of the build-up of generator
output voltage.
A frequency roll-off circuit continuously
monitors the generator underspeed
protection by reducing the generator output
voltage in proportion with the speed below a
threshold.
2 - OPERATION OF THE AVR
The AVR is powered by the terminals of the
AC generator, with 110 V-220 VAC rms at
50 Hz or 60 Hz. The sensing voltage, which
is the regulated voltage, is also based on the
input power. The AVR forms an important
part of the closed loop system comprising
the generator field, generator armature and
the AVR.
The AVR first builds up the generator voltage
from its residual levels to the rated voltage
value. When the generator is loaded, the
sensed voltage decreases and generates
an error voltage, which is required in order
for the closed loop system to work.
The AVR contains a high gain amplifier.
Depending upon the value of the amplifier
voltage (either high or low) the ramp
intersects the amplified voltage at a point,
which is either early or late in the half-cycle.
At this intersection point a starting pulse is
produced to trigger the power device.
When the power device is triggered early in
the half-cycle, more voltage is fed to the field
and when triggered late in the half-cycle,
less voltage is transmitted to the field.
In order to reduce the generator voltage at
low speed, a signal inversely proportional to
the speed is generated as an extra input.
