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Automatic Voltage Regulator
NOTICE
Important! There is no procedure for directly testing
the AVR. Best method of testing the AVR is to use a
process of elimination.
The automatic voltage regulator (AVR) Figure 10
monitors voltage/frequency of the alternator then reacts
to frequency dips or lags in the load by aiding frequency
recovery through limiting or even reducing excitation as a
function of frequency.
Figure 10. Automatic Voltage Regulator (AVR)
When the frequency dips below a predetermined threshold
as a result of a heavy instantaneous load change on the
engine the regulator reduces generator voltage and in turn
the kW load by the square of the voltage which allows the
frequency to recover more rapidly and at the same time,
regulates voltage recovery to minimize voltage overshoot.
The AVR also aids the generator to sustain fault current
for a longer time period allowing downstream protective
devices to operate.
The heavy-duty construction designed AVR utilizes three-
phase voltage sensing for generator output monitoring. The
sensing leads are center tapped at the voltage change-
over board (Figure 11A) for 180kVA generators and above
and at the voltage selector switch for 150kVA generators
and below so there is no operator interaction with the AVR
required when switching from 240 to 480 VAC.
The AVR receives its power from the three-phase open delta
configured auxiliary windings (Figure 11B) and regulates
the output to the field excitation winding (Figure 11C) to
maintain constant and stable voltage to the load.
PAGE 14 — GENERATOR SERVICE AND TROUBLESHOOTING MANUAL — REV. #0 (08/29/23)
GENERATOR THEORY
The auxiliary windings are coupled to one another in a
three-phase series connection or open-delta with two
open ends.
The four terminal points (Figure 11D) of the windings
connected to the AVR allow for automatic switching
between the output points of the open delta winding
to provide current support to the field windings as load
demand on the generator changes.
This multiple output open delta configuration allows the
AVR to utilize the first field harmonic to generate power
for normal operation and responds to transient loads and
motor starting inrush current by utilizing the zero-phase
component having the third harmonic obtained through the
open ends of the series connected windings.
The employment of the zero-phase component for field
control make it possible for the AVR to have excellent
response to motor starting inrush current and short-circuit
faults.
MQ Power Generator Advantage
MQ Power manufactured generators (alternators) have
excellent heat rise ratings and utilizes multiple output
auxiliary windings wound in the stator core that are
independent of the load winding to provide clean input
power to the AVR.
The MQ Power "Separately Derived Excitation" system
provides a high degree of immunity from the effects of non-
linear loads. The patented AVR creates a robust system
of transient load control for quicker recovery. This type
of system allows MQ Power manufactured generators
to endure the harsh rental and construction industry
environment.
The advantage of the use of the three-phase multiple output
open-delta auxiliary windings compared to the use of a
permanent magnetic generator (PMG) system, simply, it's
a more efficient and durable system and has an extremely
low incident rate whereas the PMG is an add-on system
available at an additional cost.
The add-on PMG system increases the weight and length
of the alternator and is less efficient. In addition the MQ
Power alternators have internal winding taps to provide true
120 VAC at the convenience receptacles without the use of
an additional transformer when the generator is operating
at either 240 or 480 VAC.
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