SUMMARY OF AVR CONTROLS
CONTROL FUNCTION DIRECTION
VOLTS
STABILITY
STAB SWITCH
UFRO
DROOP
VTRIM
EXC TRIP
TO ADJUST GENERATOR OUTPUT VOLTAGE
TO PREVENT VOLTAGE HUNTING
TO OPTIMISE TRANSIENT PERFORMANCE
TO SET THE UFRO KNEE POINT
TO SET THE GENERATOR DROOP TO 5% AT OPF
TO OPTIMISE ANALOGUE INPUT SENSITIVITY
TO SET OVER EXCITATION TRIP CUT OFF LEVEL
CLOCKWISE INCREASES OUTPUT VOLTAGE
CLOCKWISE INCREASE THE DAMPING EFFECT
SEE TABLE ABOVE
CLOCKWISE REDUCES THE KNEE POINT FREQUENCY
CLOCKWISE INCREASES THE DROOP
CLOCKWISE INCREASES THE GAIN OR SENSITIVITY
CLOCKWISE INCREASES THE CUT OFF LEVEL
The main functions of the AVR are:
Potential Divider and Rectifier takes a proportion of the
generator output voltage and attenuates it. The potential
divider is adjustable by the AVR Volts potentiometer and
external hand trimmer (when fitted). The output from the
droop CT is also added to this signal. A rectifier converts
the a.c. input signal into d.c. for further processing.
The DC Mixer adds the Analogue input signal the Sensing
signal.
The Amplifier (Amp) compares the sensing voltage to the
Reference Voltage and amplifies the difference (error) to
provide a controlling signal for the power devices. The
Ramp Generator and Level Detector and Driver infinitely
control the conduction period of the Power Control Devices
and hence provides the excitation system with the required
power to maintain the generator voltage within specified
limits.
The Stability Circuit provides adjustable negative ac
feedback to ensure good steady state and transient
performance of the control system.
The Low Hz Detector measures the period of each
electrical cycle and causes the reference voltage to be
reduced approximately linearly with speed below a
presettable threshold. A Light Emitting Diode gives
indication of underspeed running.
The Synchronising circuit is used to keep the Ramp
Generator and Low Hz Detector locked to the generator
waveform period.
The Low Pass Filter prevents distorted waveforms
affecting the operation of the AVR control circuit.
The Short Circuit Detector senses the presence of a
short circuit on the generator output and forces the
Power Control Devices into full, conduction. This only
occurs when the AVR is powered from an auxiliary
winding.
Power Control Devices vary the amount of exciter field
current in response to the error signal produced by the
Amplifier.
Suppression components are included to prevent sub
cycle voltage spikes damaging the AVR components
and also to reduce the amount of conducted noise on
the generator terminals.
The Over Excitation Detector continuously monitors the
exciter field voltage and provides signals, to collapse
the output voltage if an over excitation condition
persists for the specified time period.
The Power Supply provides the required voltages for
the AVR circuitry.
ADJUSTMENT OF AVR CONTROLS
VOLTAGE ADJUSTMENT
The generator output voltage is set at the factory, but
can be altered by careful adjustment of the VOLTS
control on the AVR board, or by the external hand
trimm er if fitted. If major adjustment is necessary or
you lose stability, follow the VOLTAGE SETUP
PROCEDURE’.
Terminals 1 and 2 on the AVR will be fitted with a shorting
link if no hand trimmer is required. Terminals La and Lb are
linked only for special low voltage applications.
WARNING! Do not increase the voltage above the rated
generator voltage. If in doubt, refer to the rating plate
mounted on the generator case.
WARNING! Do not ground any of the hand trimmer
terminals as these could be above earth potential. Failure
to observe this could cause equipment damage.
WARNING!
If a replacement AVR has been fitted or re-setting of the
VOLTS adjustment is required, turn the VOLTS control
fully anti-clockwise before running generator.
VOLTAGE SETUP PROCEDURE
For major adjustments and replacing the AVR.
Read and understand this procedure before
attempting to follow it.
1. Before running generator, turn the VOLTS control
fully anti-clockwise.
2. Turn remote volts trimm er (if fitted) to m idway
position.
3. Turn STABILITY control to midway position.
. Connect a suitable voltmeter (0-300V ac) across line to
5. Start generator set, and run on no load at nominal
frequency e.g. 50-53Hz or 60-63Hz.
6. If the red Light Emitting Diode (LED) is illuminated,
refer to the Under Frequency Roll Off (UFRO)adjustment.
7. Carefully turn VOLTS control clockwise until rated
voltage is reached.
8. If instability is present at rated voltage, refer to
stability adjustment, then re-adjust voltage if
necessary.
9. Voltage adjustment is now completed.
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