Stability adjustment
The AVR includes a STABILITY or damping circuit to provide
good steady state and transient performance of the generator.
A switch is provided to change the response of the stability
circuit to suit different frame size generators and applications,
(see stability table above).
The correct setting of the stability control can be found
b
y
running the generator at no load and slowly turning the
stability control anti-clockwise until the generator voltage
starts to become unstable.
The optimum or critically damped position is slightly clockwise
from this point, ( i.e. where the machine volts are stable but
close to the unstable region ).
Under Frequency Roll Off adjustment (UFRO)
The AVR incorporates an under speed protection circuit which
gives a volts per Hertz characteristic when the generator
speed falls below an adjustable threshold known as the knee
point.
The red light emitting diode (LED) gives indication that the
UFRO circuit is operating. Turning the UFRO control
clockwise lowers the frequency setting of the knee point and
extinguishes the LED.
For optimum setting, the LED should illuminate as the
frequency falls below just below nominal, i.e.47-48 Hz on a 50
Hz system and 57-58 Hz on a 60 Hz system.
The UFRO adjustment is pre-set and sealed at the works and
normally only requires the selection of 50 / 60 Hz using the
selector switch.
If the LED is flashing, refer to Over Excitation and Over-
Voltage protection adjustments.
Dip adjustment
The DIP adjustment allows the user to have some control
over the amount of frequency related voltage dip upon the
application of load.
This feature is mostly used when the generator is coupled to
turbo charged engines with limited block load acceptance,
and operates only when the speed is below the UFRO knee
point, (LED illuminated).
The circuit works by increasing the volts per Hertz slope to
give greater voltage roll off in proportion to falling speed.
With the DIP control fully anti-clockwise the generator voltage
characteristics will follow the normal volts per Hertz line as the
frequency falls below nominal.
Turning the DIP control more clockwise provides greater
voltage roll off allowing easier engine recovery.
Dwell adjustment
The DWELL operates only when the speed has fallen below
the knee point set by the UFRO adjustment. With the DWELL
control turned fully anti-clockwise, there is no DWELL and the
voltage recovery will follow the engine speed recovery.
Turning the DWELL control clockwise, limits the maximum
rate of change of voltage recovery as the engine speed
recovers.
Droop adjustment
Generators intended for parallel operation are fitted with a
quadrature droop CT which provides a correction signal for
the AVR, allowing each generator to share reactive current. It
is important to match the generator no-load voltages to less
than 1 % of each other, to ensure good reactive load sharing.
The DROOP adjustment is normally pre-set in the works to
give 5% voltage droop at zero power factor.
Clockwise increases the amount of CT signal injected into the
AVR. The generator terminal voltage falls with lagging power
factor and rises with leading power factor. With the control
fully anti-clockwise there is no droop.
If the DROOP potentiometer needs to be reset for any reason,
then the following procedure should be followed:-
Run the generator at 50% rated current at any power factor
between 0.8 lagging and 1.0 (unity). Ensure that the generator
rated current value used is the true rating without any de-rate
factor. If in doubt, check with the factory.
Measure the AVR S1 S2 voltage. This should be between 0.2
- 1.9 volts ac.
Calculate the required position of the DROOP potentiometer
(0 - 100%), where 0% = fully anti-clockwise and 100% = fully
clockwise :-
% position =
If the result is greater than 100, then it suggests that the
Droop CT ratio is incorrect.
Trim adjustment
An auxiliary input is provided to connect to accessories, (A1,
A2) and is designed to accept DC signals up to +/- 5 volts.
These terminals are usually used for connection to a power
factor controller, if required.
The DC signals presented to this input adds to, or subtracts
from the AVR sensing circuit, depending on polarity, and
allows the accessory to have an influence on generator
excitation.
The TRIM adjustment allows the user to determine or trim
how much control the accessory has over the AVR.
With the TRIM control fully anti-clockwise the accessory has
no control, clockwise gives maximum control.
Any signal connected to this input must be fully floating
and earth free.
20
S1 S2 Voltage
