4.7.1.2 EXC TRIP (Excitation Trip) AVR Types MX341 and MX321 An AVR supplied from a permanent magnet generator
inherently delivers maximum excitation power on a line to line
or line to neutral short circuit or large overload. In order to
protect the generator windings the AVR incorporates an over
excitation circuit which detects high excitation and removes it
after a pre-determined time, i.e. 8-10 seconds.
Symptoms of incorrect setting are the generator output
collapses on load or small overload, and the LED is
permanently illuminated.
The correct setting is 70 volts +/-5% between terminals X and
XX.
4.7.1.3 OVER/V (Over Voltage) AVR Type SX421, MX321 Over voltage protection circuitry is included in the AVR to
remove generator excitation in the event of loss of AVR
sensing input.
The MX321 has both internal electronic de-excitation and
provision of a signal to operate an external circuit breaker.
The SX421 only provides a signal to operate an external
breaker, which MUST be fitted if over voltage protection is
required.
Incorrect setting would cause the generator output voltage to
collapse at no-load or on removal of load, and the LED to be
illuminated.
The correct setting is 300 volts +/-5% across terminals E1,
E0. Clockwise adjustment of the OVER/V control
potentiometer will increase the voltage at which the circuit
operates.
4.7.1.4 TRANSIENT LOAD SWITCHING ADJUSTMENTS AVR Types SX421, MX341 and MX321 The additional function controls of DIP and DWELL are
provided to enable the load acceptance capability of the
generating set to be optimised. The overa ll generating set
performance depends upon the engine capability and
governor response, in conjunction with the generator
characteristics.
It is not possible to adjust the level of voltage dip or recovery
independently from the engine performance, and there will
always be a 'trade off' between frequency dip and voltage dip.
AVR Types SX421, MX341 and MX321
The dip function control potentiometer adjusts the slope of the
voltage/speed (Hz) characteristic below the knee point as
shown below :
Fig. 8
DWELL AVR Type MX321 The dwell function introduces a time delay between the
recovery of voltage and recovery of speed.
The purpose of the time delay is to reduce the generator kW
below the available engine kW during the recovery period,
thus allowing an improved speed recovery.
Again this control is only functional below the "knee point", i.e.
if the speed stays above the knee point during load switching
there is no effect from the DWELL function setting.
Clockwise adjustment gives increased recovery time.
The graphs shown above are representations only, since it is
impossible to show the combined effects of voltage regulator
and engine governor performance.