Rheostat For Remote Voltage Setting; Instructions For Manual Control Of Generators - MarelliGenerators MJB 250 Instrucciones Y Advertencias De Seguridad

dip 2
⇒ speed response becomes slower
pos.ON
dip 3
⇒ speed response becomes slower
pos.ON
dip 4
⇒ standard low speed protection
pos.ON
SLOPE - Potentiometer for changing the slow speed protection characteristic slope.
With this potentiometer, we can increase or decrease the under speed ramp slope, and set the voltage droop for a fixed reduced
speed. Setting with SLOPE is possible only with dip 4 OFF.
⇒ increase voltage droop
PAR - Potentiometer for changing the droop
In order to obtain a correct sharing of reactive power, when two or more generators are required to operate in parallel, it is
necessary to verify that the no load voltage is the same, that the bridge between "A-B" is open and the voltage droop is the
same for all generators when loaded.
When this unit is inserted the output voltage should show a 4% voltage droop when passing from no load to full load 0.8 p.f.
If the voltage is increasing as the load increases, it is necessary to reverse the leads of the current transformer at the terminals
"A-B". If necessary, the voltage droop can be reset by acting on the internal potentiometer.
In single operation the terminals A and B have to be shorted.
⇒ increase droop
OV - Potentiometer for changing the overvoltage protection threshold.
In the event a overvoltage for the sensing loss or the SCR failure occurs, the A.V.R. is able to detect such overvoltage (red led
switch on ) and a low level is provided by an Open Collector output "OUT1".This device helps to monitor the excitation system.
The time delay is about 10 seconds. It has no automatic effect on the generator excitation system.
⇒ increase the overvoltage threshold
In factory the potentiometer is calibrated in order to exclude the overvoltage protection threshold. This control device
also if correctly calibrated has to be combined with an external protection system.

5.1. Rheostat for remote voltage setting

For all generators, that rheostat can be inserted between the auxiliary terminals P and Q (FAST-ON terminals). The external rheostat
has to be inserted with its wiper in intermediate position and then the internal potentiometer of AVR (P1) has to be reset to obtain about
the nominal voltage. Rheostat features must be minimum rating 0.5W, resistance about 100Kohm.

5.2. Instructions for manual control of generators

If the voltage regulator (AVR) breaks down, a manual control system can be used, when a 24 V D.C. power supply is
available.
This source could consist of a bank of batteries or of a voltage transformer and a rectifier unit connected at the alternator output.
- disconnect the two exciter stator terminals ( whites wire + and -) from AVR;
- apply the D.C. power supply to these two wires;
- set the rheostat R to adjust the alternator output voltage.
ATTENTION!: compensate by manually increasing excitation as the load increases.
Before removing the load, reduce the excitation current.
Use the following table to select the rheostat:
pos.OFF
potentiometer
WHITE
24 V
EXCITER
STATOR
WINDING
TERMINALS
WHITE
Generator
MJB 250 – 315
MJB 355
18
⇒ proportional low speed protection V/f (see "SLOPE"
⇒ decrease voltage droop
⇒ decrease droop
⇒ decrease the overvoltage threshold
R
I max Max. resistance of rheostat
[A]
[Ω]
5 80
6 80
18
ENGLISH