– FREQ – Potentiometer for changing the low speed protection corner frequency. It is usually set at the factory in order to reduce the excitation when speed becomes lower than 90% of rated speed at 50 Hz (frequency lower than 45 Hz, or corner frequency). By removing the bridge which normally shorts the terminals Hz and 60, the speed protection acts properly for 60 Hz operation.

– AMP – potentiometer for changing the over excitation limiter; that limiter permits to protect the generator in case
of over excitation due to load conditions that could cause the rotor damage.
Even if correctly set, this function does not substitute external systems protections, it is only a completing device.

– STAB – stability potentiometer: clockwise rotation permits to increase the regulation stability (the response time
becomes larger).

– SLOPE – potentiometer to change the low speed protection characteristic slope: with the micro-switch 4 OFF, this potentiometer allows to increase or decrease the under speed ramp slope, and set the voltage droop for a fixed reduced speed.

– PAR – potentiometer to change the droop: this potentiometer allows to change the voltage droop for reactive compensation.

The regulator is provided with internal circuits in order to reduce the excitation, when running at low speed, in order to avoid damages to the excitation system of the generator (i.e. to the regulator, to the exciter field, to the rotating rectifier, main rotor).
The potentiometer FREQ fixes the corner-frequency, that is the frequency at which the limiter operates. Below that particular frequency, red LED switches-on and the voltage of the generator reduces further together with the generator
speed reduction.
By setting the micro-switch nr. 4 in OFF position, the voltage reduction is smaller and is close to be proportional to the speed reduction (voltage reduction is adjustable by the potentiometer SLOPE).

The voltage regulator is provided with internal adjustablestability circuits in order to allow stable operation in a wide
range of applications.The stability of the regulator can be set on field to adapt itto the characteristics of the plant and/or the driving engine (diesel engine, water turbine, gas turbine) in order to obtain
the best voltage response. To change the stability characteristics of the regulator, it is necessary to act on the potentiometer STAB (for fine setting of stability).
An additional coarse setting of stability can be achieved by means of the micro-switches number 1 e 2.

OVER-EXCITATION LIMITER:
This function permits to limit the over-excitation due to articular load conditions that could cause the generator
damage. As soon as the excitation voltage rises over a certain threshold, set by means of the potentiometer AMP, for a
time larger than the limiter time delay, the over-excitation limiter steps-down the excitation voltage to the threshold value.
Limiter time delay depends on the amount of the over-load: more the over-load arisen, quicker the limiter action. Limiting the excitation voltage leads to the generator excitation level decrease, partial or total, depending on the
over-load occurred. In case of excitation shutdown due to the limiter, the de-excitation condition could be not maintained.
WARNING: Even if correctly set, this function does not substitute external systems protections, it is only a completing device.
WARNING: in case of generator paralleled with a grid, an over-excitation condition detected by the limiter can lead to a generator excitation shutdown, with risk of loss of synchronism.

LIMITER ADJUSTMENT:
For a quicker limiter setting, it is possible to momentarily disable the time delay, by means of the micro-switch 3 (see
also the next paragraphs).

In order to properly adjust the limiter, the following procedure must be applied:

If the procedure is properly carried out, the excitation voltage threshold is set to a value 15-20% higher than the
excitation voltage at the maximum desired load, previously applied.
Time delay depends on the amount of the over-load occurred: it can range from 10s minimum to some minutes maximum.

DROOP KIT DEVICE:
The device is included in the voltage regulator, to allow parallel operation between similar generators: the device permits to share correctly the total reactive power required by the load among all generators operating in parallel.
The device is composed by an external current transformer (which senses the current in phase W) and the internal droop circuit of the regulator. The C.T. is placed on phase W; the phases U and V have to be connected to terminals S1 and S2.

The voltage regulator is provided with input terminals (terminals A and B) for easy connection to an external current transformer. Such terminals are normally shorted by a connection bridge, when the generator is used in single operation.
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.

An external potentiometer for fine setting of the voltage reference can be connected to the terminals P and Q.

This permits to:
– change the voltage set-point by means of remote
control device;
– set the generator voltage reference with accuracy
higher than the one of the internal regulator
potentiometer VOLT.
External potentiometer specifications are the following: Setting range Potentiometer specifications
±5% 100 kΩ – 1 W minimum
±10% 200 kΩ – 1 W minimum

As soon as the external potentiometer is connected to P and Q, a change in the global voltage reference of the regulation system will occur; the internal trimmer VOLT must be set again to the rated generator voltage.
In detail: put the external potentiometer cursor in mid position, and after that, turn VOLT counterclockwise until to reach approximately the desired generator voltage. It’s now possible to operate the fine setting of the voltage by means of the external potentiometer.

The AVR accepts a DC voltage analogue input of +/-3 V (maximum range), to control the exciter field of the generator.
This input has to be applied to terminals 6 and 8.
In case of single operation, a maximum change of +/-3 V of the analogue input leads to a maximum change of +/- 25% of the generator voltage, with respect to the rated generator voltage.
A value of 0 V at 6 and 8 terminals does not lead to any change in excitation.
The analogue input is usually used for the voltage regulator control by means of the Marelli power factor regulator, for parallel operation with the grid.

This input can be also connected to external devices not made by Marelli, for remote control of the generator voltage
or the excitation during parallel operations (voltage matching and power factor regulation), as long as those devices were equipped with suitable outputs (insulated outputs, and voltage regulation range not higher than +/-3 V).
WARNING: in the case of parallel with the grid, and voltage regulator controlled by an external device by means of the
analogue input, pay attention to the analogue input voltage value after the load rejection.
In this particular case, in order to avoid any dangerous generator over-voltages, the voltage value between the terminals 6 and 8 must be reduced to 0 V or to any values leading to a generator voltage not exceeding the rated + 5%.

DROOP KIT DEVICE:
The device is included in the voltage regulator, to allow parallel operation between similar generators: the device permits to share correctly the total reactive power required by the load among all generators operating in parallel.
The device is composed by an external current transformer (which senses the current in phase W) and the internal droop circuit of the regulator. The C.T. is placed on phase W; the phases U and V have to be connected to terminals S1 and S2.
The voltage regulator is provided with input terminals (terminals A and B) for easy connection to an external current transformer. Such terminals are normally shorted by a connection bridge, when the generator is used in single operation.
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.

MICRO-SWITCH:
To change the stability characteristics of the regulator, it is possible to use the micro-switches. In such a way it is possible obtain changes in the transient response of the regulator.

FUSE:
The voltage regulator is provided with an internal protecting fuse (which acts in case of faults on the regulator or very
large overloads on exciter circuit).

EMI SUPPRESSOR:
The AVR is provided with an internal Electromagnetic Interference filter: this interference suppression system
permits to obtain compliance with relevant EMC standards on MARELLI MOTORI generators.

Description	Part number
Fuse Ultra rapid, ceramic, 10 A – 500 V	963823010
External potentiometer 100 kW – 2 W	963824430