Over-Excitation Limiter - MarelliMotori M40FA640A/A Mark I User Manual

8. OVER-EXCITATION LIMITER

This function permits to limit the over-excitation due to particular 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 func. does not substitute ext. 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:
- when the generator is running at the rated speed, apply the maximum desired load;
select position OFF for the micro-switch 3;
-
carefully rotate counter-clockwise the potentiometer AMP, until the yellow LED lights up and the generator voltage
-
decreases to a stable value, lower than the rated voltage;
- carefully rotate clockwise AMP until the yellow LED switches-off; the generator voltage must recover the rated value;
select position ON for the micro-switch 3.
-
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.
9. 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.
10. CONTROL BY MEANS OF EXTERNAL DEVICE
10.1. External potentiometer
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.
10.2. DC voltage signal from external control device
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.
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