MarelliMotori MEC-20 Manuel Utilisateur page 29

9.4. Overexcitation limitation
The regulator is capable
operating in limitation of excitation
current, as soon as it reaches a
value which could
overheating the generator and, in
particular, the exciter field. When
this function is set properly and
there is a field overcurrent, the field
current value is returned to a safe
value within a pre-set time interval,
obtained by the curve shown in the
following figure. This characteristic
is calculated starting
setting of a field current maximum
value (I ) that the regulator can
Fset
continually sustain without any
protection intervention.
When the field current
exceeds I , the field overcurrent
Fset
limiter starts limiting after a time
interval dependent on the field
overcurrent value
reached.
The greater the overcurrent value, the less intervention time.
The intervention consists in a decrease of the field current up to the maximum continuity value set, at which it is
maintained until the following conditions occur:
❖
A sufficient time has passed to eliminate generator overheating.
❖
The operating conditions bring the excitation current value requested by the regulation to below the I
value.
The maximum field current value that the regulator can deliver is 3 x I
s. The maximum field current permitted from the regulator cannot exceed 15A, no matter the I
example, for I set at 6A, the maximum deliverable current will be 15A, not 18A.
Fset
9.5. External potentiometer
A remote generator voltage setting device (potentiometer) can be attached to terminals P and Q in place of the
usually present jumper.
The external potentiometer allows the same type of calibration offered by VFINE to be performed, or ±5%,, with
potentiometer 10 kΩ, 1 W minimum.
Once the external potentiometer is connected to P and Q, there will be a change in the overall voltage reference
of the regulation system, for which you will need to re-set the internal VOLT trimmer of the regulator.
In particular: set the external potentiometer cursor in a middle position, after that, turn VOLT until you reach,
approximately, the desired generator voltage.
At this point you can perform the fine voltage calibration using the external potentiometer.
IMPORTANT NOTE: in the event in which there is no external potentiometer or jumper attached to terminals P
and Q, the regulator will not be able to deliver excitations to the generator field.
9.6. Analogue input
A control device can control the voltage reference through the terminals 6 and 8 by a dc voltage signal (Vdc).
Terminal 6 represents the positive, terminal 8 represents the negative.
The equivalent impedance of the input is 1.1kΩ.
When dip switch 4 - BLOCK A is in the ON position, the input is suitable for use with the Power Factor Controller
M50FA400A manufactured by Marelli Motori.
When dip switch 4 - BLOCK A is in OFF position, the input is suitable for use with any external device capable of
providing a signal within the range +/- 5Vdc (corresponding to a voltage reference variation of +/-15%).
963857154_U
of
lead to
with the
value
that was
(3p.u.) for a maximum time (fixed) of 10
Fset
Fset
setting. For
Fset
29