VLT
®
FCM Series
Note that the bus reference can only be preset
via serial communication.
NB!:
It is best to preset terminals that are not
being used to
No function
[0].
Differentiator gain limit
If very rapid variations occur in an application in either
the reference signal or the feedback signal, the deviation
between the reference/setpoint and the process
’
s
actual mode will change quickly. The differentiator
can then become too dominant. This is because it
is reacting to the deviation between the reference
and the process
’
s actual mode, and the quicker the
variance changes the more powerful the differentiator
’
s
frequency contribution becomes. The differentiator
’
s
frequency contribution can therefore be limited in such a
way that both a reasonable differentiation time for slow
changes and an appropriate frequency contribution
for quick changes can be preset. This is done using
parameter 443
Process PID Differentiator gain limit
.
Lowpass filter
If there is a lot of noise in the feedback signal, these
can be dampened using an integrated lowpass filter.
A suitable lowpass filter time constant is preset.
If the lowpass filter is preset to 0.1 s, the cut-off
frequency will be 10 RAD/sec, corresponding to (10 / 2
x
π
) = 1.6 Hz. This will mean that all currents/voltages
that vary by more than 1.6 oscillations per second
will be dampened. In other words, there will only
be regulation on the basis of a feedback signal
that varies by a frequency of less than 1.6 Hz. The
appropriate time constant is selected in parameter
444
Process PID lowpass filter time
.
Inverse regulation
Normal regulation means that the motor speed is
increased when the reference/setpoint is greater than
the feedback signal. If it is necessary to run inverse
regulation, in which the speed is reduced when
the reference/setpoint is greater than the feedback
signal, parameter 437
PID normal/inverse control
must be programmed at
Inverted
.
Anti Windup
In the factory the process regulator is preset with
an active anti windup function. This function means
that when either a frequency limit, a current limit or
a voltage limit is reached, the integrator is initialised
at a frequency corresponding to the present output
frequency. This is a means of avoiding the integration
of a variance between the reference and the process
’
s
actual mode that cannot be deregulated by means of
a change of speed. This function can be deselected
in parameter 438
Process PID anti windup
.
Starting conditions
In some applications the optimal setting of the
process regulator will mean that a relatively long
period of time will pass before the required process
condition is achieved. In these applications it can
be a good idea to define an output frequency to
which the frequency converter must run the motor
before the process regulator is activated. This is
done by programming a start frequency in parameter
439
Process PID start frequency
.
■
Handling of feedback
Feedback handling is depicted in this flowchart.
The flowchart shows which parameters can affect the
handling of feedback and how. A choice can be made
between voltage, current and pulse feedback signals.
✭
= factory setting. () = display text [] = value for use in communication via serial communication port
MG.03.H3.02 - VLT is a registered Danfoss trademark
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