Handling Of Feedback - Danfoss VLT Decentral FCD 300 Operating Instructions Manual

3 Programming
Reference type
Pulse
Voltage
Current
Preset
references
Bus reference
Note that the bus reference can only be preset via serial communication.
3
NB!
It is best to preset terminals that are not being used
No function
to
Differentiator gain limit
If very rapid variations occur in an application in either the reference sig-
nal or the feedback signal, the deviation between the reference/setpoint
and the process's actual mode will change quickly. Thedifferentiator 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 the speed regulation in pa-
Speed PID Differentiator gain limit
rameter 420
Process PID Differentiator gain limit
parameter 443
Lowpass filter
If there is a lot of noise in the feedback signal, these can be dampened
using an integratedlowpass 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

3.6.3 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.
64
Terminal Parameters
29, 33 305, 307, 327, 328
53 308, 309, 310
60 314, 315, 316
215-218
68+69
[0].
and Process regulation in
.
MG.04.B8.02 - VLT is a registered Danfoss trademark
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 Speed Regulation in parameter
Speed PID lowpass filter time
421 and in Process Regulation in parameter
Process PID lowpass filter time
444 .
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 refer-
ence/setpoint is greater than the feedback signal, parameter 437
normal/inverted control must be programmed at
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 cur-
rent 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
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 pro-
gramming a start frequency in parameter 439
cy
.
VLT® Decentral FCD
PID
Inverted .
Process PID
Process PID start frequen-