time for slow changes and an appropriate frequency contribution for quick
changes can be preset. This is done using the speed regulation in pa-
rameter 420
Speed PID Differentiator gain limit
and Process regulation in
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 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
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
421
Speed PID lowpass filter time
and in Process Regulation 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 refer-
ence/setpoint is greater than the feedback signal, parameter 437
PID
normal/inverted 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 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
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 pro-
gramming a start frequency in parameter 439
Process PID start frequen-
cy
.
4.6.3 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.
NB!
Parameters 417-421 are only used, if in parameter 100
Configuration
the selection made is
Speed regulation,
closed loop
[1].
417
Speed PID proportional gain
Value:
0.000 (OFF) - 1.000
0.010
Function:
Proportional gain indicates how many times the fault (deviation between
the feedback signal and the setpoint) is to be amplified.
Description of choice:
Quick regulation is obtained at high amplification, but if the amplification
is too high, the process may become unstable in the case of overshooting.
418
Speed PID integral time
Value:
20.00 - 999.99 ms (1000 = OFF)
100 ms
Function:
The integral time determines how long the PID regulator takes to correct
the error. The greater the error, the quicker the integrator frequency
contribution will increase. The integral time is the time the integrator
needs to achieve the same change as the proportional amplification.
4 Programming
VLT
®
Decentral FCD 300 Design Guide
94
MG.90.S1.02 - VLT
®
is a registered Danfoss trademark
4