Process Control Relevant Parameters - Danfoss VLT FC 360 Design Manual

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2.5.3 Process Control Relevant Parameters

2 2
Parameter
7-20 Process CL Feedback 1 Resource
7-22 Process CL Feedback 2 Resource
7-30 Process PID Normal/ Inverse Control
7-31 Process PID Anti Windup
7-32 Process PID Start Speed
7-33 Process PID Proportional Gain
7-34 Process PID Integral Time
7-35 Process PID Differentiation Time
7-36 Process PID Diff. Gain Limit
7-38 Process PID Feed Forward Factor
5-54 Pulse Filter Time Constant #29 (Pulse
term. 29), 5-59 Pulse Filter Time Constant #33
(Pulse term. 33), 6-16 Terminal 53 Filter Time
Constant (Analog term 53), 6-26 Terminal 54
Filter Time Constant (Analog term. 54)
Table 2.7 Process Control Parameters
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VLT AutomationDrive FC 360 Design Guide
Description of function
Select from which Source (i.e. analog or pulse input) the Process PID should get its
feedback
Optional: Determine if (and from where) the Process PID should get an additional
feedback signal. If an additional feedback source is selected the two feedback signals will
be added together before being used in the Process PID Control.
Under [0] Normal operation the Process Control will respond with an increase of the motor
speed if the feedback is getting lower than the reference. In the same situation, but under
[1] Inverse operation, the Process Control will respond with a decreasing motor speed
instead.
The anti windup function ensures that when either a frequency limit or a torque limit is
reached, the integrator will be set to a gain that corresponds to the actual frequency. This
avoids integrating on an error that cannot in any case be compensated for by means of a
speed change. This function can be disabled by selecting [0] Off.
In some applications, reaching the required speed/set point can take a very long time. In
such applications it might be an advantage to set a fixed motor speed from the frequency
converter before the process control is activated. This is done by setting a Process PID
Start Value (speed) in 7-32 Process PID Start Speed.
The higher the value, the quicker the control. However, too large value may lead to
oscillations.
Eliminates steady state speed error. Lower value means quick reaction. However, too small
value may lead to oscillations.
Provides a gain proportional to the rate of change of the feedback. A setting of zero
disables the differentiator.
If there are quick changes in reference or feedback in a given application - which means
that the error changes swiftly - the differentiator may soon become too dominant. This is
because it reacts to changes in the error. The quicker the error changes, the stronger the
differentiator gain is. The differentiator gain can thus be limited to allow setting of the
reasonable differentiation time for slow changes.
In application where there is a good (and approximately linear) correlation between the
process reference and the motor speed necessary for obtaining that reference, the Feed
Forward Factor can be used to achieve better dynamic performance of the Process PID
Control.
If there are oscillations of the current/voltage feedback signal, these can be dampened by
means of a low-pass filter. This time constant represents the speed limit of the ripples
occurring on the feedback signal.
Example: If the low-pass filter has been set to 0.1 s, the limit speed will be 10 RAD/s (the
reciprocal of 0.1 s), corresponding to (10/(2 x π)) = 1.6 Hz. This means that all currents/
voltages that vary by more than 1.6 oscillations per second will be damped by the filter.
The control will only be carried out on a feedback signal that varies by a frequency
(speed) of less than 1.6 Hz.
The low-pass filter improves steady state performance but selecting a too large filter time
will deteriorate the dynamic performance of the Process PID Control.
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