Figure 3.3: Process PID Control diagram
The following parameters are relevant for process control.
Parameter
Description of function
Par. 7-20
Process CL Feedback 1 Resource
Select from which source (i.e., analog or pulse input) the process PID should receive its feedback
Par. 7-22
Process CL Feedback 2 Resource
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 process PID control.
Par. 7-30
Process PID Normal/Inverse 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.
Par. 7-31
Process PID Anti Windup
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”.
Par. 7-32
Process PID Controller Start Value
In some applications, reaching the required speed/setpoint can take a very long time. In such
applications, it might be an advantage to set a fixed motor speed from the adjustable frequency
drive before the process control is activated. This is done by setting a Process PID Start Value
(speed) in par. 7-32
Process PID Controller Start Value
.
Par. 7-33
Process PID Proportional Gain
The higher the value, the quicker the control. However, a value that is too large may lead to
oscillations.
Par. 7-34
Process PID Integral Time
Eliminates steady state speed error. Lower value means quick reaction. However, a value that is
too small may lead to oscillations.
Par. 7-35
Process PID Differentiation Time
Provides a gain proportional to the rate of change of the feedback. A setting of zero disables the
differentiator.
Par. 7-36
Process PID Differentiation Gain Limit
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.
Par. 7-38
Process PID Feed Forward Factor
In applications 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.
Par. 5-54
Pulse Filter Time Constant #29
(Pulse
term. 29), par. 5-59
Pulse Filter Time Constant
#33
(Pulse term. 33), par. 6-16
Terminal 53 Filter
Time Constant
(Analog term 53), par. 6-26
Ter-
minal 54 Filter Time Constant
(Analog term. 54)
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 0.1s, the limit speed will be 10 RAD/sec. (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 filter time that is too long
will deteriorate the dynamic performance of the process PID control.
FC 300 Design Guide
3 Introduction to AutomationDrive FC 300
MG.33.BC.22 - VLT
®
is a registered Danfoss trademark
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