Danfoss FC 300 Design Manual page 34

3 Introduction to AutomationDrive FC 300
The following parameters are relevant for the Process Control
Parameter
Process CL Feedback 1 Resource
Par. 7-20
Process CL Feedback 2 Resource
Par. 7-22
Process PID Normal/ Inverse Control
Par. 7-30
3
Process PID Anti Windup
Par. 7-31
Process PID Start Speed
Par. 7-32
Process PID Proportional Gain
Par. 7-33
Process PID Integral Time
Par. 7-34
Par. 7-35 Process PID Differentiation Time
Process PID Diff. Gain Limit
Par. 7-36
Process PID Feed Forward Factor
Par. 7-38
Pulse Filter Time Constant #29
Par. 5-54
term. 29), par. 5-59 Pulse Filter Time Constant
#33
(Pulse term. 33), par. 6-16
Time Constant
(Analog term 53), par. 6-26
minal 54 Filter Time Constant
34
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 par. 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.
(Pulse 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
Terminal 53 Filter
feedback signal.
Ter-
Example: If the low-pass filter has been set to 0.1s, the limit speed will be 10 RAD/sec. (the
(Analog term. 54) 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.
MG.33.BC.02 - VLT
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FC 300 Design Guide