Danfoss VLT AutomationDrive FC 300 Design Manual page 33

FC 300 Design Guide
The following parameters are relevant for the Process Control
Parameter
Feedback 1 Source Par. 7-20
Feedback 2 Source Par. 7-22
Normal/inverse control Par. 7-30
Anti Windup Par. 7-31
Control Start Value Par. 7-32
Proportional Gain Par. 7-33
Integral Time Par. 7-34
Differentiation Time Par. 7-35
Differentiator Gain Limit Par. 7-36
Feed Forward Factor Par. 7-38
Lowpass Filter Time Par. 5-54 (Pulse term.
29), Par. 5-59 (Pulse term. 33), Par. 6-16
(Analog term 53), Par. 6-26 (Analog term.
54)
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 fre-
quency. 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.
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 dom-
inant. 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 damp-
ened 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 a too large filter
time will deteriorate the dynamic performance of the Process PID Control.
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3. Introduction to FC 300
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