Danfoss FC 300 Design Manual page 37

FC 300 Design Guide
The following parameters are relevant for 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 If there are quick changes in reference or feedback in a given appli-
Feed Forward Factor Par. 7-38
Low-pass 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 receive its feedback
Optional: Determine if (and from where) the process PID should get
an additional feedback signal. If an additional feedback source is se-
lected, the two feedback signals will be added together before being
used in 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 adjustable frequency drive 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, a value that
is too large may lead to oscillations.
Eliminates steady state speed error. Lower value means quick reac-
tion. However, a value that is too small may lead to oscillations.
Provides a gain proportional to the rate of change of the feedback. A
setting of zero disables the differentiator.
cation - 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 differ-
entiator gain is. The differentiator gain can thus be limited to allow
setting of the reasonable differentiation time for slow changes.
In applications where there is a good (and approximately linear) cor-
relation between the process reference and the motor speed neces-
sary 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 0.1s, 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.
MG.33.B9.22 - VLT ® is a registered Danfoss trademark.
3. Introduction to FC 300
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