Danfoss FC 300 Design Manual page 32

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)
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Introduction to FC 300
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 ad-
ditional 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 re-
spond with a decreasing motor speed instead.
The anti windup function ensures that when either a frequency limit or a tor-
que 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.
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. How-
ever, 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 differ-
entiator gain can thus be limited to allow setting of the reasonable differen-
tiation 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 dy-
namic 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 a too
large filter time will deteriorate the dynamic performance of the Process PID
Control.
MG.33.B7.02 - VLT is a registered Danfoss trademark