Danfoss VLT AutomationDrive FC 300 Design Manual page 36

FC 300 Design Guide
The following parameters are relevant for the Process Control
Parameter
Feedback 1 Resource Par. 7-20
Feedback 2 Resource 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)
The following is an example of a Process PID Control used in a ventilation system:
36
Introduction to FC 300
Description of function
Select from which resource (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, optimum setting of the process regulator will mean
that it takes an excessive time for the desired process value to be reached.
In such applications it might be an advantage to fix a motor frequency to
which the frequency converter is to bring the motor before the process
regulator is activated. This is done by programming a Process PID Start
Value (frequency) in this parameter.
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.
If there are oscillations of the current/voltage feedback signal, these can be
dampened by means of a lowpass filter. This time constant represents the
frequency limit of the ripples occurring on the feedback signal. Example:
If the lowpass filter has been set to 0.1s, the limit frequency will be 10
RAD/sec. (the reciprocal of 0.1 s), corresponding to (10/(2 x π)) = 1.6
Hz. This will mean that all currents/voltages that vary by more than 1.6
oscillations per second will be removed by the filter. In other words, control
will only be carried out on a feedback signal that varies by a frequency of
less than 1.6 Hz. In other words; 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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