Danfoss VLT AutomationDrive FC 300 Design Manual page 38

FC 300 Design Guide
The following parameters are relevant for 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
Process PID Anti Windup Par. 7-31
Process PID Controller Start Value Par.
7-32
Process PID Proportional Gain Par. 7-33 The higher the value, the quicker the control. However, too large a value
Process PID Integral Time Par. 7-34
Process PID Differentiation Time Par.
7-35
Process PID Differentiator Gain Limit
Par. 7-36
Process PID Feed Forward Factor Par.
7-38
Par. 5-54 (Pulse Filter Time Constant
#29), Par. 5-59 (Pulse Filter Time
Constant #33), Par. 6-16 (Terminal
53 Filter Time Constant), Par. 6-26
(Terminal 54 Filter Time Constant)
38
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 adjustable frequency drive 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.
may lead to oscillations.
Eliminates steady state speed error. Lower value means quick reaction.
However, too small a 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 applications 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 in the current/voltage feedback signal, these can be
damped by means of a low-pass filter. This time constant represents the
frequency limit of the ripples occurring on the feedback signal. Example: If
the low-pass 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 too large a filter time will deteriorate the dynamic performance
of the Process PID Control.
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