Danfoss FC 300 Design Manual page 36

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
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)
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, 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 a value
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 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.B6.22 - VLT is a registered Danfoss trademark