GE AF-650 GP Design Manual page 32

The following parameters are relevant for the Process Control
Parameter
Par. PI-20 Process CL Feedback 1 Resource
Par. PI-22 Process CL Feedback 2 Resource
Par. PI-30 Process PID Normal/ Inverse Control
Par. PI-31 Process PID Anti Windup
Par. PI-32 Process PID Start Speed
Par. PI-33 Process PID Proportional Gain
Par. PI-34 Process PID Integral Time
Par. PI-35 Process PID Differentiation Time
Par. PI-36 Process PID Diff. Gain Limit
Par. PI-38 Process PID Feed Forward Factor
Par. E-64 Pulse Filter Time Constant #29 (Pulse term.
29), par. E-69 Pulse Filter Time Constant #33 (Pulse
term. 33), par. AN-16 Terminal 53 Filter Time Con-
stant (Analog term 53), par. AN-26 Terminal 54 Filter
Time Constant (Analog term. 54)
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 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 appli-
cations 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. PI-32 Process
PID Start Speed.
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 dif-
ferentiator.
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 ref-
erence 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 deteri-
orate the dynamic performance of the Process PID Control.
AF-650 GP Design Guide
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