GE AF-650 GP Design And Installation Manual page 75

Application Setup Examples
Parameter
PI-34 Process PID Integral Time
PI-35 Process PID Differentiation Time
PI-36 Process PID Diff. Gain Limit
PI-38 Process PID Feed Forward Factor
E-64 Pulse Filter Time Constant #29 (Pulse
term. 29), E-69 Pulse Filter Time Constant #33
6 6
(Pulse term. 33), AN-16 Terminal 53 Filter
Time Constant (Analog term 53),
AN-26 Terminal 54 Filter Time Constant
(Analog term. 54)
Table 6.20
6-20
TM
AF-650 GP Design and Installation Guide
Description of function
Eliminates steady state speed error. Lower value means quick reaction. However, a value
that is too small 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/s (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 filter time that is too
long will deteriorate the dynamic performance of the process PID control.
DET-767A