Lenze SCF frequency inverter - Manual (Page 67)

Lenze 13466146 EDBSF01 v24 EN
67
Appendix
B.8 Tuning the PI Control
Once the PI control is configured properly, it needs to be tuned in order to maintain the process
setpoint. First, set the Integral Gain (Parameter 65) to zero, and increase the Proportional Gain
(Parameter 64) until the system becomes unstable, then lower the gain until the system stabilizes
again. Set the Proportional Gain about 15% less than that value that stabilizes the system. If only
Proportional Gain is used, and the system is operating in a steady-state condition (setpoint is fixed
and process variable has settled to a fixed value), there will always be a certain amount of error in
the system. This is called the steady-state error.
Integral Gain (Parameter 65) is used to force the steady-state error to zero by increasing the output
speed command with respect to time. Over time, the error will be forced to zero because the Integral
term will continue to change the speed command, even after the Proportional term reaches steady
state and no longer affects the speed command. The Integral Gain affects the rate of rise of the output
speed command from the Integral term. Small amounts of Integral Gain can cause large changes
in PI performance, so care must be taken when adjusting Integral Gain. Too much Integral Gain will
result in overshoots, especially if large step changes in error occur.
The other parameter setting that affects the response of the PI control is Parameter 66 - PI ACCEL/
DECEL. This sets the acceleration and deceleration rate of the setpoint reference into the PI unit.
When the setpoint changes, this function will “filter” the input to the PI unit by ramping the setpoint
reference from the previous value to the new value. This will help prevent overshoots that can occur
when the PI control attempts to respond to step changes in setpoint, resulting in smoother operation.
If PI ACCEL/DECEL is set to 0.0 seconds, it is effectively disabled.
B.9 Additional Status Displays
The PI option contains additional messages that may appear on the display:
PI OPTION DISPLAYS
DISPLAY DESCRIPTION
ACP
AUTO CONTROL PAD: The p and q buttons are used to adjust the setpoint in PI
mode.
nCP
MANUAL CONTROL PAD: The p and q buttons are used for manual speed control
when PI mode is enabled but the drive is operating in open-loop mode.
FL
FOLLOWER LOST: If a 4-20 mA signal is used as a speed reference, PI setpoint
reference, or PI feedback, and the signal drops below 2 mA, the drive will trip and
display “FL” to indicate that the signal is lost.
SLP
SLEEP MODE: The drive is in Sleep Mode. The motor is at zero speed. The drive will
wake up when system demand requires a motor speed above SLEEP THRESHOLD
(P75).