LEESON SM-Plus Series Sub-Micro Inverters - Manual (Page 63)

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:
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PI OPTION DISPLAYS
DISPLAY DESCRIPTION
ACP AUTO CONTROL PAD: The ▲ and ▼ buttons are used to adjust the
setpoint in PI mode.
nCP MANUAL CONTROL PAD: The ▲ and ▼ 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).