Lenze SCF frequency inverter - Manual (Page 59)

Lenze 13466146 EDBSF01 v24 EN
59
Appendix
B PI Setpoint Control Option
The following describes the PI Setpoint Control software option for the SCF drive. This software option
has additional parameters compared to the standard SCF drive. Also, some of the parameters found
in the standard drive have changed in the PI version.
PI Setpoint Control allows the SCF drive to maintain a process setpoint, such as PSI or CFM, without
using an external controller. When PI is activated the SCF will operate in a closed-loop fashion,
automatically adjusting the motor speed to maintain the process setpoint.
PI setpoint control requires feedback from the process in order to compare the actual process value
to the setpoint. The difference between this value and the setpoint is called the error. The SCF
drive will increase or decrease the motor speed in an attempt to minimize the error. By constantly
adjusting the motor speed, the PI control will drive the process toward the setpoint. Refer to the PI
block diagram below:
SETPOINT
PROCESS
VARIABLE
(PSI, CFM, etc)
Process Variable
Feedback
Motor
Error Speed
Command
P
I
B.1 Direct Acting vs. Reverse Acting Systems
The PI function must be set to match the type of system that it will control. A direct (or normal) acting
system requires an increase in motor speed if the process variable decreases. A reverse acting system
requires a decrease in motor speed if the process variable decreases. Parameter 61 - PI MODE must
be programmed to the setting that matches the system (normal or reverse acting) and also the type
of feedback signal that is used (0-10 VDC or 4-20 mA).
B.2 Feedback Devices
A transducer or transmitter is required to monitor the process variable and provide feedback to
the PI unit in order to compare the process value with the desired setpoint. A transducer outputs
a signal corresponding to a fixed range of the process variable. A transmitter provides offset and
gain adjustments to allow the output signal to be adjusted to correspond to different ranges of the
process variable. Typical output signals for transducers and transmitters are 0-10 VDC or 4-20 mA.
NOTE
If a 4-20 mA signal is used for feedback, and the signal is lost, the drive will display "FL" to
indicate the lost signal. The signal is considered lost if it drops below 2 mA
.