AC Tech SCF SERIES, Installation And Operation Manual

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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.