Lenze MC1000 Series - Manual (Page 74)

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19 MC1000 PID SET POINT CONTROL
PID Set point Control allows the MC1000 to maintain a process set point, such
as PSI or CFM, without using an external controller. When PID is activated, the
MC1000 will operate in a closed-loop fashion, automatically adjusting the motor
speed to maintain the set point.
PID set point control requires feedback from the process in order to compare the
process variable “value” to the set point. The difference between the process variable
“value” and the set point is called the error. The MC1000 will increase or decrease
the motor speed in an attempt to minimize the error. By constantly adjusting the
motor speed, the PID control will drive the process toward the set point. Refer to
the PID block diagram below:
SET
POINT
Process Variable
Feedback (transducer)
Motor
Error Speed
Command
P
I
D
Process
19.1 FEEDBACK DEVICES
A transducer or transmitter is required to monitor the process variable and provide
feedback to the PID unit in order to compare the process variable feedback to the
set point. 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: 0-5 VDC, 0-10 VDC, or 4-20
mA. The feedback device must be externally powered, as the drive does not have
a power supply for such devices. Program Parameter 74 - PID FB (PID FEEDBACK
SOURCE) for the appropriate terminal (TB-5A or TB-5B), and connect the feedback
device as described below:
POT
The positive signal wire (wiper) is connected to TB-5A, and the
“high” lead is connected to TB-6.
0-5, 0-10 VDC Connect the positive signal wire to TB-5A.
4-20 mA Connect the positive signal wire to TB-5B.
The common, or negative signal wire, is connected to TB-2 (circuit common).
Feedback devices can be direct or reverse acting. A direct acting device outputs
a signal that increases as the process variable increases. A reverse acting device
outputs a signal that decreases as the process variable increases. The programming
of Parameters 75 - FB @ MIN and 76 - FB @ MAX depend on the type of feedback
device being used.
When using a direct acting transducer, Parameter 75 - FB @ MIN should be set to
the value of the process variable feedback corresponding to the minimum feedback
signal (0 VDC or 4 mA), and Parameter 76 - FB @ MAX should be set to the value
of the process variable feedback corresponding to the maximum feedback signal
(5 or 10 VDC, or 20 mA).