Lenze MC3000 Series - Manual (Page 71)

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19 MC3000 PID SETPOINT CONTROL
PID Setpoint Control allows the MC3000 to maintain a process setpoint, such as PSI or
CFM, without using an external controller . When PID is activated, the MC3000 will operate
in a closed-loop fashion, automatically adjusting the motor speed to maintain the setpoint .
PID setpoint control requires feedback from the process in order to compare the process
variable “value” to the setpoint . The difference between the process variable “value” and
the setpoint is called the error . The MC3000 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 setpoint . Refer to the PID block diagram below:
SETPOINT
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 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: 0-5 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) .