SJ700
2
Inverter
Oper
ati
ons
and Monitor
ing
4–57
To use the PID Second Stage Output feature, you will need to choose upper and lower limits for
the PV, via C053 and C052 respectively. As the timing diagram below shows, these are the
thresholds Stage #1 inverter uses to turn ON or OFF Stage #2 inverter via the [FBV] output.
The vertical axis units are percent (%) for the PID setpoint, and for the upper and lower limits.
The output frequency, in Hz, is superimposed onto the same diagram.
When system control begins, the following events occur (in sequence in the timing diagram):
1.
Stage #1 inverter turns ON via the [FW] Run command.
2.
Stage #1 inverter turns ON the [FBV] output, because the PV is below the PV low limit
C053. So, Stage #2 is assisting in loop error correction from the beginning.
3.
The PV rises and eventually exceeds the PV high limit C052. Stage #1 inverter then turns
OFF the [FBV] output to Stage #2, since the boost is no longer needed.
4.
When the PV begins decreasing, only Stage #1 is operating, and it is in the linear control
range. This region is where a properly configured system will operate most often.
5.
The PV continues to decrease until it crosses under the PV low limit (apparent external
process disturbance). Stage #1 inverter turns ON the [FBV] output, and Stage #2 inverter is
assisting again.
6.
After the PV rises above the PV low limit, the [FW] Run command to Stage #1 inverter
turns OFF (as in a system shutdown).
7.
Stage #1 inverter enters Stop Mode and automatically turns OFF the [FBV] output, which
causes Stage #2 inverter to also stop.
Network
Detection Signal
The intelligent output [Ndc] Network Detection Signal indicates the status of ModBus-RTU
communications (not ASCII protocol). The [Ndc] terminal turns ON when the external device
on the ModBus stops transmitting for a time that exceeds parameter C077, Communication
Time-out Before Trip. If a time-out occurs, the output [Ndc] remains ON until ModBus-RTU
communication resumes. Use parameter C076 Action Upon Communication Error Selection to
select the desired inverter response to a time-out event.
PV high limit
t
0
t
C052
C053
Stage #1 [FW]
t
[FBV] to Stage #2 [FW]
PV low limit
1
0
1
PID setpoint (SP)
PID feedback (PV)
Output frequency
1, 2
3
4
5
Example:
6
7
Opt. Code
32
Symbol
[Ndc]
Valid for
Outputs
[11 to [15],
[ALx]
Required
Settings
C076, C077
Default
terminals
Requires
config.
Master
t
0
C077
1
Slave (inverter)
Watchdog timer
= xx.xx sec.
0
1
Alarm output [ALx]
Time-out
C076 = 00 or 01 (trip)
Network detection [Ndc]
