Moeller DF51 - Feedback Value Check Signal (FBV)

10/05 AWB8230-1541GB
PID control
139
Feedback value check signal (FBV)
The FBV (Feedback Value Check) signal is issued when the actual
(process) value (PV) drops below the lower limit value (PNU C053)
in RUN mode. It remains active until:
• the actual value exceeds the upper limit value (PNU C052);
• the frequency inverter changes from RUN mode to STOP mode
(deceleration with the set ramp time).
With PNU C021 or C022 you can set value 07 (FBV) for a digital
outputs (11, 12) or, with PNU C026, for signalling relay K1 (K11-
K12).
With the feedback value check signal (FBV), the DF51’s PID
controller can provide a direct “two-stage control”, as commonly
used for ventilation and air conditioning applications.
Example: ventilation system with two fans (frequency inverter).
Under normal operating conditions, the maximum output power
of fan 1 (M1) is sufficient to maintain the actual value (PV) at the
reference value. When fan 1 is fully utilized and additional airflow
is required, a second fan (M2) with constant power is a simple
solution.
The control sequence for the example cited here is illustrated by
the graph in Fig.145. The process variable and the limit values are
shown as a percentage here. The output frequency (Hz) is also
shown.
• Start of fan motor M1 with signal FWD. The actual value (PV)
lies below the limit value specified with PNU C053. The FBV
output (11, 12, K1) therefore switches to also start fan motor
M2.
• The actual value rises and reaches the upper limit (PNU C052).
The FBV output is automatically switched off (= fan M2 Off).
Fan M1 remains in operation and works in linear control mode.
In a correctly set up system, this is the normal operating range.
• If the actual value drops below the limit value (PNU C053), the
FBV output is switched and fan M2 is activated again to support
fan M1.
• When the FWD signal is removed from frequency inverter 1, the
inverter goes from RUN to STOP mode decelerates the drive
over the set ramp time.
• When frequency inverter 1 is stopped, the FBV output is
automatically de-energized so that fan M2 also stops.
Figure 145:PID control, feedback value check signal (FBV)
a Output frequency (Hz)
a Actual value (process variable – PV)
FWD: Start signal, clockwise rotating field
FBV: Feedback value check signal, limit values PNU C052, C053
exceeded
h
The upper and lower actual value limits (PNU C052,
C053) are “process signals”: they can not be used for
monitoring the feedback value check signal. FBV is not a
fault signal.
C052
C053
FWD
FBV
t
%
b
a
Figure 146:Block diagram, ventilation with “two-stage control”
1: Frequency inverter with PID control for fan motor M1
2: Motor starter (frequency inverter, soft starter, contactor) for fan
motor M2
FWD: Start signal, drive 1 (clockwise rotating field)
FBV: Feedback value check signal from drive 1 for actuating drive 2
PV: Process variable (airflow m
3
/h) as normalized actual value signal
Start: Start signal, drive 2
PV
1
2
FWD
M2
M1
FBV
Start
0... 10 V/ 4... 20 mA