Hitachi SJ7002 Series Instruction Manual page 218

4–56
Using Intelligent Output Terminals
PID Feedback
Second Stage
Output
31
Opt. Code
[FBV]
Symbol
[11 to [15],
Valid for
[ALx]
Outputs
Required
C052, C053
Settings
Monitor
D004
Settings
Default
Requires
config.
terminals
The inverter has a built-in PID loop feature for two-stage control, useful for certain applications
such as building ventilation or heating and cooling (HVAC). In an ideal control environment, a
single PID loop controller (stage) would be adequate. However, in certain conditions, the
maximum output energy from the first stage is not enough to maintain the Process Variable
(PV) at or near the Setpoint (SP). And, the output of the first stage is in saturation. A simple
solution is to add a second stage, which puts an additional and constant amount of energy into
the system under control. When sized properly, the boost from the second stage brings the PV
toward the desired range, allowing the first stage PID control to return to its linear range of
operation.
The two-stage method of control has some advantages for particular applications.
• The second stage is only ON in adverse conditions, so there is an energy savings during
normal conditions.
• Since the second stage is simple ON/OFF control, it is less expensive to add than just dupli-
cating the first stage.
• At powerup, the boost provided by the second stage helps the process variable reach the
desired setpoint sooner than it would if the first stage acted alone.
• Even though the second stage is simple ON/OFF control, when it is an inverter you can still
adjust the output frequency to vary the boost it provides.
Refer to the example diagram below. Its two stages of control are defined as follows:
• Stage 1 - Inverter #1 operating in PID loop mode, with motor driving a fan
• Stage 2 - Inverter #2 operating as an ON/OFF controller, with motor driving a fan
Stage #1 provides the ventilation needs in a building most of the time. On some days, there is a
change in the building's air volume because large warehouse doors are open. In that situation,
Stage #1 alone cannot maintain the desired air flow (PV sags under SP). Inverter #1 senses the
low PV and its PID Second Stage Output at [FBV] terminal turns ON. This gives a Run FWD
command to Inverter #2 to provide the additional air flow.
Stage #1
Inverter #1
[U, V, W]
PV
[O], [OI], or [O2]
[FBV]
NOTE: The [FBV] is designed for implementing two-stage control. The PV high limit and PV
low limit parameters, C052 and C053, do not function as process alarm thresholds. Terminal
[FBV] does not provide a PID alarm function.
Fan #1
Fan #2
PID Second Stage Output
Process Variable
Air flow Sensor
Stage #2
Inverter #2
[U, V, W]
[FW]