Trane PID Control User Manual
®
Chapter 4 Applications
42
CNT-APG002-EN
Determining the staging points
This section describes how to find the points at which stages are turned
on and off.
on and off.
Start with these guidelines:
•
To avoid having a stage turn off at the lowest extreme, always have at
least one stage on at 10% of the output range. Turn that stage off
when the control value is less than 10%. Due to hysteresis (the pro-
gramming of equipment to react in a different way depending on
whether the control value is increasing or decreasing), this stage may
be on only when the output is decreasing.
least one stage on at 10% of the output range. Turn that stage off
when the control value is less than 10%. Due to hysteresis (the pro-
gramming of equipment to react in a different way depending on
whether the control value is increasing or decreasing), this stage may
be on only when the output is decreasing.
•
To avoid having a stage turn on at the PID maximum value, have all
stages on at 90% of the output range.
stages on at 90% of the output range.
•
To reduce equipment cycling, stages should overlap.
•
As a starting point, assume that the overlap range is the same for all
stages. You can adjust the staging points later to optimize the system.
stages. You can adjust the staging points later to optimize the system.
To determine the staging points:
1.
Use the following formula to find the overlap range:
(assuming the system has three fans).
2.
To create overlap, the first stage should turn on at the lowest extreme
plus 2 times the overlap range and turn off at the lowest extreme, or:
plus 2 times the overlap range and turn off at the lowest extreme, or:
For a three-fan system, the first stage should turn on at 50% and turn
off at less than 10%.
off at less than 10%.
3.
For each subsequent stage, the on and off points are described by:
Although not discussed in this section, equipment minimum-on and -off
times become a factor as more stages are added to the system. The higher
stages may be on for shorter periods of time. System behavior may
become erratic if a stage control is dominated by minimum-on and -off
times instead of the calculated control value.
times become a factor as more stages are added to the system. The higher
stages may be on for shorter periods of time. System behavior may
become erratic if a stage control is dominated by minimum-on and -off
times instead of the calculated control value.
Overlap range
highest extreme
lowest extreme
–
stage count
1
+
--------------------------------------------------------------------------------------------
=
Overlap range
90%
10%
–
stage count
1
+
-----------------------------------------
80%
3
1
+
-------------
20%
=
=
=
Stage 1
On: control value
10%
2
overlap range
×
(
)
+
50%
=
≥
Off: control value
10%
<
=
Stage n
On: control value
10%
n
1
+
(
) overlap range
×
+
≥
Off: control value
10%
n
1
–
(
) overlap range
×
+
<
=