Proportional Band; Neutral Zone - Carel PRK300L3FK User Manual

ENG
6.2.1

Proportional band

The operating principle is normal proportional or proportional + integral
control (P, P+I).
The control set point is central, consequently - for proportional control
only - operation is schematised in the following fi gure:
100%
Differential
Setpoint
For example, for 4 devices with the same capacity and proportional only
control, start-up occurs as shown in the fi gure:
C1 C2
Differential
Setpoint
With P+I control, added to the eff ect of the proportional action described
above is the integral action, used to achieve a null control error in steady
operation, as shown in the fi gure:
Regulation variable
P+I
P
Setpoint
The integral action depends on the time and the deviation from the set
point. This modifi es the request if the control value does not approach
the set point for some time.
The integral time setting represents how fast integral control is
implemented:
•
low values determine fast and intense control action
•
high values determine slower and more stable control action
It is recommended to not set a value that is too low for the integral time,
to avoid instability.
Note: the set point is in the centre of the activation band, therefore
when reaching the set point some devices are on, even with purely
proportional control.
pRack +0300025EN rel. 1.3 - 17.12.2015
Regulation request
Regulation
Differential
probe value
Fig. 6.b
Devices activation
C3 C4
Suction
Differential
pressure
Fig. 6.c
time
Fig. 6.d
6.2.2

Neutral zone

The operating principle is schematised in the following fi gure:
Regulation request
Decrease
zone
100%
Force off Diff.
Inside the neutral zone the capacity request sent by the controller is
constant (except when there is a modulation device and modulation is
enabled inside the neutral zone, as described in the following paragraph)
and the value satisfi es the temperature control request in those specifi c
operating conditions, therefore within this zone no device is stopped or
started.
In the decrease zone, the request also decreases at a rate that depends on
the deviation from the set point, and vice-versa in the increase zone the
request increases proportionally to the deviation.
For the increase and decrease zones, the following can be used:
•
Fixed times: the request decreases or increases constantly as time
elapses.
•
Variable times: the request decreases or increases more quickly
(according to the settings) as the deviation from the set point increases.
Note: The previous fi gure shows the increase and decrease with
fi xed times.
For control in Neutral zone, the parameters shown in the fi gure must be
set:
NZ timings
Decrease Neutral
zone zone
Max time 0%
Min time 0%
Diff.
Decr. Diff.
Setpoint
As well as the decrease and increase diff erentials, 4 times need to be set,
two for each zone, which represent the maximum and minimum time
to reach the request, equal to 0% or 100%, for the decrease and increase
respectively.
Tutorial: the decrease/increase times (minimum and maximum)
represent the time needed to change from maximum to minimum
capacity and vice-versa, and not the time between the deactivation/
activation of the individual device. For example, in the case of 4 devices
with the same capacity, an increase time of 180 s means that one device
is activated every 45 s.
34
Neutral Increase
zone zone
Regulation
Diff.
probe value
Setpoint
Fig. 6.e
Increase
zone
Max time 100%
Min time 100%
Regulation
Diff.
Incr. Diff.
probe value
Fig. 6.f