µRack
Cod. CAREL +03P220431 rel. 0.0 del 08/09/05 - preliminary version
10
The devices are activated when the measured value exceeds the limit to the right (measured value greater than SP + DZN, see Figure 4.3). The number of devices
to be activated varies according to the time elapsed outside of the dead band. The first device will start immediately, while the others will wait the set time
between starts (r07).
Similarly, the devices are stopped when the measured value falls below the dead band (measured value less than the set point), and remains there for a period
equal to the time between device stop requests. In this case too, the first device stops immediately, while the others wait the delay time between stops (r09).
Also see the paragraph on Time settings.
The program will switch the devices on according to the start-up logic configured and the availability of the devices
DOffZ
DOnZ
NZ
SP
DZN
RP
Fig. 4.2
Compressor dead band with variable times
The user can decide to set a variable time between calls, depending on whether the pressure is moving away from the dead band. In particular, the activation /
deactivation time of the outputs decreases as the distance from the dead band increases. To set this function, the following parameters must be configured:
• Maximum compressor on time (parameter r08)
• Minimum compressor on time (parameter r07)
• Pressure differential within which the time varies. (parameter r11)
• Maximum compressor off time (parameter r10)
• Minimum compressor off time (parameter r09)
Fig. 4.3
Key:
InPress Suction pressure DTNZ Differential within which the time varies
STPM Control set point TOnMax Maximum compressor on time
RBM Control band TOnMin Minimum compressor on time
NZ Dead band TOffMax Maximum compressor off time
DOnZ Device activation zone TOffMin Minimum compressor off time
DOffZ Device deactivation zone
In the activation phase, the following cases are possible:
1. Pressure equal to point b
same call time as the “maximum compressor on time”
2. Pressure between point b and point b + DTNZ
type of call between “Max on time” and “Min on time”
3. Pressure greater than or equal to point b + DTNZ
same call time as “Min on time”
In the deactivation phase, on the other hand, the following cases are possible:
1. Pressure equal to point STPM
same call time as the “maximum compressor off time”
2. Pressure between point STPM and point STPM - DTNZ
type of call between “Max off time” and “Min off time”
3. Pressure greater than or equal to point STPM - DTNZ
same call time as “Min off time”
N.B. To make the device call time constant in the activation phase, simply set the times TOnMax and TonMin to the same value. The same is true for the
deactivation phase.
Key:
DOffZ Device deactivation zone
DOnZ Device activation zone
NZ Dead band
DZN Dead band differential
RP Suction pressure read
SP Set point
STPM
RBM
NZDOffZ DOnZ
InPress [ºbar]
TOnMinTOffMin
TOnMaxTOffMax
DTNZ
B
DTNZ
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