PPI Zenex Plus, User Manual, Page: 18 / 27

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Zenex Plus User Manual
Compressor Time Delay
Once the compressor is switched off, a time delay is desired before it is turned ON again. The time delay prevents the possible
damage due to short cycling. When the compressor switches off, it spins backward as pressure equalizes. If compressor is
energized while it is still spinning backward, it continues to run backward until it trips on internal overloads. This may cause
damage to the compressor.
The time delay cycle is executed every time the compressor is turned off. The compressor turning off may be a result of power
failure or the on-off control algorithm executed by the controller. The controller in “Auto Mode” monitors the Process Value (PV)
against the Set-Point (SP) and attempts to switch the compressor ON or OFF depending upon whether the PV is above or
below the SP.
The time delay starts counting down from the instance the compressor is switched off. The compressor is inhibited from
switching-on until the delay elapses regardless of the difference between the PV and SP. Once the time delay is elapsed, the
control algorithm switches the compressor ON as and when the PV is above SP.
Figure 5.4
PV (Temperature)
SP
Compressor Switch-ON Level
Compressor Switch-OFF Level
Compressor Switch-ON Level = 20.0 + 1.0 = 21.0°C
Temperature SP = 20.0°C
Compressor Set-point = 1.0°C
Compressor Hysteresis = 1.2°C
Compressor Switch-OFF Level = 21.0 - 1.2 = 19.8°C
Compressor Status
OFF
ON
20.0
19.8
21.0
°C
Case 2 Case 3 Case 1
The figure 5.4 above illustrates 3 cases. Case (1) illustrates power-up delay. In case (2); the time elapses before PV rises
above the SP. The compressor is thus switched ON as soon as the PV rises above the SP. In case (3); the PV rises above the
SP while the time delay is still in progress. The compressor is switched ON as soon as the delay time elapses.
Table 5.2 lists various parameters required for setting-up appropriate compressor switching strategy and associated
parameters.
For parameter description the user is requested to read the preceding section.
Once the air temperature falls below Compressor Switch-ON Level by an amount set by the parameter 'Compressor
Hysteresis', the compressor is turned OFF. That is;
Compressor Switch - OFF Level = (Compressor Switch-ON Level) – (Compressor Hysteresis)
The hysteresis inserts a dead band between the Compressor Switch-ON Level and Compressor Switch-OFF Level to avoid
frequent switching of the compressor.