Override Staging /Destaging; Minimum Speed Destaging; Fixed Speed Only Operation - Danfoss VLT Series Operating Instructions Manual

1 Extended and Advanced Cascade Controller
Option
If the cascade controller is still unable to maintain the system pressure with all of the variable speed pumps on at maximum, it will begin to stage on
fixed speed pumps. A fixed speed pump will be staged on when the pressure goes below the setpoint by the Normal Operation Range (27-20) percentage
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and stays there for the Staging Delay (27-23) time . This repeats for all of the fixed speed pumps.
Destaging:
If the system pressure increases, the speed of all of the Drives decrease to match the system's reduced demand for flow. While maintaining pressure, if
the Drive goes below the Stage off Speed (27-33) and stays there for the Destaging Delay (27-24) time, a variable speed pump will be staged off. This
repeats for all of the variable speed pumps except the last one.
If the system pressure is still too high with only one Drive running at minimum speed, it will begin to destage fixed speed pumps. A fixed speed pump
will be destaged when the pressure goes above the setpoint by the Normal Operating Range (27-20) percentage and stays there for the Destaging Delay
(27-24) time . This repeats for all of the fixed speed pumps. This leaves only one variable speed pump running. If the system demand continues to drop
the system will enter sleep mode.

1.7.8 Override Staging /Destaging

Normal staging and destaging handles most of the situations in typical applications. However sometimes it is necessary to respond rapidly to changes in
system feedback pressure. In these cases the cascade controller is equipped to immediately stage and destage pumps in response to large changes
system demand.
Staging:
When the system pressure drops by more than the Override Limit (27-21), the cascade controller will immediately stage on a pump to meet the demand
for more flow.
If the system pressure continues to stay below the Override Limit (27-21) for the Override Hold Time (27-25) time, the cascade controller will then stage
on the next pump. This repeats until all of the pumps are on or until the system pressure drops below the Override Limit.
Destaging:
When the system pressure increases rapidly above the Override Limit (27-21), the cascade controller will immediately destage a pump to try to reduce
the pressure.
If the system pressure continues to stay above the Override Limit (27-21) for the Override Hold Time (27-25) time, the cascade controller will destage
another pump. This will repeat until only the lead pump is left on or until the pressure stabilizes.
The Override Limit parameter 27-21 is set as a % of the Maximum Reference. It defines a point above and below the system Setpoint where Override
staging and destaging will occur.

1.7.9 Minimum Speed Destaging

To reduce emergency usage the cascade controller will destage a pump if the Lead Pump is running at minimum speed for Min Speed Destage Delay
(27-27).

1.7.10 Fixed speed only operation

Fixed Speed only operation is a feature designed to keep critical systems operating in the rare event that all of the variable speed pumps are unavailable
to the cascade controller. In this situation the cascade controller will attempt to maintain system pressure by turning on and off fixed speed pumps.
Staging:
If all the variable speed pumps are unavailable and the system pressure goes below the Fixed Speed Only Operating Range (27-22) for the Staging Delay
(27-23) time, then a fixed speed pump will be turned on. This repeats until all of the pumps are on.
De-staging:
If all of the variable speed pumps are unavailable and the system pressure goes above the Fixed Speed Only Operating Range (27-22) for the Destage
Delay (27-24) time, a fixed speed pump will be turned off. This repeats until all of the pumps are off.
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MI.38.C3.02 - VLT is a registered Danfoss trademark
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VLT Operating Instructions