System Status And Operation - Danfoss VLT AQUA Design Manual

6. Application Examples
Lead Pump Alternation
The motors must be of equal size. This function makes it possible to cycle the drive between the pumps in the system (maximum of 2 pumps). In this
operation the run time between pumps is equalized reducing the required pump maintenance and increasing reliability and lifetime of the system. The
alternation of the lead pump can take place at a command signal or at staging (adding another pump).
The command can be a manual alternation or an alternation event signal. If the alternation event is selected, the lead pump alternation takes place every
time the event occurs. Selections include whenever an alternation timer expires, at a predefined time of day or when the lead pump goes into sleep mode.
Staging is determined by the actual system load.
A separate parameter limits alternation only to take place if total capacity required is > 50%. Total pump capacity is determined as lead pump plus fixed
speed pumps capacities.
Bandwidth Management
In cascade control systems, to avoid frequent switching of fixed speed pumps, the desired system pressure is kept within a bandwidth rather than at a
constant level. The Staging Bandwidth provides the required bandwidth for operation. When a large and quick change in system pressure occurs, the
Override Bandwidth overrides the Staging Bandwidth to prevent immediate response to a short duration pressure change. An Override Bandwidth Timer
can be programmed to prevent staging until the system pressure has stabilized and normal control established.
6 When the Cascade Controller is enabled and the drive issues a trip alarm, the system head is maintained by staging and destaging fixed speed pumps.
To prevent frequent staging and destaging and minimize pressure fluxuations, a wider Fixed Speed Bandwidth is used instead of the Staging bandwidth.
6.1.9. Pump Staging with Lead Pump Alternation
With lead pump alternation enabled, a maximum of two pumps are con-
trolled. At an alternation command, the PID stops, the lead pump ramps
to minimum frequency (fmin) and after a delay ramps to maximum fre-
quency (fmax). When the speed of the lead pump reaches the destaging
frequency, the fixed speed pump will be cut out (destaged). The lead
pump continues to ramp up and then ramps down to a stop and the two
relays are cut out.
After a time delay, the relay for the fixed speed pump cuts in (staged) and this pump becomes the new lead pump. The new lead pump ramps up to
maximum speed and then down to minimum speed. When ramping down and reaching the staging frequency, the old lead pump is now cut in (staged)
on the mains as the new fixed speed pump.
If the lead pump has been running at minimum frequency (fmin) for a programmed amount of time, with a fixed speed pump running, the lead pump
contributes little to the system. When programmed value of the timer expires, the lead pump is removed avoiding water heating problems.

6.1.10. System Status and Operation

If the lead pump goes into Sleep Mode, the function is displayed on the Local Control Panel. It is possible to alternate the lead pump on a Sleep Mode
condition.
When the cascade controller is enabled, the operation status for each pump and the cascade controller is displayed on the Local Control Panel. Information
displayed includes:
• Pumps Status, is a read out of the status for the relays assigned to each pump. The display shows pumps that are disabled, off, running on the
frequency converter or running on the mains/motor starter.
• Cascade Status, is a read out of the status for the Cascade Controller. The display shows the Cascade Controller is disabled, all pumps are off,
and emergency has stopped all pumps, all pumps are running, fixed speed pumps are being staged/destaged and lead pump alternation is
occurring.
• Destage at No-Flow ensures that all fixed speed pumps are stopped individually until the no-flow status disappears.
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VLT AQUA Drive Design Guide