When the Cascade Controller is enabled and running normally and the frequency converter 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 controlled. At an alternation command, the lead pump will ramp to minimum frequency
(fmin) and after a delay will ramp to maximum frequency (fmax). When the speed of the lead pump reaches the destaging frequency, the fixed speed
pump will be cut out (de-staged). 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 the programmed value of the timer expires, the lead pump is removed, avoiding a deal heat water circulation
problem.
6.1.10 System Status and Operation
If the lead pump goes into Sleep Mode, the function is displayed on the LCP. 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 LCP. 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/de-staged and lead pump alternation is
occurring.
•
De-stage at No-Flow ensures that all fixed speed pumps are stopped individually until the no-flow status disappears.
6 Application Examples
VLT
®
HVAC Drive Design Guide
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MG.11.B9.02 - VLT
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is a registered Danfoss trademark
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