Pump Staging With Lead Pump Alternation; System Status And Operation - Danfoss VLT AQUA Drive FC 202 Design Manual

Application Examples
Lead Pump Alternation
The motors must be of equal size. This function makes it
possible to cycle the frequency converter between the
pumps in the system (maximum of two pumps). In this
operation, the run time between pumps is equalised
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. The actual system load
6 6
determines staging.
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 stabilised and normal control
established.
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 minimise pressure
fluctuations, a wider fixed speed bandwidth is used instead
of the staging bandwidth.
6.1.6 Pump Staging with Lead Pump
Alternation
With lead pump alternation enabled, a maximum of two
pumps are controlled. At an alternation command, the PID
stops, the lead pump ramps to minimum frequency (f
and after a delay, it ramps to maximum frequency (f
When the speed of the lead pump reaches the de-staging
frequency, the fixed speed pump is cut out (de-staged).
The lead pump continues to ramp up and then ramps
down to a stop and the 2 relays are cut out.
144
Design Guide
Illustration 6.8 Lead Pump Alternation
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
(f
min
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.

6.1.7 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:
)
min
).
max
Danfoss A/S © Rev. 06/2014 All rights reserved.
Alternation
command/PID stops
f
max
Destaging freq.
f
min
f
max
Staging freq.
Mains operation
) for a programmed amount of time, with a fixed
•
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 that 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.
Mains operation
Time
PID contr.
starts
5s
Time
MG20Z202