Specifications

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 to take place only
if total capacity required is > 50%. Total pump capacity is
determined as lead pump plus fixed speed pump
capacities.
Bandwidth Management
In cascade control systems, the desired system pressure is
kept within a bandwidth rather than at a constant level to
avoid frequent switching of fixed speed pumps. 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.
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
Time
Time
Mains operation
Destaging freq.
Alternation
command/PID stops
Staging freq.
Mains operation
PID contr.
starts
130BA364.10
5s
f
max
f
max
f
min
Illustration 6.8 Pump Staging with Lead Pump Alternation
With lead pump alternation enabled, a maximum of 2
pumps are controlled. At an alternation command, the
lead pump will ramp to minimum frequency (f
min
) and
after a delay will ramp to maximum frequency (f
max
). When
the speed of the lead pump reaches the destaging
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.
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
) 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 water
circulation problem.
6.1.10 System Status and Operation
If the lead pump goes into sleep mode, the function is
displayed in 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 mains/motor starter
•
Cascade Status is a read out of the status for the
cascade controller. The display shows the
following conditions:
-
Cascade controller is disabled
-
All pumps are off
-
An emergency has stopped all the
pumps
-
All pumps are running
-
Fixed speed pumps are being staged/
destaged
-
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.
Application Examples VLT® HVAC Drive FC 102 Design Guide
MG16C102 - Rev. 2013-08-20 159
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