demand normally stays below the maximum capacity of
the lead pump
4.2.7 Staging/De-staging in Mixed Pump
Configurations
Two methods are used to decide when pumps should be
staged or de-staged.
•
Speed of the frequency converters
•
Feedback pressure exceeding the normal
operating range
In a mixed pump configuration with more than one
frequency converter both methods are used.
In the following example, feedback is referred to as
pressure.
Staging
When the master drive receives a start command a
variable speed pump is selected, and started using one of
the available frequency converters.
If the system pressure drops, the speed of the frequency
converter increases to meet the demand for more flow.
While maintaining the pressure, if the frequency converter
exceeds
27-31 Stage On Speed [RPM]
(or
27-32 Stage On
Speed [Hz]
) and remains above that speed for
27-33 Stage
Off Speed [RPM]
(or
27-34 Stage Off Speed [Hz]
) time, the
next variable speed pump is staged on.
If the Cascade Controller is unable to maintain the system
pressure with all variable speed pumps running at
maximum, it will stage on fixed speed pumps. A fixed
speed pump will be staged on when the pressure goes
below the setpoint by
27-20 Normal Operating Range
percentage and stays there for the Staging Delay time
(
27-23 Staging Delay
).
De-staging
If the system pressure increases, the speed of the
frequency converter decreases to maintain pressure. If the
frequency converter drops below
27-33 Stage Off Speed
[RPM]
(or
27-32 Stage On Speed [Hz]
) and stays there for the
time chosen in
27-24 Destaging Delay
, a variable speed
pump will be staged off.
If the system pressure is too high with only one frequency
converter running at minimum speed, it will de-stage fixed
speed pumps. A fixed speed pump will be de-staged when
the pressure goes above the setpoint in
27-20 Normal
Operating Range
and stays there for the time chosen in
27-24 Destaging Delay
. If the system demand continues to
drop the system will enter sleep mode.
4.2.8 Override Staging/De-staging
Normal staging and de-staging handles most of the
situations in typical applications but the Cascade Controller
in stage and de-stage pumps immediately in response to
large changes in system demand.
Staging
When the system pressure drops below the override limit
(
27-21 Override Limit
), the Cascade Controller will
immediately stage on a pump to meet the demand for
more flow.
If the system pressure continues to stay below
27-21 Override Limit
for
27-25 Override Hold Time
, the
Cascade Controller will then stage on the next pump. This
repeats until all pumps are on or until the system pressure
drops below the Override Limit.
De-staging
When the system pressure exceeds
27-21 Override Limit
,
the Cascade Controller immediately de-stages a pump to
reduce the pressure.
If the system pressure continues to stay above
27-21 Override Limit
for
27-25 Override Hold Time
, the
Cascade Controller de-stages another pump. This will
repeat until only the lead pump is left on or until the
pressure stabilizes.
27-21 Override Limit
is set as a % of the Maximum
Reference. It defines a point above and below the system
Setpoint where Override staging and de-staging will occur.
4.2.9 Minimum Speed De-staging
To reduce emergency usage the Cascade Controller will
de-stage a pump if the lead pump is running at minimum
speed for
27-27 Min Speed Destage Delay
.
4.2.10 Fixed Speed only Operation
Fixed speed only operation keeps critical systems
operating even if all variable speed pumps are unavailable
to the Cascade Controller. In this situation, the Cascade
Controller maintains 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
27-22 Fixed Speed Only
Operating Range
for the Staging Delay time (
27-23 Staging
Delay
), a fixed speed pump will be turned on.
Configuring the System
VLT
®
AQUA Drive MCO 101/MCO 102
20
MI38C402 - VLT
®
is a registered Danfoss trademark
4
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