Danfoss VLT Series Design Manual page 116

6. Application Examples
6.1.8. BASIC Cascade Controller
6
The BASIC Cascade Controller is used for
pump applications where a certain pressure
("head") or level needs to be maintained over
a wide dynamic range. Running a large pump
at variable speed over a wide for range is not
an ideal solution because of low pump effi-
ciency and because there is a practical limit of
about 25% rated full load speed for running a
pump.
In the BASIC Cascade Controller the frequen-
cy converter controls a variable speed motor
as the variable speed pump (lead) and can
stage up to two additional constant speed
pumps on and off. By varying the speed of the
initial pump, variable speed control of the en-
tire system is provided. This maintains con-
stant pressure while eliminating pressure
surges, resulting in reduced system stress and
quieter operation in pumping systems.
Fixed Lead Pump
The motors must be of equal size. The BASIC
Cascade Controller allows the frequency con-
verter to control up to 3 equal size pumps
using the drives two built-in relays. When the
variable pump (lead) is connected directly to
the drive, the other 2 pumps are controlled by
the two built-in relays. When lead pump al-
ternations is enabled, pumps are connected to
the built-in relays and the drive is capable of
operating 2 pumps.
Lead Pump Alternation
116
The motors must be of equal size. This func-
tion makes it possible to cycle the drive be-
tween the pumps in the system (maximum of
2 pumps). In this operation the run time be-
tween pumps is equalized reducing the re-
quired pump maintenance and increasing
reliability and lifetime of the system. The al-
ternation of the lead pump can take place at
a command signal or at staging (adding an-
other 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. Se-
lections 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 Band-
width overrides the Staging Bandwidth to pre-
vent immediate response to a short duration
pressure change. An Override Bandwidth Tim-
er 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 drive issues a trip
alarm, the system head is maintained by stag-
ing and destaging fixed speed pumps. To pre-
vent frequent staging and destaging and
minimize pressure fluxuations, a wider Fixed
Speed Bandwidth is used instead of the Stag-
ing bandwidth.
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MG.11.B2.02 - VLT is a registered Danfoss trademark
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VLT HVAC Drive Design Guide