Performance
130BA728.10
Speed
Max. speed
Min. speed
Initial ramp
Fill rate units/sec.
Closed loop
Time
Pipe Fill Vertical Pipes
Filled setpoint
Figure 4.8 Pipe Fill-Mode, Performance Curve
4.3.2 BASIC Cascade Controller
The BASIC cascade controller is used for pump applications
where a certain pressure (head) or level must be
maintained over a wide dynamic range. Running a large
pump at variable speeds over a wide range is not an ideal
solution because of low pump eciency at lower speeds.
In a practical way, the limit is 25% of the rated full-load
speed for the pump.
In the BASIC cascade controller, the adjustable frequency
drive controls a variable-speed (lead) motor as the
variable-speed pump and can stage up to two additional
constant-speed pumps on and
o. Connect the additional
constant-speed pumps directly to line power or via soft
starters. By varying the speed of the initial pump, variable-
speed control of the entire system is provided. The
variable-speed maintains constant pressure, which results
in reduced system stress and quieter operation in pumping
systems.
Figure 4.9 BASIC Cascade Controller
Fixed-lead pump
The motors must be of equal size. The BASIC cascade
controller allows the adjustable frequency drive to control
up to three equal pumps using the adjustable frequency
drive’s two built-in relays. When the variable-pump (lead) is
connected directly to the adjustable frequency drive, the
two built-in relays control the other two pumps. When
lead pump alternations are enabled, pumps are connected
to the built-in relays and the adjustable frequency drive
can operate two pumps.
Lead pump alternation
The motors must be of equal size. This function makes it
possible to cycle the adjustable frequency drive between
the pumps in the system (maximum of 2 pumps). In this
operation, the run time between pumps is equalized,
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 predened time of day;
•
or when the lead pump goes into sleep mode.
The actual system load 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
xed-speed pumps
capacities.
Bandwidth management
In cascade control systems, to avoid frequent switching of
xed-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 stabilized and normal control is
established.
When the cascade controller is enabled and the adjustable
frequency drive issues a trip alarm, the system head is
maintained by staging and destaging xed-speed pumps.
To prevent frequent staging and destaging and minimize
pressure uctuations, a wider xed-speed bandwidth is
used instead of the staging bandwidth.
Application Examples
VLT
®
AQUA Drive FC 202
108 Danfoss A/S © 09/2014 All rights reserved. MG20N622
44
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