6.1.8 BASIC Cascade Controller
Figure 6.7
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 range
is not an ideal solution because of low pump efficiency,
and because there is a practical limit of about 25% rated
full load speed for running a pump.
In the BASIC Cascade Controller, the Adjustable frequency
drive 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 entire system is
provided. This maintains constant 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 Adjustable frequency drive to control
up to three equal size pumps using the drives two built-in
relays. When the variable pump (lead) is connected directly
to the Adjustable frequency drive, the other two pumps
are controlled by the two built-in relays. When lead pump
alternations is enabled, pumps are connected to the built-
in relays and the Adjustable frequency drive is capable of
operating 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 two pumps). In this
operation, the run time between pumps is equalized, thus
reducing the required pump maintenance and increasing
reliability and system lifetime. 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. 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. 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 has been established.
When the Cascade Controller is enabled and running
normally and the Adjustable frequency drive issues a trip
alarm, the system head is maintained by staging and
destaging fixed-speed pumps. To prevent frequent staging
and de-staging and minimize pressure fluctuations, a wider
fixed-speed bandwidth is used instead of the staging
bandwidth.
6.1.9 Pump Staging with Lead Pump
Alternation
Figure 6.8
With lead pump alternation enabled, a maximum of two
pumps are controlled. At an alternation command, the
lead pump will ramp to minimum frequency (fmin), and
after a delay, it will ramp to maximum frequency (fmax).
When the speed of the lead pump reaches the de-staging
frequency, the fixed-speed pump will be cut out (de-
staged). The lead pump continues to ramp up and then
ramps down to a stop and the two relays are cut out.
Application Examples
VLT
®
HVAC Drive Design Guide
MG11BB22 - VLT
®
is a registered Danfoss trademark
6-5
6
6
