Fixed Lead Pump
The motors must be of equal size. The BASIC Cascade Controller allows the frequency converter 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 alternations is enabled, pumps are connected to the built-in relays and the drive is capable of operating 2 pumps.
Lead Pump Alternation
The motors must be of equal size. This function makes it possible to cycle the 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 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 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 the drive 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.
6.1.8 Pump Staging with Lead Pump Alternation
With lead pump alternation enabled, a maximum of two pumps are con-
trolled. At an alternation command, the PID stops, the lead pump ramps
to minimum frequency (f
min
) and after a delay, it ramps to maximum fre-
quency (f
max
). 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.
VLT
®
AQUA Drive Design Guide
6 Application Examples
MG.20.N5.02 - VLT
®
is a registered Danfoss trademark
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