Basic Cascade Controller - Danfoss VLT HVAC Design Manual

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VLT HVAC Drive Design Guide 6 Application Examples

6.1.8 BASIC Cascade Controller

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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 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 frequency 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 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 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 frequency converter, 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 frequency converter is capable of operating 2 pumps.
Lead Pump Alternation
The motors must be of equal size. This function makes it possible to cycle the frequency converter 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.
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