Cascade Controller - Danfoss VLT FC 102 Series Design Manual

Application Examples

7.1.8 Cascade Controller

7 7
Figure 7.7 A Pump Application
The 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 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 Cascade Controller
allows the adjustable frequency drive to control up to five
equal size pumps using the adjustable frequency drives,
two built-in relays and terminal 27, 29 (DI/DO). When the
variable pump (lead) is connected directly to the
adjustable frequency drive, the other four pumps are
controlled by the two built-in relays and terminal 27, 29
(DI/DO). Lead pump alternation cannot be selected when
lead pump is fixed.
140
Design Guide
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 (when 25-57 Relays per Pump =1,
maximum pump is 4. When 25-57 Relays per Pump =2,
maximum pump is 3). 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 lag 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, when the lead pump goes into sleep mode.
Staging is determined by the actual system load.
25-55 Alternate if Load <= 50%= 1, if load >50% alternation
does not happen. If load <=50% Alternation happens.
When 25-55 Alternate if Load <= 50% = 0, Alternation
happens no matter with Load. Total pump capacity is
determined as lead pump plus lag 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 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 destaging and minimize pressure fluctuations, a wider
fixed-speed bandwidth is used instead of the staging
bandwidth.
Danfoss A/S © Rev. 06/2014 All rights reserved. MG11BC22