Basic Cascade Controller; Pump Staging With Lead Pump Alternation - Danfoss VLT HVAC Drive FC 102 Design Manual

Application Examples

6.1.8 BASIC Cascade Controller

Constant Speed
Pumps (2)
Variable Speed
Pumps (1)
Motor starter
6 6
Illustration 6.7 BASIC Cascade Controller
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 frequency converter
controls a variable speed motor as the variable speed
pump (lead) and can stage up to 2 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.
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 two built-in relays. When the
variable pump (lead) is connected directly to the drive, the
other 2 pumps are controlled by the 2 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
When the motors are of equal size, lead pump alteration
enables the frequency converter 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).
164
Design Guide
Pressure Sensor
Frequency Converter
with
Cascade Controller
Danfoss A/S © Rev. 05/2014 All rights reserved.
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 to take place only
if total capacity required is > 50%. Total pump capacity is
determined as lead pump plus fixed speed pump
capacities.
Bandwidth Management
In cascade control systems, the desired system pressure is
kept within a bandwidth rather than at a constant level to
avoid frequent switching of fixed speed pumps. 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 frequency converter 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.
6.1.9 Pump Staging with Lead Pump
Alternation
Alternation
command/PID stops
f
max
Destaging freq.
f
min
f
max
Staging freq.
Mains operation
Illustration 6.8 Pump Staging with Lead Pump Alternation
With lead pump alternation enabled, a maximum of 2
pumps are controlled. At an alternation command, the
lead pump will ramp to minimum frequency (f
after a delay will ramp to maximum frequency (f
the speed of the lead pump reaches the destaging
frequency, the fixed speed pump is cut out (de-staged).
The lead pump continues to ramp up and then ramps
down to a stop and the 2 relays are cut out.
Mains operation
Time
PID contr.
starts
5s
Time
) and
min
). When
max
MG16C202