Danfoss VLT AHF 005 Design Guide

5.4 Basic Cascade Controller
The basic cascade controller is used for pump applications
where a certain pressure (head) or level must 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 eciency at lower speed. In
a practical way, the limit is 25% of the rated full-load
speed for the pump.
In the basic cascade controller, the drive controls a variable
speed (lead) motor as the variable speed pump and can
stage up to 2 more constant speed pumps on and
o.
Connect the additional constant speed pumps directly to
mains or via soft starters. By varying the speed of the
initial pump, variable speed control of the entire system is
provided. The variable speed maintains constant pressure,
which results in reduced system stress, and quieter
operation in pumping systems.
Constant Speed Pumps (2)
Variable Speed
Pumps (1)
Motor starter
Drive with
Cascade Controller
Pressure Sensor
e30ba362.11
Illustration 5.22 Basic Cascade Controller
Fixed lead pump
The motors must be of equal size. The basic cascade
controller allows the drive to control up to 3 equal pumps
using the 2 built-in relays in the drive. When the variable
pump (lead) is connected directly to the drive, the 2 built-
in relays control the other 2 pumps. When lead pump
alternations are enabled, pumps are connected to the
built-in relays and the drive can operate 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 predened time of day.
•
When the lead pump goes into sleep mode.
The actual system load determines staging.
A separate parameter limits alternation only to take place if
total capacity required is >50%. Total pump capacity is
determined as lead pump plus xed speed pumps
capacities.
Bandwidth management
In cascade control systems, to avoid frequent switching of
xed-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 is
established.
When the cascade controller is enabled and the drive
issues a trip alarm, the system head is maintained by
staging and destaging xed-speed pumps. To prevent
frequent staging and destaging and minimize pressure
uctuations, a wider xed speed bandwidth is used instead
of the staging bandwidth.
5.4.1.1 Pump Staging with Lead Pump
Alternation
With lead pump alternation enabled, a maximum of 2
pumps are controlled. At an alternation command, the PID
stops, the lead pump ramps to minimum frequency (f
min
)
and, after a delay, it ramps to maximum frequency (f
max
).
When the speed of the lead pump reaches the destaging
frequency, the xed-speed pump is cut out (destaged). The
lead pump continues to ramp up and then ramps down to
a stop and the 2 relays are cut out.
Time
Time
Mains operation
Destaging freq.
Alternation
command/PID stops
Staging freq.
Mains operation
PID contr.
starts
130BA364.10
5s
f
max
f
max
f
min
Illustration 5.23 Lead Pump Alternation
After a time delay, the relay for the
xed-speed pump cuts
in (staged) and this pump becomes the new lead pump.
The new lead pump ramps up to maximum speed and
then down to minimum speed. When ramping down and
reaching the staging frequency, the old lead pump is now
cut in (staged) on the mains as the new xed-speed pump.
Product Features
VLT
®
HVAC Drive FC 102
38 Danfoss A/S © 11/2017 All rights reserved. MG16C302
55

Danfoss VLT AHF 005 Specifications

General

Brand	Danfoss
Model	VLT AHF 005
Category	DC Drives
Language	English