Cascade Controller Option
■ Operation modes
■ Cascade Control Operation
With the cascade controller option card, the AFD
(Adjustable Frequency Drive) can automatically control
up to five motors. Staging motors on or off is done
cyclically, in accordance with operating hours. This
function assures equal use over time and eliminates
concern about starting a seldom used motor. The
cascade controller includes four Form C relays having
contacts rated 250 V, 2 A (non-inductive) that are
used to control the motor contactors. The controller
option card installs in the AFD control card cassette
and can be ordered factory installed. The cascade
controller is effective in applications where multiple
motors are used to control a common flow, level or
pressure involving pumps, fans, and blowers.
F
eedback Signals
An important advantage to the cascade controller
option is that it is based upon the AFDs advanced
PID controller. This means that programming is
done in selected units of measurement appropriat
e
to the application and that the feedback and
setpoints can be displayed. Unlike drives that base
staging upon frequency, feedback allows prec
ise
control in response to actual system demand. The
AFDs PID controller accommodates two feedback
signals and two setpoints which allow
s regulating
a system with different setpoint zones.
In pumping applications, when a pressure feedback
signal is not practical, the feedback signal may be
taken near the pump by measuring flow. When the
flow rate is low, the pressure needed is low. At
increased flow, the pumps need to provide greater
pressure to compensate for the increased pressure
drop in the piping. The setpoint should be adjusted
to match the flow in these cases. While difficult to
accomplish with standard PID controllers, the cascade
controller provides an easy solution. By programming
one setpoint for minimum flow and the other for
maximum flow, the AFD calculates intermediate
setpoints based on the flow required.
Cascade control operation depends on the general
system design. Two operational modes are available:
1. Standard Cascade Control, with one speed
controlled pump/fan and up to four fixed speed
pumps/fans. Lead pump alternation makes it
possible to average out the use of the pumps. This
is done by cycling the leadpump in the system.
Up to four pumps can be controlled this way.
Programming for this mode is described in Chapter
6, Standard Cascade Control Mode Setup.
2. Master / Slave Cascade Control has all pumps/fans
speed controlled through the Master drive.
Programming is described in Chapter 7,
Master/Slave Cascade Control Mode Setup.
Standard Cascade Control Mode
MG.60.I5.22 - VLT is a registered Danfoss trademark
6
(8 pages)
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