Operation Modes; Cascade Control Operation - Danfoss Cascade Operating Instructions Manual

Operation modes

Cascade Control Operation

With the cascade controller option card, the frequency
converter 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 frequency converter 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.
Feedback Signals
An important advantage to the cascade controller
option is that it is based upon the VLTs advanced
PID controller. This means that programming is
done in selected units of measurement appropriate
to the application and that the feedback and
setpoints can be displayed. Unlike drives that base
staging upon frequency, feedback allows precise
control in response to actual system demand. The
VLTs PID controller accommodates two feedback
signals and two setpoints which allows regulating
a system with different setpoint zones.
6
Cascade Controller Option
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 VLT 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. 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.I3.02 - VLT is a registered Danfoss trademark