Download Danfoss VLT 6000 Instruction manual

Cascade Controller Option
■ Master/Slave Cascade Control Mode
Master/slave systems control multiple pumps in
parallel at the same output frequency. Pumps are
staged on and off as required to meet the system’s
demand. The master/slave control mode of operation
provides maximum system efficiency.
In master/slave control, each motor has its own
adjustable frequency drive which responds to control
from a master drive which contains the cascade
option card. The master drive sends a pulse speed
reference signal to the drives under control to ensure
that all operate at the same speed. The motors
must be of equal size. In some applications, it may
be advisable for a second drive with a cascade
card to act as an auxiliary controller.
Sequential staging of drives on or off is provided by
the master drive in response to system feedback,
maintaining precise setpoint control. Pressure surge
and water hammer are eliminated. Because all
of the operating pumps run at the same speed,
the possibility of running a pump continuously into
a closed check valve is all but eliminated. This
reduces pump maintenance by minimizing the
wear on pump seals and bearings.
Danfoss has available the Multiple Unit Staging
Efficiency Calculator (MUSEC), a free software program
available on the Danfoss website. By entering pump
and system data, MUSEC provides the programmer
with master drive on and off staging frequencies at
optimal efficiency for each pump: for example, three
motors providing flow at reduced speed rather than
two at full speed. This often results in 10% to 15%
additional savings over similar control systems. For
a free download, go to www.danfoss.com/drives
and go to software download.
Best efficiency is attained when a pressure transmitter
is placed at the farthest significant load in the system.
If this is not practical and the pressure transmitter
is placed close to the discharge of the pumps, or
when pump data and system characteristics are
not accessible, alternative ways of programming
the cascade controller are provided.
Master/Slave Cascade Control Mode
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MG.60.I5.22 - VLT is a registered Danfoss trademark