Master/Follower Application Note - Danfoss VLT Series Operating Instructions Manual

1 Extended and Advanced Cascade Controller
Option

1.10 Master/Follower Application Note

1
Application Description
The system used as example contains 4 equal sized pumps in a water distribution system. They are each connected to a Danfoss VLT
pressure transmitter with an analogue output format of 4-20mA is used as feedback and connected to the drive named the 'master drive'. The master
drive also includes the Danfoss
system.
Arguments for using a 'master / follower' setup instead of the standard cascade control mode could be:
• In old and weak pipe system where huge pressure surges can lead to leakage, the high performance of the master / follower mode can be a
real benefit.
• In constant pressure water systems the pumps can be operated in the most energy efficient way by using Master / follower operation.
• In systems with large variances in flow, the fast reacting Master / Follower mode will safely and fast maintain a constant pressure.
• Very easy installation - no need for external equipment. The drives can be delivered in IP55 or even IP66, which means no need for panels,
except for fuses.
Issues to keep in mind
Compared to traditional cascade control the number of running pumps is controlled by speed instead of feedback. To obtain the highest energy saving
the stage on and off speed must be set correctly according to the system. To understand the principle better, please note figure 1.
The stage on and off speed is set by the user for each stage. The right
speed depends on the application and the system. In VLT
ware version higher than 1.1, the speed will be auto-tuned by the drive.
The right settings can also be determined by using the Danfoss PC soft-
ware called MUSEC, which is downloadable from our homepage:
www.danfoss.com
For a start the settings showed in table 1.1 can be used in most applica-
tions.
Stages
Stage 1
Stage 2
Stage 3
Stage 4
Table 1.2: Example of stage on and stage off speed
38
®
VLT Extended Cascade Controller Option MCO-101
®
AQUA soft-
Stage on speed [Hz]
(Par. 27-32)
40
44
46
48
MI.38.C3.02 - VLT
. The purpose of the system is to maintain a constant pressure in the
Illustration 1.15: Total power consumption.
Stage off speed [Hz]
(Par. 27-34)
Min. speed
38
40
42
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is a registered Danfoss trademark
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VLT Operating Instructions
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AQUA Drive. A