Flow Compensation For Applications With Cascade Controller - Danfoss VLT AQUA Operating Instructions Manual

Configuring the System
De-staging
If all variable speed pumps are unavailable and the system
pressure goes above 27-22 Fixed Speed Only Operating
Range for the De-stage Delay time (27-24 Destaging Delay),
a fixed speed pump will be turned off.
4.2.11 Flow Compensation for Applications
with Cascade Controller
To place the pressure sensor close to the system output is
not always an advantage. The frequency converter controls
the pump speed to produce a constant pressure at the
system outlet. The constant pressure ensures the required
maximum flow. At reduced flow, the energy that went into
producing this excess pressure is wasted.
The Flow Compensation feature adjusts the internal
reference based on the number of pumps in operation. It
uses the feedback to estimate the set-point required at
various rates of flow. As Illustration 4.3 shows, the
calculation of the theoretical curve results in step wise
adjusted set-points following this curve. Set-point 1 is the
minimum pressure required when the system is running
with only one pump operating and under minimum load
conditions. Set-point 2 is used, when all pumps are
running.
Illustration 4.3 Calculation of Theoretical Curve
The range of the curve is determined by the no/low flow
point and the uncompensated set-point (e.g. in
20-21 Setpoint 1).
Besides configuration of the Cascade Controller, the
required programming steps are:
•
Enable Flow Compensation in 22-80 Flow
Compensation
•
Program the Pressure at No-Flow Speed in
22-87 Pressure at No-Flow Speed
•
Program the uncompensated setpoint (e.g. in
20-21 Setpoint 1)
®
VLT AQUA Drive MCO 101/MCO 102
If the pressure feedback signal originates at the far end of
the system, the frequency converter does not need to
compensate for system pressure changes due to flow and
the default feedback process can be used.
®
MI38C402 - VLT is a registered Danfoss trademark
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