Danfoss VLT Series Design Manual page 130

6. Application Examples
pump (lead) is connected directly to the drive, the other 2 pumps are controlled by the two built-
in relays. When lead pump alternations are enabled, pumps are connected to the built-in relays
and the drive is capable of operating 2 pumps.
Lead Pump Alternation
The motors must be of equal size. This function makes it possible to cycle the drive between the
pumps in the system (maximum of 2 pumps). In this operation, the run time between pumps is
equalized, thus reducing the required pump maintenance and increasing reliability and system
lifetime. The alternation of the lead pump can take place at a command signal or at staging (adding
another pump).
The command can be a manual alternation or an alternation event signal. If the alternation event
is selected, the lead pump alternation takes place every time the event occurs. Selections include:
whenever an alternation timer expires, at a predefined time of day, or when the lead pump enters
sleep mode. Staging is determined by the actual system load.
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A separate parameter limit's alternation is only to take place if the total capacity required is >
50%. Total pump capacity is determined as the sum of the capacities of the lead pump and the
fixed speed pumps.
Bandwidth Management
To avoid frequent switching of fixed speed pumps in cascade control systems, the desired system
pressure is kept within a bandwidth rather than at a constant level. Staging bandwidth provides
the required bandwidth for operation. When a large and quick change in system pressure occurs,
the override bandwidth overrides the staging bandwidth to prevent immediate response to a short
duration pressure change. An override bandwidth timer can be programmed to prevent staging
until the system pressure has stabilized and normal control has been established.
When the cascade controller is enabled and running normally and the drive issues a trip alarm,
the system head is maintained by staging and de-staging fixed speed pumps. To prevent frequent
staging and de-staging and minimize pressure fluctuations, a wider fixed speed bandwidth is used
instead of the staging bandwidth.
6.1.9. Pump Staging with Lead Pump Alternation
With lead pump alternation enabled, a maximum of two pumps are controlled. At an alternation
command, the lead pump will ramp to minimum frequency (fmin), and after a delay, it will ramp
to maximum frequency (fmax). When the speed of the lead pump reaches the de-staging fre-
quency, the fixed speed pump will be cut out (de-staged). The lead pump continues to ramp up,
then ramps down to a stop, and the two relays are cut out.
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MG.11.B4.22 - VLT ® is a registered Danfoss trademark.
VLT ® HVAC Drive Design Guide