Danfoss VLT AQUA Drive Design Manual page 118

6. Application Examples
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.
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.
6
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 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 maxi-
mum of two pumps are controlled. At an al-
ternation command, the PID stops, the lead
pump ramps to minimum frequency (fmin),
and after a delay, ramps to maximum fre-
quency (fmax). When the speed of the lead
pump reaches the de-staging frequency, 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.
After a time delay, the relay for the fixed speed pump cuts in (staged), and this pump becomes
the new lead pump. The new lead pump ramps up to maximum speed and then down to minimum
speed. When ramping down and reaching the staging frequency, the old lead pump is now cut in
(staged) on the lines as the new fixed speed pump.
If the lead pump has been running at minimum frequency (fmin) for a programmed amount of
time with a fixed speed pump running, the lead pump contributes little to the system. When the
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MG.20.N2.22 - VLT ® is a registered Danfoss trademark.
VLT ® AQUA Drive Design Guide