Application note AN-ODV-2-068
22 October 2014
AN-ODV-2-068 Using Optiflow Multi Pump Cascade Control
3
Assist Pump Start and Stop Speeds – Example Calculation
Below is an example of a pump flow curve, showing the available head and flow at various speeds, and the relative power
consumed. A typical system curve is also shown, representing the required operating range of the pump up to maximum speed,
along with the pump efficiency at various points.
From the curve above, we can see that a single pump operating at maximum speed and the chosen system point produces a
flow level of around 720m
3
/h. This is shown by the
Red line A
. As this point is closest to the optimum pump efficiency, the best
system efficiency will be achieved by operating with only one pump up to the maximum flow of a single pump; hence in this
case, the Assist Pump Start Speed parameter P8-17 should be close to maximum speed, e.g. 1475 Rpm / 49.9Hz.
Assuming we now have two identical pumps in parallel, to achieve the same flow demand and system operating point, each
pump is required to produce 50% of the flow, 360m
3
/h each. This is shown by the
Green line B
. From this point, we can estimate
the pump speed required to produce this flow level, based on the shown Rpm curves. By following the green curve, this point
can be estimated as approximately 1320 Rpm. The Assist Pump Stop Speed parameter, P8-18 should therefore be approximately
1320 Rpm / 44.0Hz in this case. These parameter settings can then be optimised during system test and commissioning.
To avoid rapid cycling of pumps, it is always advisable to set P8-18 slightly below the estimated value, to ensure that the final
Assist Pump only stops when the flow demand drops below the level where a single pump can meet the demand, and repeated
start / stop cycling of assist pumps is avoided.
Using Standby Mode In Multi-Pump Systems
Standby mode can be used to automatically switch off the pumps when there is no flow requirement in the system. If Standby
Mode operation is required
Standby Mode must be enabled on all the drives in the system, by setting P2-27 >0. Refer to the drive User Guide for
more information on P2-27. The same value should be used on all drives on the system for consistent operation.
Standby Mode activates when the output frequency of the drive remains below the threshold value programmed in P3-
14 for the time set in P2-27. For Standby Mode to operate correctly, P3-14 must be less than P8-18.
The PID Error Wake Up Level P3-13 must be set in the Master drive only. This is the % difference allowed between the
chosen system setpoint and the measured feedback level at which the drive should restart the first pump.
Example of Standby Mode Parameter Values Calculation
Referring to the pump curves shown earlier, it can be seen that following the
Blue Line C
, the minimum operating speed for the
pump at which the system operating point can be met can be estimated. In this case, the value would be approximately 1250
Rpm. This value can be used for the Standby Mode Activation Speed, P3-14.
A
B
C
