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Can anyone suggest a best size of variable speed drive for a pump?

A question from Layali:

we have 6 water wells used for irrigation, we need 4 vertical turbine pumps 300 hp and 2 vertical turbine pumps 100 hp, these pumps will tie on PV system so we need variable speed drive (VSD). I need your help to choose the best size of VSD for the above mentioned pump , are there pumps applicable to install VSD on it ? the last question if the pumps have been fed from PV system , is the pressure change or drop while the radiation was been little? please help me to understand the pressure affect by solar system ?

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Dear Layali Although you list the sizes of the existing pumps you do not give the capacity of the solar array which I presume is enough to drive these pumps and also the variation over the day although that is usually a half sine wave form.

Are the pumps deep borehole pumps with a submerged motor or vertical line shaft pumps with the motor above ground? These have somewhat different characteristics for VSD operation. The submerged type tend to have a minimum speed requirement often 30% of the fixed speed. At least with the above ground motor type it is usual to derate the motor and select a larger frame size when specifying for variable speed so that the motor cooling is adequate at a lower speed. if the pumps are existing then the pump rating may have to be reduced.

for both types at these ratings the variable speed drive will be a drive unit contained in a panel, either separate or joined into a motor control centre depending on the nature of your site. these have some specific safety considerations that you should be aware of in that they can have parts at high voltage for some time after disconnection and so it is necessary to wait after disconnection for the capacitors to discharge before opening.

What the variable speed drive will not do is to control the pump speed to match the power available. To do that will require a controller such as a programmable logic controller (PLC)to monitor the balance of supply and demand and provide the start/stop and pump speed required signals to the VSDs.

One point, is it necessary to have all pumps operating on variable speed? There may be savings to be made by having some pumps operating on fixed speed and others on VSD.

The change in output over the speed range is a bit hard to say without the pump information and system information. I would expect that the system is predominantly the static lift plus the spray nozzles and so relatively fixed.

Sorry that this can't be more helpful.

Steve Oxtoby

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Dear Layali

Here is a response to your enquiry from John Cody, another of our RedR Water and Sanitation experts:

"There is insufficient information to provide a specific answer to your question; To select an appropriate pump you will have to match pump characteristics to the distribution network characteristics (static heads and friction losses at a design flow rate), selecting a pump that will operate near its design point in the network. If you select VSD pumps you will add significant complexity, capital and O&M costs to your system, for which there is little justification in your query. If you are looking at VSD to compensate for fluctuations in the PV supply then I think it would likely to be cheaper and operationally more efficient to install a reservoir in the system to allow your delivery network to be designed as a gravity or constant head pumped system, using pumps to provide the required heads by pumping to an elevated reservoir. The elevated reservoir will allow you to compensate for fluctuations in the PV supply without the need for pump controllers, and simple valves can be used to control the distribution network.

"The PV supply will be a function of elevation of the sun, orientation, cloud cover and maintenance of the PV panels. It would be standard to provide a reservoir within the system to even out diurnal and seasonal fluctuations in the power supply. A water reservoir is almost certainly cheaper over the lifetime of a system than battery storage for example. A reservoir would also allow for irrigation at night, which may significantly improve water use efficiency and reduce long term salinisation issues."

Please let us know what you would like to do and how we might be able to help.

Kind regard, Leigh

LC Jones RedR KnowledgePoint Moderator

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