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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