VFD is far more expensive than soft starter. However, like in everything else, COST-to- BENEFIT applies to here also. From a technical standpoint, VFDs are useful when the component of FRICTION head is predominant in the TOTAL HEAD. If your conditions dictate the need for operation across a wide range of your system head curve, which will indicate that varying pump speeds is required and the selection of a VFD will show a benefit in cost. VFD can give precise control of flow variation as compared to a control valve at discharge of the pump. Eventually having less affect on pumps health in long run & results in cost saving
The second aspect is cost-economics. If the pump is fairly small (less than 50 hp), the cost to benefit may not be attractive to go for a VFD application.
Soft starters can do a bit more as, many are programmable, and the time to reach full speed or slow down on shutdown may show some benefits in protecting your piping system. soft starters would reduce the current at the start but they can’t help in varying the speed of pump and a flow control valve would be required at discharge to vary the flow...again depends on how much flow range you are going to cover & how frequent you need change the flows.
VFD is definitely a better option but off course would be expensive.
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