In which case, the use of the Variable speed drive is cost-effective?

The totoal life cycly costs is largely affected by energy costs and maintenance cost of system requiring the Variable speed drive (VSD). If the system is such that it does not require constant flow, it is very possilble that using VSD will save more energy to prove cost effective and provide most efficient flow. 

If the flow rate does vary the VSD or VFD will prove better lifetime cost saving and better payback than regular measures like (1) resizing the pump, (2) replacing the impeller and diffuser elements or (3) reducing impeller diameters, or even a (4) new pump.

Simple payback for total investement with VSD is usually shorter than without VSD. 

Thank you for the invitation

The use of the VFD depend on the application as you use it in the applications that need to be variable . So it should be a demand in the field,

To be cost effective you have to select the right rating of the device that matching the load power   and use the device that is suitable to your load application ex. Pump, fan , conveyors