Variable Speed Drive for Krebs Pump

It depends upon the electricity tariff in use. If a peak-demand tariff is in use then the VSD will pay for itself in weeks as a result of softer motor-starting and the lower peak demand that results.

If a per-kWh tariff is in use, the VSD might be programmed so as to reduce the terminal voltage when the pumps are running below maximum demand.

You mention that your (unspecified) pumpage is a presumably crushed coal/water slurry, specific gr 1.73, at a rate of 730m^3/hr. You do not mention if your operation requires a constant or veriable Q (pump output) If you require a variable pump output, then the installation of a VSD is essential and the savings (if any) are secondary. If you do not require a variable Q, then why speak of a VSD in the first place. To use a VSD just to obtain a soft start is neither practical nor cost effective. There are other technologies specially designed for this purpose, more efficient and less costly.