Pump Motor Sizing & Current Calculator

Hydraulic power is the useful energy in the liquid: flow × head × SG ÷ 367. Shaft power divides that by the pump efficiency, which is typically 50–70 % for chemical pumps at duty. The motor then adds its own efficiency, usually 85–92 %, and the next standard IS 1231 frame above the result is selected — the sequence runs 0.37, 0.55, 0.75, 1.1, 1.5, 2.2, 3.7, 5.5, 7.5 kW and so on.

A 25 m³/h pump at 40 m handling 50 % caustic (SG 1.2) needs about 5.03 kW at the shaft and a 7.5 kW motor frame — a frame larger than a water-duty calculation would suggest, because SG scales the power directly.

Full-load current is the nameplate value for the motor frame at rated output — a 7.5 kW motor at 415 V draws roughly 13.6 A full load. Duty current is what the pump actually draws at your operating point, in the example about 10.2 A. The difference matters for overload relay settings: set the relay between the duty and full-load values, never above full load.

Direct-on-line (DOL) is fine up to about 5.5 kW. Above that, star-delta limits the starting current to roughly one-third, protecting the supply and the pump. Above about 22 kW, or where starting current is a concern, a VFD gives soft starting plus speed control. The calculator recommends the starter from the frame size — and the affinity calculator shows what the VFD would save in running cost.

When a drive runs a 50 Hz motor above rated speed, power rises with the cube of the ratio — 60 Hz means roughly 1.73× the power and a larger frame is usually required. Always verify motor frame, coupling and NPSH when changing speed or frequency. Teflow's engineers confirm motor sizing against the duty point before retrofit work — call +91 98251 62709 for a check.