During a typical test, one person opens the valve fully and calls it the heaviest motor load. Another throttles the valve, sees lower current and assumes every pump behaves that way. Determine maximum loading from the particular hydraulic design and its power curve. A trend common to some radial-flow pumps cannot be applied to all mixed-flow, axial-flow or specialised designs.
Read power alongside flow and head
The head-flow curve describes hydraulic delivery. The power curve describes the required input at different operating points. First establish whether the engineering curve refers to mechanical power at the pump shaft or electrical input to the complete set.
Wire input includes motor and possibly other losses. It is not interchangeable with shaft power. Nor can motor loading be inferred from flow multiplied by head alone, because efficiency changes along the curve.
At shutoff, useful external hydraulic output approaches zero, but internal friction, recirculation and heating can remain. Zero delivery therefore does not mean zero electrical demand or unlimited safe operation.
Different designs put the maximum in different places
For many low-specific-speed radial designs, power is relatively low at small flow and rises as flow increases. Some higher-specific-speed designs can demand more power toward the low-flow end. Other designs limit power with a curve that peaks and then levels or falls.
The approved curve for the actual version is the relevant evidence. The label submersible pump does not identify the most heavily loaded point.
Consider illustrative electrical inputs of 500, 620 and 590 watts at three increasing-flow points. The middle point is the highest of those three observations. The rightmost point cannot be called the maximum merely because its flow is largest. These figures are not product data, and an unmeasured peak may still exist between the selected points.
Maximum power and worst temperature need not coincide
Higher electrical input can increase the heat burden, but temperature also depends on water temperature, immersion, cooling flow and internal loss distribution. A lower-input low-flow point may still be unsuitable for extended operation.
After identifying the power peak, assess thermal behaviour over the permitted range according to the design. A normal current reading does not independently establish safe cooling or mechanical operation.
Starting is another condition. A steady power curve does not prove adequate starting torque or supply capacity. A jam, locked rotor or loss of water is not a normal curve point that may be sustained indefinitely. Protective operation does not replace the approved operating limits, and protection must not be bypassed to complete a test.
Verify the range the installation can reach
Request flow-head and power information for the same speed, frequency, impeller and motor revision. Ask how shaft and electrical power were established. Test the intended operating region and use engineering assessment to identify the important loading and thermal conditions.
Changing impeller geometry requires its own assessment; a power curve from the earlier hydraulic version should not automatically accompany the revised pump.
Installation changes also move duty. A larger hose, lower discharge or opened bypass can increase flow. Restriction, valve changes or blockage can move operation toward lower flow. A sample that was satisfactory at one middle point has not thereby demonstrated acceptable behaviour at both ends.
Controlled electrical and performance testing should be carried out by suitable personnel. End users should not open energised equipment to investigate a loading concern.
Make the acceptance statement specific
A useful conclusion names the tested range, observed power behaviour, thermal conditions and remaining limits. “Passed with the valve open” is too vague unless that condition has been shown to represent the relevant design requirement. The aim is to establish adequate motor and cooling margin across the real task.
For a speed-controlled version, identify the speed represented by each curve. Reduced speed changes flow, head and power together; lower input is not automatically an equal-task energy saving. Assess drive, motor cooling and mechanical limits across the permitted speed-duty combinations. A power peak on one speed curve need not cover the complete variable-speed operating envelope.
Questions about pump loading
Is a fully open valve always the highest electrical demand?
No. Use the particular power curve and installed system conditions.
Can a pump run indefinitely below nameplate current?
Not on that evidence alone. Cooling, liquid suitability, mechanical limits and the permitted flow range still apply.
Can a shaft-power curve be used directly for electricity cost?
No. Include motor and other relevant losses, with the measurement boundary clearly identified.