“The test bench uses a default motor efficiency. Why is the shaft-power number so low? Would entering 100 percent measure the real efficiency?” First identify the source of each displayed value. If software calculates shaft power by multiplying electrical input by assumed motor efficiency, that field is a calculation. Editing the assumption does not add an independent measurement.
Separate measurements from calculated fields
A pump bench may measure flow, pressure or total head, electrical input and speed, but equipment configurations differ. A named display field does not prove that a corresponding sensor is installed.
Check the bench manual, channel configuration and equations. Mark each quantity as measured, manually entered or calculated. Real electrical input is also different from voltage multiplied by current, and an electronic drive requires suitable power measurement for its waveform.
Total head needs a correct measurement boundary. These basic quantities must be reliable before wire-to-water efficiency can support comparisons. A display with many decimal places does not establish measurement quality.
One overall efficiency allows different loss allocations
Consider illustrative values, not product data: electrical input is 400 watts and hydraulic output is 120 watts. Wire-to-water efficiency is therefore thirty percent.
Assuming fifty-percent motor efficiency gives a calculated shaft power of 200 watts and a pump hydraulic-stage efficiency of sixty percent. Assuming seventy-five-percent motor efficiency gives 300 watts of calculated shaft power and forty-percent hydraulic-stage efficiency.
Both combinations produce the same thirty-percent overall efficiency. The measured electrical and hydraulic endpoints do not uniquely identify how losses divide between motor and pump. Choosing the assumption that makes the display look plausible does not establish the actual motor efficiency.
This is an information limit, not a criticism of the pump. An additional independent measurement or applicable motor-efficiency information is needed to resolve it.
What entering 100 percent can do
If the software uses the multiplication described above, entering 100 percent makes calculated shaft power equal electrical input. A pump-efficiency field using that value as its denominator may then display the wire-to-water ratio.
That is a mathematical assumption of zero motor loss. It does not mean the real motor achieved perfect efficiency. Nor should the behaviour be assumed for every bench: field definitions, units and correction routines can differ.
Preserve the original settings, export raw measurements and check the equation using a known numerical example. Identify the assumption in reports generated after a setting change. Otherwise, batches calculated with different defaults can appear to have different shaft performance even when the measured endpoints are unchanged.
Add evidence that closes the gap
Suitable torque and speed measurement can establish mechanical shaft output: power is torque multiplied by angular speed. Alternatively, an efficiency-based estimate needs applicable information for the particular motor, load, speed, supply and thermal condition.
A rated efficiency at one point, or an assumed value from another motor, cannot simply cover every operating point. Submersible-motor assessment must also retain appropriate cooling and sealing conditions.
For an engineering estimate, a justified efficiency interval can produce an estimated shaft-power interval. Label its assumptions and uncertainty. Without that independent basis, a precise-looking actual value would be misleading. Purchasing acceptance for rated shaft output should specify its evidence method rather than rely on a bench default.
Make the report transparent
List measured flow, total head and real electrical input separately from hydraulic power and any assumed motor efficiency. Identify sample revision, instruments, measurement location and software settings. Where torque is measured, connect calculated mechanical output to the actual torque and speed channels.
Mechanical and electrical testing belongs with competent personnel using a controlled arrangement. Opening or modifying an energised submersible pump on site is not an appropriate way to obtain a shaft reading.
Separate an output estimate from the purchasing decision. If acceptance concerns delivered flow, head, electrical consumption and temperature, those measured quantities may answer it directly. Do not edit a default simply to populate a shaft-power field. If the decision specifically requires shaft output or motor certification evidence, obtain the relevant independent evidence. Define the decision before judging whether the available bench channels are sufficient.
Questions about calculated shaft power
Can motor efficiency be calculated from overall efficiency alone?
Not uniquely. Independent information about the hydraulic stage or mechanical output is still missing.
Does a lower default make the physical pump worse?
No. It changes results dependent on that assumption, not the pump itself. Check the measured quantities and operating condition separately.
Can rated motor efficiency be used at every duty?
No. Part-load condition, supply, speed and temperature can change efficiency. Confirm that the data covers the intended point.