A site record says average voltage is 400 volts, so the supply is normal. Yet one phase draws more current and the three-phase pump motor runs hot. An average can conceal differences among the three line-to-line voltages. Competent personnel should record the complete supply and current set under the same load, rather than relying on one apparently reassuring number.
A good average can hide unequal values
Consider illustrative line voltages of 390, 400 and 410 volts. Their average is exactly 400 volts. Using maximum deviation from the average divided by that average gives 2.5 percent voltage unbalance.
This is an example of a calculation method, not a universal acceptable limit. It does not mean every motor phase experiences equivalent conditions.
Other unbalance measures, including sequence-component methods, also exist. Values from different methods should not be compared without identification. Retain the three original line voltages, method, supply frequency and test condition rather than reporting only one percentage.
Unequal voltage can produce disproportionate effects
Unbalanced supply can create unfavourable magnetic-field and current conditions, increasing losses and heating and affecting torque. Current unbalance can be more pronounced than the voltage difference, but a fixed multiplier cannot predict every motor and duty.
One high phase current does not prove that the external supply is the cause. Winding condition, connection resistance and measurement error can also matter. A normal average current can likewise conceal an abnormal individual phase.
Connect supply-side and equipment-side observations with load and temperature. A photograph of one clamp-meter reading cannot independently judge motor quality or identify the fault path.
Start with what changed
Recent work on distribution equipment, contactors, connectors or cables can provide a useful starting point. A loose or deteriorated connection, or a change in other supply loads, can affect the actual conditions reaching the motor.
A normal control-circuit indicator does not establish that every main-power contact is sound. Starting and steady operation may also expose different problems, so preserve the relevant timing rather than relying only on one settled reading.
Live measurements require competent personnel, suitable instruments and the appropriate safety procedure. Opening a panel, measuring phases, changing connections or tightening contacts is not a general end-user troubleshooting task.
Before dismantling, isolate relevant energy sources, prevent automatic restart and establish the required safe condition. Do not bypass phase-loss or overload protection to keep the pump operating.
Restore conditions before judging the pump
Where the supply is outside the equipment's permitted conditions, correct the supply or connection problem. Raising a protective setting to maintain flow is not an acceptable substitute.
Whether derating or shutdown is necessary depends on the actual motor and drive requirements. There is no general small percentage that can be declared harmless for every submersible pump.
After correction, repeat line voltages, phase currents, real input power and temperature under comparable water level, pipework and valve conditions. If abnormalities remain with suitable supply, continue to winding, mechanical-load and cooling checks. This order helps separate an installation problem from a hydraulic or motor-design issue.
Use appropriate methods with a drive
For a variable-frequency-drive system, distinguish grid input from motor output. The output waveform is not a simple sine wave, and an ordinary instrument may not be suitable for every parameter.
Use the drive, motor and test procedure's relevant methods and limits. A rule intended for direct mains supply should not automatically be transferred to every drive-output measurement.
An intermittent connection may require time-resolved recording by competent personnel. Relate pump voltage and current events to other large loads starting. The recorder and sampling method must suit the event being investigated. A value averaged over several minutes cannot establish that a brief disturbance lasting a fraction of a second never occurred.
Questions about three-phase balance
Does correct phase sequence prove balanced voltage?
No. Rotation sequence and voltage-magnitude balance are separate conditions.
Can two readings describe the complete unbalance?
Retain all three line-to-line voltages and the corresponding phase currents. A missing reading can conceal the important difference.
Can unbalance be ignored if the casing feels cool?
Assess the equipment's permitted conditions. A short-term external temperature observation does not establish acceptable long-term motor operation.