For a simple voltage-magnitude unbalance estimate, divide the largest absolute deviation from the average line-to-line voltage by that average, then multiply by 100. This is a screening measure, not a substitute for symmetrical-component analysis.
Start with the symptom
The pattern matters. Keep the equipment record and do not make repairs from a symptom alone.
| Observed symptom | Possible fault area | Evidence to collect |
|---|---|---|
| Unequal line-to-line values | Supply imbalance, single-phase loads or poor connection | Three measurements from the same operating interval |
| Motor phase currents vary strongly | Voltage imbalance, unequal winding or connection issues | Current trend, nameplate and supply records |
| Motor runs hotter than normal | Extra losses from imbalance or unrelated cooling/load problems | Load, cooling history and temperature trend |
| Imbalance appears only under load | Voltage drop along unequal paths or changing network load | Incoming and motor-terminal history where available |
What may be happening
How the simple percentage is calculated
Suppose a qualified engineer has recorded 411 V, 415 V and 419 V between the three line pairs during the same operating condition. The average is 415 V. The largest deviation is 4 V. The simple percentage unbalance is 4 / 415 × 100, or approximately 0.96%. Different standards can define unbalance differently, so identify which method is used in a report.
Current imbalance can be much greater than voltage imbalance
Fluke notes that motor current unbalance can be several times the voltage unbalance. This is why a seemingly small change in supply voltage can still matter. The effect depends on the machine and system conditions.
Look for the source of the variation
Uneven single-phase load distribution, transformer connections, damaged contacts, long feeders and changing plant operations are possible contributors. A comparison between historical supply and motor-terminal data can help narrow the investigation. No single reading reveals the full root cause.
Questions that help narrow the fault
These are questions to discuss with the authorized maintenance team. Use existing logs and approved records where possible.
- Are all three values line-to-line and captured together?
- Was the motor operating at the same load for each reading?
- Are there recorded current imbalance or heating symptoms?
- Does the condition change with a specific plant load?
- Which unbalance definition does the power-quality report use?
What to do next
Do not judge balance from one voltage measurement. Record all three values, the operating condition and the measurement method. Specialist power-quality analysis is appropriate when symptoms persist or critical equipment is affected.
Frequently asked questions
Can 1% voltage unbalance matter to a motor?
It can. Current unbalance may be noticeably larger than voltage unbalance, and heating risk depends on load and equipment.
Does this formula calculate negative-sequence voltage unbalance?
No. It is the maximum-deviation magnitude estimate. Negative-sequence calculations require phasor information.
Technical references
Use these for background. Refer to the manual and approved instructions for the actual equipment model.
Record your fault symptoms
Electrical Doctor can collect equipment information and prepare a preliminary fault summary. It does not authorize a repair or switching operation.
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