Calculation reviewed: 4 October 2026 · Engineering Desk Tools
How to use the Motor voltage & current imbalance calculator
Compare three-phase readings and identify the largest deviation from the average. Start with measurements or documented values for the equipment and reporting period you actually want to assess. The calculator does not fill in a typical installation for you. Its blank fields help keep a demonstration value from being mistaken for your own operating data.
Choose the required options, enter the values in the labelled units and select Calculate. If a field is rejected, check its range and unit before changing it. The result is accompanied by the equation and assumptions so you can decide whether it answers your engineering question.
Inputs and measurement basis
- Voltage L1–L2 (V). Line-to-line voltage measured at the motor terminals under steady load. The value must be greater than zero.
- Voltage L2–L3 (V). Line-to-line voltage measured at the same time and load. The value must be greater than zero.
- Voltage L3–L1 (V). Line-to-line voltage measured at the same time and load. The value must be greater than zero.
- Current L1 (A). Line current measured at the same time as the voltages.
- Current L2 (A). Line current measured at the same time as the voltages.
- Current L3 (A). Line current measured at the same time as the voltages.
Record the measurement source and operating condition. Retain unrounded values for verification.
Method and interpretation
The calculation compares magnitudes against their average. Measure the three line-to-line voltages and the three line currents under comparable steady operating conditions. The maximum deviation is taken as an absolute value so both an unusually high and an unusually low reading can determine the percentage.
Imbalance (%) = 100 × maximum absolute deviation from the three-reading average / average.
Enter line-to-line RMS voltages and line currents measured under the same steady load. This magnitude-based calculation is not the IEC negative-sequence voltage unbalance factor. Compare results with the motor or drive manufacturer limits; it does not diagnose the fault or set a trip threshold. Current average must be greater than zero.
Worked example
This example does not populate the form. Enter your own installation data.
Calculated result: 2.5 % voltage imbalance. Largest voltage deviation: 10 V. Largest current deviation: 2 A. Review connections, supply balance and load conditions against equipment guidance.
Check the arithmetic independently using the displayed equation. Retain intermediate precision and compare the final value with your equipment documentation.
Quick-reference table
These illustrative rows retain the worked example assumptions except for the varied input. They are not certified ratings or manufacturer data. Use the form for intermediate values.
Common mistakes
Mixing line-to-line and phase-to-neutral voltages; collecting readings under changing load; confusing this magnitude method with a phase-angle or negative-sequence assessment. Before relying on the output, check the decimal format, unit scale and source of each value. Recheck unusual results against the formula.
Frequently asked questions
Are the example values used automatically?
No. Inputs remain blank when the tool opens or when you clear them. The worked example and table are reading material below the calculator. Enter the actual values you want to check each time, and label saved reports so their context is not lost.
What does the result mean?
The results are the percentage by which the largest voltage and current readings deviate from their averages. Compare them with the motor and drive limits, and have a qualified person investigate a high current imbalance.
Can I use a zero or a negative value?
The allowed range follows the meaning of each input. A divisor needs a positive value; other fields may allow zero. Check the form labels and error message rather than forcing an invalid value. A rejected input must be corrected before a result can be shown.
Is this a final design or equipment approval?
No. Enter line-to-line RMS voltages and line currents measured under the same steady load. This magnitude-based calculation is not the IEC negative-sequence voltage unbalance factor. Compare results with the motor or drive manufacturer limits; it does not diagnose the fault or set a trip threshold. Current average must be greater than zero. Use the calculation as support for a checked decision, and review equipment documentation and applicable requirements before changing an installation or operating setting.
How can I save or share a calculation?
Calculate first, then use Copy result or Print report. Some maintenance tools also provide a CSV summary. Reports contain your own entered data, so review them before sharing. You can share the public page URL without including plant records or confidential measurements.
How do I report an unexpected result?
Use the Contact page and include the tool URL, input values, units, expected output and the reference you used. Explain the measurement basis. Exclude confidential information.
Related tools and records
Use related calculations with consistent units, periods and equipment scope.
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