Electrical · Updated 4 October 2026
Motor voltage and current imbalance: measurement checks
Compare phase readings without confusing magnitude imbalance with a full power-quality assessment.
Measure a comparable operating condition
Take the three line-to-line RMS voltage readings and three line-current readings with the motor at a steady load. Label the readings clearly so a phase-to-neutral voltage cannot be mixed into the same set. Measurements should be made by suitably qualified personnel using equipment and procedures appropriate for the installation.
Calculate the average and maximum deviation
For 400, 410 and 390 V, the average is 400 V. Absolute deviations are 0, 10 and 10 V. The maximum deviation divided by the average, multiplied by 100, is 2.5%. For currents of 10, 12 and 8 A, the average is 10 A and the magnitude imbalance is 20%.
Interpret the two results separately
Voltage and current percentages describe separate sets of readings. One should not be substituted for the other or assumed to determine a universal trip threshold. This magnitude calculation does not use phase-angle information and is not the IEC negative-sequence voltage unbalance factor. Check the motor and drive documentation for the applicable limits and measurement method.
Keep a maintenance record
Record load condition, measurement time, supply arrangement, instrument identifier and the actual readings. Check connections and operating conditions through approved procedures rather than assuming the calculator identifies the root cause. Repeat measurements after an intervention under comparable load. A calculated improvement is useful evidence only when the before and after measurements are comparable.