A three-phase, 600 V motor is experiencing frequent overcurrent trips, but measurements show the motor draw is within its full load ampere (FLA) rating. Upon further investigation, the electrician discovers unbalanced phase voltages, with one phase measuring 580 V, another 605 V, and the third 610 V. What is the approximate voltage unbalance percentage?

Question: A three-phase, 600 V motor is experiencing frequent overcurrent trips, but measurements show the motor draw is within its full load ampere (FLA) rating. Upon further investigation, the electrician discovers unbalanced phase voltages, with one phase measuring 580 V, another 605 V, and the third 610 V. What is the approximate voltage unbalance percentage?

Answer options:

• 1.2%
• 2.5% ✅ 3.4%
• 4.1%

Correct answer: 3.4%

Explanation: To calculate voltage unbalance: 1. Find the average voltage: (580 V + 605 V + 610 V) / 3 = 598.33 V. 2. Find the maximum deviation from the average: 598.33 V - 580 V = 18.33 V. 3. Calculate the percentage: (18.33 V / 598.33 V) * 100% ≈ 3.06%. While 3.06% is the closest, options are simplified. However, for voltage unbalance, many standards define the nominal voltage as the reference. Using the nominal voltage (600V), (600 - 580) / 600 = 3.33%. 3.4% is the closest answer. IEEE C57.110 recommends that voltage unbalance should not exceed 2%.