A technician is performing a diagnostic on a three-phase motor. Using a clamp-on ammeter, the technician measures the phase currents as follows: Phase A = 40A, Phase B = 38A, Phase C = 42A. What does this indicate about the motor's load condition?
- AThe motor is operating within a reasonable balance.
- BThere is a significant phase imbalance.
- CThe motor is drawing excessive reactive current.
- DThe motor is severely overloaded.
Show answer & explanationAnswer & explanation
Correct answer: A. The motor is operating within a reasonable balance.
To determine phase imbalance, calculate the average current: (40A + 38A + 42A) / 3 = 40A. Then find the maximum deviation from the average: 42A - 40A = 2A. The percentage imbalance is (Maximum Deviation / Average Current) * 100 = (2A / 40A) * 100 = 5%. The NEC generally considers a voltage imbalance of up to 5% acceptable, and current imbalance is typically 6-10 times the voltage imbalance. A current imbalance of 5% is usually considered within reasonable limits for a motor, indicating it is operating with a reasonable balance.
Why the other options are wrong
- B. A significant phase imbalance typically involves much larger differences, often exceeding 10% or more. A 5% current imbalance is generally considered acceptable.
- C. Current imbalance does not directly indicate excessive reactive current. Excessive reactive current would be indicated by a low power factor, which is not provided.
- D. The average current is 40A. Without knowing the motor's full-load current rating, we cannot definitively say it's overloaded, but the question is about balance, not overload itself.
3-Phase Current Imbalance
Current imbalance in a 3-phase system is the variation in current magnitude between the phases. A small imbalance (typically up to 5-10%) is considered reasonable, while larger imbalances can indicate issues.
- Calculated as (Max Deviation from Avg / Avg Current) * 100%.
- Voltage imbalance is the primary cause of current imbalance.
- High current imbalance can lead to motor overheating and premature failure.
- NEC recommends limiting voltage imbalance to 5%.
Memory trick: Current balance: Check the average, then the deviation.