A 575 V, three-phase induction motor is consistently tripping its overload relays after running for only a few minutes, even though the driven load seems normal. An amperage check shows currents of 25 A, 30 A, and 28 A on the three phases. The motor nameplate FLA is 28 A. What is the most likely cause of the tripping?

Question: A 575 V, three-phase induction motor is consistently tripping its overload relays after running for only a few minutes, even though the driven load seems normal. An amperage check shows currents of 25 A, 30 A, and 28 A on the three phases. The motor nameplate FLA is 28 A. What is the most likely cause of the tripping?

Answer options:

• The motor is significantly underloaded, causing regeneration. ✅ There is an unbalanced voltage supply to the motor.
• The overload heater elements are oversized.
• The motor bearings are worn out, causing mechanical drag.

Correct answer: There is an unbalanced voltage supply to the motor.

Explanation: The current readings (25 A, 30 A, 28 A) show a significant unbalance (phase B is higher than the other two). Voltage unbalance can cause current unbalance, leading to overheating in the motor windings, even if the average current is at or below FLA. This overheating will trip the overload relays.