A three-phase 550 V, 15 kW (20 hp) induction motor in a sawmills stops suddenly during operation. The electrician checks the motor control center (MCC) and finds that the thermal overload relay for this motor has tripped. After resetting, the motor starts and runs for a few minutes before tripping again. What is the most likely cause?

Question: A three-phase 550 V, 15 kW (20 hp) induction motor in a sawmills stops suddenly during operation. The electrician checks the motor control center (MCC) and finds that the thermal overload relay for this motor has tripped. After resetting, the motor starts and runs for a few minutes before tripping again. What is the most likely cause?

Answer options: ✅ An overloaded motor due to a mechanical issue or excessive load.

• Incorrect motor rotation due to a phase reversal.
• Low voltage in one phase, causing an unbalanced current.
• A short circuit within the motor windings.

Correct answer: An overloaded motor due to a mechanical issue or excessive load.

Explanation: Repeated tripping of the thermal overload relay indicates an excessive current draw, most commonly caused by the motor attempting to drive a mechanical load that is too high, or a mechanical issue with the motor itself (e.g., seized bearings). While low voltage could also cause increased current, short circuits typically trip circuit breakers or fuses more quickly and are less likely to reset and run for a 'few minutes'. Phase reversal would prevent the motor from starting correctly or cause it to run in the wrong direction, not cause repeated overload trips.