A technician is troubleshooting a three-phase motor control circuit. The motor hums but does not start when the START pushbutton is pressed, and the thermal overload relay is not tripped. A voltmeter across the motor terminals reads 240 V between L1-L2, L2-L3, and L1-L3. Which of the following is the MOST likely cause?

Question: A technician is troubleshooting a three-phase motor control circuit. The motor hums but does not start when the START pushbutton is pressed, and the thermal overload relay is not tripped. A voltmeter across the motor terminals reads 240 V between L1-L2, L2-L3, and L1-L3. Which of the following is the MOST likely cause?

Answer options:

• A welded contactor contact in one phase. ✅ An open circuit in one phase of the motor winding.
• Incorrect wiring of the START/STOP pushbuttons.
• A faulty auxiliary contact on the motor contactor.

Correct answer: An open circuit in one phase of the motor winding.

Explanation: If the motor hums but doesn't start, it suggests that the motor is receiving power but cannot establish a rotating magnetic field, often due to a single-phasing condition. An open circuit in one phase of the motor winding (or supply) would lead to 2 phases energized and one open, causing humming and no rotation. Welded contacts would prevent the motor from stopping, not starting. Pushbutton wiring or auxiliary contacts typically affect the control circuit, preventing contactor engagement rather than causing humming.