A electrician is troubleshooting a 400 V, 3-phase, 7.5 kW (10 hp) motor that fails to start. When the 'start' button is pressed, the motor hums loudly but does not rotate. A quick check with a clamp-on ammeter shows very high current draw across all three phases. What is the MOST probable cause?

Question: A electrician is troubleshooting a 400 V, 3-phase, 7.5 kW (10 hp) motor that fails to start. When the 'start' button is pressed, the motor hums loudly but does not rotate. A quick check with a clamp-on ammeter shows very high current draw across all three phases. What is the MOST probable cause?

Answer options:

• Overload relay contacts are welded closed.
• One phase of the supply voltage is missing. ✅ The motor's bearings are seized.
• The control circuit fuse is blown.

Correct answer: The motor's bearings are seized.

Explanation: If the motor hums but doesn't rotate, and draws high current on all phases, it indicates that power is reaching the windings, but something is physically preventing rotation. Seized bearings would present exactly these symptoms. A single missing phase would typically cause the motor to single-phase and hum, but not draw high current on all three phases (the missing phase would draw zero). Welded overload contacts wouldn't prevent starting, and a blown control fuse would prevent the contactor from energizing entirely.