Elevator Mechanic Practice Exam

The condition of a stalling, humming, or growling 3-phase AC motor is often indicative of issues related to the rotor. A broken rotor bar can result in an imbalance in the magnetic field within the motor, leading to insufficient torque generation. When the motor tries to start, it may not reach the required operational speed, which can cause it to stall and subsequently produce a humming sound. This noise typically arises from the motor attempting to draw excessive current to overcome the lack of torque, resulting in the vibrations associated with that condition.

While incorrect wiring, loose connections, and voltage spikes can lead to motor issues, the specific symptoms described – stalling and humming – strongly align with the mechanical issues caused by a broken rotor bar. Correctly identifying the rotor's condition is critical for effective troubleshooting and repair of the motor.