Ace the Elevator Mechanic Exam 2024 – Rise to the Challenge and Elevate Your Career!

The scenario that is most likely to cause a 3-phase AC motor to growl or hum loudly is an improper voltage supply. When a motor is supplied with voltage that is either too low or too high, it struggles to operate efficiently. A low voltage condition can lead to increased current draw as the motor attempts to compensate for the lack of power, while a high voltage can cause excessive stress on the motor windings. Both situations create vibration and noise in the motor, often manifesting as a growling or humming sound. Other options, while they may cause issues, are less directly associated with a loud, growling noise. Loose rotor bars can lead to mechanical issues, such as vibration and potential damage, but the immediate hum or growl is more commonly linked to voltage problems. Mechanical misalignment may cause noise, but this generally manifests as grinding or squealing rather than a hum. Overheating can cause the motor to become less efficient and may lead to failure, but it is not primarily characterized by a growling noise. Therefore, an improper voltage supply is the most fitting cause for a loud hum or growl in a 3-phase AC motor.