Which of the following is a sign of overheating rotor bars?

Which of the following is a sign of overheating rotor bars?

• A. Reduced starting torque.
• B. Noise.
• C. Discoloration.
• D. All of the above.

Answer: (C)

Overheating rotor bars leave a clear, visible trace: discoloration on the rotor surfaces. When the rotor overheats, the metal (often copper in a squirrel-cage rotor) oxidizes and the protective coatings can burn or scorch, producing reddish-brown to darker discoloration. This color change is a direct physical sign you can observe during inspection and strongly points to overheating of the rotor bars. Other symptoms like reduced starting torque or abnormal noise can occur with rotor problems, but they are not as specific to overheating as discoloration. They can also arise from other motor faults, so they’re not as reliable as a visible color change for indicating overheated rotor bars.

Overheating rotor bars leave a clear, visible trace: discoloration on the rotor surfaces. When the rotor overheats, the metal (often copper in a squirrel-cage rotor) oxidizes and the protective coatings can burn or scorch, producing reddish-brown to darker discoloration. This color change is a direct physical sign you can observe during inspection and strongly points to overheating of the rotor bars.

Other symptoms like reduced starting torque or abnormal noise can occur with rotor problems, but they are not as specific to overheating as discoloration. They can also arise from other motor faults, so they’re not as reliable as a visible color change for indicating overheated rotor bars.