Common Failures of Cage Rotor Motors and How to Prevent Them

Common Problems in Cage Rotor Motors During Operation

Compared with wound rotor motors, cage rotors generally have better quality and safety. However, in applications involving frequent starts and high rotational inertia, cage rotors can still experience reliability issues.

Among them, cast aluminum rotors show relatively higher reliability. Their rotor bars are well coupled with the iron core, providing stronger resistance to the heating effect during motor startup. Nevertheless, quality defects such as shrinkage cavities and thin bars caused during the casting process, as well as rotor bar breakage due to overheating, should not be overlooked. These problems are especially serious when the bar material quality is poor or the casting process is inadequate.

When issues occur with cast aluminum rotors, they can usually be identified through surface inspection and related quality indicators. If rotor bar breakage occurs, it will lead to severe overheating. Localized or widespread blue discoloration appears on the rotor surface, and in severe cases, molten aluminum beads may form due to overheating. Such problems typically occur in the middle sections of the rotor bars. Overheating can also deform the rotor end rings, and in extreme situations, the end fan blades may be thrown out radially, potentially damaging the stator windings.

For special designs such as double-cage rotors, deep-bar rotors, or bottle-shaped rotors (used to improve starting performance), bar breakage usually occurs near the welding joints close to the end rings. Rotor bar fractures are mainly caused by long-term thermal stress, alternating electromagnetic forces, centrifugal forces, and tangential stresses. These repeated effects lead to bending and fatigue damage, making the bar-to-end-ring areas the most vulnerable.

During motor startup, the skin effect causes uneven heating of the rotor bars, subjecting them to bending stress directed toward the rotor axis. In normal operation, the rotor bars and end rings are subjected to centrifugal forces, producing bending stress away from the axis. These stresses compromise the reliability of both ends of the rotor bars. To enhance welding quality, medium-frequency brazing technology has been increasingly applied in the manufacturing of large rotors.

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