Compared with wound rotors, cage rotors have relatively better quality and safety, but cage rotors will also have quality problems in situations with frequent starting and large rotational inertia.
Relatively speaking, the quality reliability of cast aluminum rotors is better, the rotor bars are well coupled with the rotor core, and the ability to resist heat generation during motor startup is stronger. However, the quality defects such as shrinkage holes and thin bars that occur during the aluminum casting process, as well as the problem of bar breakage caused by rotor heating cannot be ignored, especially for the case of poor bar material and poor aluminum casting process, the problem is more serious.
When there is a problem with the cast aluminum rotor, it can generally be judged from the outer surface of the rotor and some other quality phenomena. When the rotor has a broken bar problem, it will definitely heat up seriously, and the rotor surface will have obvious blueing phenomenon partially or completely. In severe cases, there will be small aluminum beads formed by heat flow. This problem mostly occurs in the middle part of the bar. When the aluminum cast rotor heats up, the rotor end ring will also deform. In severe cases, the wind blades at the end of the rotor will be thrown out radially and damage the stator winding.
For double squirrel cage rotors, deep groove rotors, bottle-shaped rotors, etc., which are used to improve starting performance, once the rotor bars break, the breakage position mostly occurs at the welding point near the end ring. The rotor bar breakage is due to the repeated effects of long-term thermal stress, alternating electromagnetic force, centrifugal force and tangential stress, which will cause bending and fatigue damage to the bars. The bars and end rings are most likely to have problems. During the motor starting process, due to the skin effect, the rotor bars are heated unevenly, and the rotor bars are subjected to bending stress toward the axis; when the motor is operating normally, the rotor bars and end rings are subjected to centrifugal force, and the bars produce bending stress away from the axis. These stresses will threaten the reliability of both ends of the rotor bars. In order to improve the rotor welding quality, medium-frequency brazing technology has gradually been applied to the welding process of large rotors.