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You are right that aluminum certainly has a higher coefficient of thermal expansion. Whether that translates into an inherent weakness I'm not sure (I'd think the design attempts to account for the material properties). In older days it was in vogue to specify copper rotors but my general understanding is that these days there is not a significant difference in reliability with respect to bar to end ring joints. We have had a problem in each type of construction. Aluminum rotor is more susceptible to melting from short term abuse such as many-starts-in-short interval or motor not tripped quick enough during locked-rotor condition, but standard protection should prevent those scenarios. If you do end up having to repair the rotor, the copper technology is typically easier for a repair shop to repair. For one thing they are more familiar with the copper brazing because it's similar to what they do in the stator endwinding connections, and for another they are more familiar with copper rotor repair overall simply because it is a more common rotor type. Aluminum fabricated is more specialized and harder to repair.Siemens said:While there are many similarities of AlBar [aluminum fabricated] to CuBar [copper fabricated] construction, there are two notable differences: the end connector of an AlBar rotor is welded to the rotor bars (as opposed to brazed), and, the end connector of an AlBar rotor clamps the rotor punchings (as opposed to end heads). It should be noted that AlBar rotors can also be built with a construction method similar to CuBar rotors, but this method is more expensive and not as common.