A simple substitution of copper for aluminum in the rotor conductor bars will generally provide a significant reduction in I2R losses, and thereby an increase in motor efficiency. However, if the design of the rotor and stator are re-evaluated to account for the better electrical conductivity of the copper, the even greater improvements in efficiency can be obtained.
This section of the web page provides information about the electrical properties of the copper alloys used to die cast the rotors, the type of efficiency gains that can be achieved with copper rotors, and the tools that are available to optimize the design of the copper die cast rotors.