Normalized Analytical Model of Stresses in a Surface Mounted Permanent Magnet Rotor

With the current trend towards high-power density electric machines, rotors are being pushed closer to their structural limits. While finite element analysis tools are capable of calculating rotor stresses, there exists a need for accurate and rapid analytical modeling for use in population-based machine design optimizations. This paper presents an analytical model for calculating the stresses due to centrifugal and thermal loading in a surface mounted permanent magnet rotor retained using a composite rotor sleeve. The primary contribution of this paper is the normalization of this model by the outer rotor radius and its use to identify normalized design trends. Using rotor tip speed, the results from the normalized model can be generalized to analyze machines for a wide range of rated speeds. A method for determining optimal rotor sleeve geometry is presented and a generalized design case study is used to validate this model with a finite element analysis tool. Finally, the relationship between maximum rotor tip speed, magnet thickness, and temperature rise is demonstrated using the normalized model. This relationship illustrates an important trade off between magnetic loading and rotor tip speed.