Shear stress concentrations in permanent magnet rotor sleeves

There is a strong trend in using permanent magnet synchronous machines for very high speed, high power applications due to their high efficiencies, versatility and compact nature. To increase power output for a given speed, rotor design becomes critical in order to maximize rotor volume and hence torque output for a given electrical loading and cooling capability. The two main constraints on rotor volume are mechanical, characterized by stresses in the rotor and resonant speeds of the rotor assembly. The level of mechanical stresses sustained in rotors increases with their radius and speed and, as this is pushed higher, previously minor effects become important in rotor design. This paper describes an observed shear stress concentration in sleeved permanent magnet rotors, caused by the Poisson effect, which can lead to magnet cracking and rotor failure. A simple analytical prediction of the peak shear stress is presented and methods for mitigating it are recommended.