Comparison of Two Analytical Methods for Calculating the Maximum Mechanical Stress in the Rotor of High-Speed-Assisted Synchronous Reluctance Machines

Embedded applications like electric and hybrid vehicles are increasingly using electrical machines with a very high power/mass ratio. In this context, the design of such machines, very often operating at high speeds, becomes essential. However, in order to remain economically attractive, these machines cannot use rare earth magnets but at best include (low cost) ferrites. In this context, a significant number of publications have shown that synchronous reluctance machine (SynRM) may be a good choice. However, this type of machine gives poor performances due to the difficulty to get high saliency ratio. This saliency ratio may be improved by the use of low-cost ferrites that saturate the rotor magnetic bridges. The determination of the height of the magnetic bridges becomes then a problem of the prime importance to ensure both high-speed integrity and high saliency ratio. This article proposes a fast model based on the "Beam Theory Method (BTM)," which is intended to be integrated in an electromechanical optimization process. This method will be compared to another one that has been previously introduced for this type of problem: the equivalent ring method. The maximum mechanical stress at critical regions of the rotor of the assisted SynRM can then be calculated for different rotating speeds and rotor iron bridge thicknesses. By adopting the concept of stress concentration, both methods are compared with the results of finite element analyses. Finally, a prototype of the assisted SynRM is used to validate these results from finite element simulations.