High speed permanent magnet machine design with minimized stack-length under electromagnetic and mechanical constraints

Certain aircraft and military applications require high speed machines with low stack-length and the lowest possible weight. Hence, the accommodation of the highest bore diameter may seem the natural option. However, a rotor design with high diameter results in significant increase in mechanical stresses in the employed magnet retention. In a sleeved magnet retention mechanism, the sleeve thickness can be increased in order to accommodate the stresses. However, this will result is significant drop in air-gap flux-density and will not yield the high power density expected by the machine. This paper presents an analytical technique that combines the sleeve stress model and the air-gap flux density model to calculate the optimal rotor diameter to achieve the maximum power of the machine design for a minimum stack-length. The technique is applied to both a Carbon Fibre sleeve version and a metallic sleeve version. The analytical calculation of the stresses is validated with mechanical finite-element simulations. The machine design with the analytical calculations is validated with electromagnetic finite element simulations. The results confirm the rotor design strategy and the design technique.