Design considerations for permanent magnet synchronous motors for flux weakening applications

The required parameter values of a permanent magnet synchronous motor designed for very large power-speed range are determined based on a linear dq model. It is shown that unconventional motor constructions with the daxis inductance equal to, or bigger than, the praxis inductance can provide superior torque, power factor and control characteristics. The initial sizing stage of a design procedure is described. Analytical equations are developed and used to illustrate the influence of the rotor configuration, electromagnetic loading and airgap length on the performance of a relatively large range of medium-sized motors. Suggestions regarding the improvement of efficiency, power factor, overload performance and PM protection against permanent demagnetisation are also given. The results of the analytical design procedure have been checked against finite element computations and experimental data.