Hybrid Excitation PM Synchronous Motors: Part I – Per Unit Analysis

A synchronous machine is analysed, characterised by a rotor including both Permanent Magnets (PMs) and excitation winding. The PMs produce a fixed flux while the excitation winding is adopted to modulate the rotor flux. The nominal point and flux-weakening performance of such a synchronous machine is compared to that of a more commonly used PM machine, to the aim of highlighting advantages and drawbacks. For the sake of achieving a more general comparison, a per unit system is introduced. Thanks to this normalization all the combinations of the machine parameters are taken into account, for both standard PM and HEPM machine. Fixing the same performance at nominal speed, it is shown that the HEPM machine exhibits a higher capability and power factor equal to one during flux-weakening operations. The possibility to modulate the rotor flux allows to overcome the speed limit of the standard PM machines and to keep a high torque and power in a wide range of speeds.