Optimization of Rotor Salient Pole Reluctance for Typical Field Modulated Electric Machines

The objective of this article is to investigate the possibility to improve the torque characters of electric machines featuring rotor salient pole reluctance from the perspective of the general airgap field modulation theory. The torque production mechanisms of two typical field modulated machines, namely, the magnetically geared machine (MGM) and the flux-reversal permanent magnet (FRPM) machine, are discussed based on the Maxwell stress tensor (MST) method, where the mono-harmonic mechanism of rotor PM machine like the MGM is revealed. Several common theoretical salient rotor pole configurations, like the upright/inverted trapezoidal configuration, are introduced in detail, based on which general and modified rotor configuration advices for the two electric machines are proposed to provide initial design and future optimization guidance. It is proven that there is a possible opportunity to make the torque characters better via reasonable selection of rotor configuration and topological parameters. Electromagnetic performances validation based on 2-D finite-element analysis, such as improved airgap magnetic field harmonic distributions, load flux linkage/electromotive force (EMF), and general torque performances of two field modulated machines is provided to demonstrate the effectiveness of theoretical analysis. It is also verified by experimental measurements on the FRPM machines with different pole pair combination.