Principle of Flux-Switching PM Machine by Magnetic Field Modulation Theory Part II: Electromagnetic Torque Generation

Abstract

Based on the back electromotive force generation principle in part I of the series papers, the electromagnetic torque generation mechanism of flux-switching permanent magnet (FSPM) machines is rethought in this article as part II from the perspective of air-gap modulation magnetic field. By magnetic field modulation theory, the air-gap armature magnetic field due to winding currents is modeled. Considering the unipolarity of the stator core, a novel concept of ‘fictitious modulation reluctance (FMR) rotor’ for FSPM machines is proposed. Then, by combining with the air-gap permanent magnet (PM) field and the FMR rotor, generation mechanisms of harmonic PM torque and reluctance torque are investigated. The requirements for FSPM machines to produce a stable electromagnetic torque are determined, i.e., <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">|mP_w</i> ± <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">vP_r</i> | = <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pm</sub> , <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m_rvM_fm</i> ≠0, and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ω_e</i> = <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P_rω_r</i> . Besides, the harmonics operation states and harmonic torque characteristics of FSPM machines are analyzed and calculated based on the Maxwell stress tensor method. Further, the average torque versus current density predicted by finite element is validated by testing on an exampled 12s/10p FSPM one.