Axial-flux PM synchronous machines with air-gap profiling and very high ratio of spoke rotor poles to stator concentrated coils

Permanent magnet machines including a magnetic gearing effect are attractive for low-speed high-torque applications. This paper proposes a dual-stator axial-flux machine topology with a high ratio for the number of rotor poles to concentrated stator coils. The stator consists of a relatively small number of teeth, with each tooth including multiple smaller/auxiliary teeth. The rotor employs spoke-type permanent magnets, which result in high flux concentration, thereby further improving the torque density. The paper discusses the principle of operation of the proposed machine topology. Finite element results on example designs with 6 stator teeth, each having 2 and 3 auxiliary teeth, and with 20, 32, and 34 rotor poles are presented, respectively. Other feasible slot-pole combinations are also identified.

I. Introduction

Permanent magnet (PM) machines with an integrated magnetic gearing effect are useful for low-speed direct-drive applications because of their potential for very high torque density. Such PM machine topolgies, often referred to as derivations of the Vernier-type concept, have been proposed over the years, going back to 1990 [1] and with more recent researches published, for example, in [2]–[8]. Due to the high magnetic leakage and relatively large equivalent air-gap, the power factor is inherently low in these types of electrical machines and in order to improve performance dual stator arrangements have been proposed, e.g. [3], [4], [9].