Ω Shaped Axial-Flux Permanent-Magnet Machine for Direct-Drive Applications with Constrained Shaft Height G. De Donato, F. Giulii Capponi, G. Borocci and F. Caricchi Dept. of Astronautical, Electrical and Energetic Engineering University of Rome “La Sapienza” Rome, Italy giulio.dedonato@uniroma1.it L. Beneduce, L. Fratelli and A. Tarantino Ansaldobreda Naples, Italy beneduce.luigi@ansaldobreda.it Abstract— This contribution investigates an original solution that can be used in the design of AFPM machines, whenever a constrained shaft height requirement penalizes a standard design. The machine is tailored for electrical traction, but the ideas that are set forth are valid for any application with constrained shaft height. Two design solutions are investigated that comply with the constrained shaft height: a standard Torus AFPM machine and an asymmetrically wound Torus machine, named “Ω AFPM” due to the shape of the stator. It is shown that the Ω AFPM machine has lower losses and a higher efficiency. FE simulations and experimental tests on a full-scale prototype confirm the validity of the proposed solution. I. INTRODUCTION Over the past decade, axial-flux permanent-magnet (AFPM) machines have gained increasing acceptance in industrial applications ranging from renewable energy systems to transportation, which can benefit from these machines’ extreme axial compactness coupled with high torque density and high efficiency. AFPM machines are generally regarded as ideally suited whenever low speed and high torque are required, such as in direct-drive applications, [1]-[4]. On the other hand, some researchers have also proposed AFPM machines for high speed applications, such as flywheel energy storage, [5]-[7] , and hard disk spindle drives, [8]. One of the basic AFPM machine topologies is the Torus topology, composed of a stator positioned between two rotor discs that are rigidly connected to the machine shaft. On the surface of each disc, permanent magnets (PMs) are mounted in order to produce an axially directed magnetic field in the machine air gaps. The stator core is made of spirally wound laminated electrical steel lamination. The stator coils are accommodated either inside radially directed slots (slotted configuration) or directly wound around the stator core (slotless configuration). Among the two configurations, the slotted one has a larger per unit inductance, which allows these machines to have flux weakening ranges as high as 2:1, and a larger torque density, up to 20 Nm/kg. Thus, slotted AFPMs are of great interest for electrical traction, [2]. Different solutions that allow to extend the flux weakening range of AFPM machines beyond such limits have been investigated in the literature, [9]-[11], but are not used in this research. This contribution investigates an original solution that can be used in the design of AFPM machines, whenever a constrained shaft height requirement penalizes a standard design. In this paper, the machine is tailored for electrical traction, but the ideas that are set forth are valid for any application with constrained shaft height. Initially, a fully wound, slotted Torus type AFPM machine with core-wound coils is designed to fit the shaft height constraint and to produce 67.5 kW at 320 rpm. It is shown that such a design has unacceptably high losses and low efficiency. Therefore a modified design is proposed with an increased average diameter to improve performances; in order to comply with the shaft height constraint, a portion of the winding is removed, leading to an asymmetrically shaped stator winding. Due to the shape of the wound stator, as will be apparent in the following section, this machine has been named “Ω”. It will be shown that such a solution leads to lower losses and a higher efficiency compared to the fully wound design. Since the Ω design has a portion of the stator periphery that is left unwound, it is bound to experience flux-fringing effects that will cause a drop in the torque production capability of the machine. To show this effect and to be able to quantify the reduction in performance, selected no load and rated load finite element analyses (FEA) are reported. A full scale prototype of the Ω AFPM machine has been built and experimental tests have been carried out at reduced power, due to limitations of the test bench. The reported results are in line with the theoretical and simulative predictions and confirm the Ω AFPM machine as a valid alternative for direct-drive applications with constrained shaft height. II. AFPM MACHINE DESIGN SOLUTIONS FOR CONSTRAINED SHAFT HEIGHT The design of an electrical traction motor starts with the selection of the rated parameters of the machine, based on a number of operating points that the machine is required to fulfil, such as those shown in the curves in fig.1. Fig.1a