This work has been supported by Protean Electric Ltd. All FEA simulations have been done using JMAG Designer. Comparison of parallel slots against parallel teeth in an In-Wheel Halbach array motor Iago Martinez Ocaña School of Engineering Newcastle University Newcastle upon Tyne, UK i.martinez-ocana@newcastle.ac.uk Chengwei Gan Protean Electric Ltd Farnham, UK Chengwei.Gan@proteanelectric.com Barrie Mecrow School of Engineering Newcastle University Newcastle upon Tyne, UK barrie.mecrowb@newcastle.ac.uk Simon Brockway Protean Electric Ltd Farnham, UK Simon.Brockway@proteanelectric.com Nick J. Baker School of Engineering Newcastle University Newcastle upon Tyne, UK nick.baker@newcastle.ac.uk Chris Hilton Protean Electric Ltd Farnham, UK Chris.Hilton@proteanelectric.com Abstract— This paper will investigate the use of a Halbach array in an in-wheel motor. It highlights the benefits of introducing a Halbach array to reduce the rotor-core-back depth and compares two different stator topologies for the application: a parallel teeth and a parallel slots. Keywords— In-wheel motors, Halbach array, automotive traction, parallel teeth, parallel slots I. INTRODUCTION In-wheel electric motors have been developed in recent years to provide integrated direct drive traction in electric vehicles. Classical automotive components, including drive shafts, gears and differentials can be eliminated by integrating the drive and the motor in the wheel, see Fig. 1 Although the use of an in-wheel motor can increase the un-sprung mass, this has a minimal effect upon steering and handling as the suspension system is modified to suit. The removal of components gives overall efficiency, weight and simplicity gains. Using in-wheel motors allows the use of true torque vectoring control at each wheel, electronic differential, traction control and more efficient regenerative braking. In addition, the integration of inverter and motor with the wheel frees more space in the vehicle for use in other ways. The work presented in this article is focused on increasing the torque density of the in-wheel motor through the use of a Halbach array rotor topology with two alternative stator topologies, a parallel slots topology and a parallel teeth topology. II. REFERENCE MACHINE The reference machine used to asses performance is an existing surface mount permanent magnet machine integrated into a wheel. This machine has been developed and manufactured for mass production: it has been extensively tested with dynamometers and withinin road vehicles, so it is now very well characterized [1], [2],[3]. It already has a very high torque density, but there is always a desire to increase it further. Two areas must be considered: the maximum continuous operating point, which is generally thermally limited, and the short-term overload, which is limited by the capability of the inverter and magnetic saturation in the machine. The outer rotor diameter of the machine and the axial length is constrained as this is dependent on the wheel size. This motor has been designed to be used in an 18 inch wheel frame with the electrical constraints presented in [4]. III. ROTOR DESIGN A Halbach array comprises a set of magnets which guide the magnetic flux round the rotor flux path. At the pole centre the magnets are radially magnetised, whilst between poles the transition magnets are circumferentially magnetised. In this work a simple arrangement of only two magnets per pole has been chosen to ease the manufacturing process: one magnet radially magnetised and one circumferentially magnetised. Previous work [4] has shown the benefit of adding more transitions per pole, however for this study a single transition per pole was chosen due to its simplicity for manufacture. The Halbach arrangement generally has a larger proportion of the magnetic flux path passing through the magnet material: this can result in either a higher air-gap flux density or, alternatively, a longer air-gap without any loss of performance. A second benefit is the reduction or even elimination of any rotor core-back iron, as some or all of the circumferential portion of rotor flux flow is through the magnets: this can give more efficient use of space as, while limiting the outer rotor diameter, the air-gap diameter can be increased. A reduced rotor core-back reduces the mass and inertia of the rotating element and has a positive effect upon dynamic performance. However, the most important aspect of an increased air-gap diameter is the provision of more space for the stator – an advantage which will be considered later. As expressed in (1), where D is the air-gap diameter, Lstk the Fig. 1: Protean integrated drive ***NCL Eprint version*** Published in: 2019 IEEE International Electric Machines & Drives Conference (IEMDC) Date of Conference: 12-15 May 2019 Date Added to IEEE Xplore: 05 August 2019 ISBN Information: DOI: 10.1109/IEMDC.2019.8785216