A New Flux-Concentrating Rotor of Permanent Magnet Motor for Electric Vehicle Application

This article presents a new interior permanent magnet (IPM) rotor structure/ assembly, which inherits from the Halbach magnet configuration and spoke-type IPM rotor to achieve a high saliency ratio and high torque/ power density. With this rotor structure, the torque density and power density are improved obviously, while the rotor inner diameter can be maximized to reduce core material and increases internal space in the rotor. Accordingly, the electromagnetic performance of three motors consists of a unified stator but different rotors, namely flux-concentrating rotor, Halbach array permanent magnet (PM) rotor and spoke-type IPM rotor is compared by finite element analysis. Nevertheless, due to the segmented structure of flux-concentrating PM rotor without magnetic bridges, the mechanical challenges are analyzed in detail for electric vehicle application to hold the pole pieces validate the structural feasibility at 7200 r/min. Finally, a 72-slot/ 16-pole fractional-slot PM motor with new flux-concentrating rotor is manufactured and tested to verify its performance and feasibility. It can be concluded that the new rotor is superior to the spoke-type IPM and Halbach array PM rotor in terms of torque density and PM utilization.

I. Introduction

Since the flux-concentrating effect of two tangentially magnetized permanent magnets (PMs) in magnetic core essentially improves the air-gap flux density, the spoke-type interior permanent magnet (IPM) motors exhibit a higher torque density. In recent decades, increasing research attention has been drawn on the various structures of spoke-type IPM motors [1]–[6]. Among them, the V-shaped IPM motor has been widely concerned and applied due to good flux-weakening capacity and high saliency ratio, but the strengthening rib in the rotor will lead to increase of the flux leakage and reduce the PM utilization. Meanwhile, for the spoke-type IPM motor, the flux-concentrating property allows the use of low-performance PMs becomes an option for spoke-type IPM motor [7]–[11]. Some studies have demonstrated that the output performance of the motors using Fe-PMs holds the similar line to that of the IPM motors using Nd-PMs [12]–[15]. However, it is should be noted that some issues still exists for the spoke-type IPM motors: 1)

The leakage flux does exist, especially around the rotor inner circumference, even though various nonmagnetic materials are employed to reduce magnetic leakage on the rotor shaft.

2)

An additional nonmagnetic holder is still needed to bind the modularized rotor laminations, which will limit the rotor inner circumference and increase the weight of the rotor.

3)

Compared with V-shaped IPM designs, the relatively lower saliency ratio of spoke-type IPM rotor in integer and fractional slot machines will deteriorate the torque density due to a lower reluctance torque.