Comparative Study on Hybrid Excitation Flux Switching Motors without and with Variably Magnetizable Permanent Magnets for Electrified Vehicle Propulsion

Takeshi Okada,Takashi Kosaka,Hiroaki Matsumori,Nobuyuki Matsui

doi:10.3390/wevj12020058

Takeshi Okada, Takashi Kosaka + Show 2 more

Open Access

https://doi.org/10.3390/wevj12020058

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Abstract

A demand for high efficiency traction motors has been accelerated by the promotion of electrified vehicles, such as battery and fuel cell electric vehicles. As a part of development of the high efficiency traction motor, this paper reports a comparative study on two kinds of hybrid excitation flux switching motors (HEFSM) as a variable flux machine. One is the conventional HEFSM, which consists of a stator with constantly magnetized-permanent magnets, field excitation coils (FECs) and three-phase armature windings, and a rotor with salient poles like a switched reluctance motor. The other is a HEFSM employing variably magnetizable-permanent magnets (VM-PMs) that replace a part in the FEC slot area in the conventional one. Based on the variable magnetization nature of VM-PMs, the latter HEFSM promises that the replacement of magnetomotive force (mmf) of FECs with that of the VM-PMs makes the motor efficiency better at both low- and high-speed under the low-torque condition, that is, at both urban driving or highway cruising. To verify that, finite element analysis- (FEA)-based design simulations, as well as experimental performance evaluations for the two kinds of HEFSM, were conducted under reasonable dimensional and electrical constraints. As a result, it is shown that the latter HEFSM can achieve higher motor efficiency at the low-torque and high-speed region while keeping the motor efficiency at the low-torque and low-speed region.

Highlights

The hybrid excitation flux switching motor (HEFSM) as a VFM has realized better motor efficiency at light-load and high-speed operating regions while keeping the torque and power densities comparable with the interior permanent magnet synchronous motors (IPMSMs) installed on Toyota third-generation Prius [8]
The new HEFSM employing variably magnetizable-permanent magnets (VM-permanent magnets (PMs)) was designed as an effective countermeasure against the motor efficiency improvement in both the low- and high-speed region, in which a part of the mmf of field excitation coils (FECs) in the conventional HEFSM was replaced with a VM-PM
In the low-speed region, the amount of armature and FEC current required for a given torque command can be reduced by strengthening the magnetization state of VM-PMs, whereas, in the high-speed region, iron loss as well as back-emf in an open circuit can be decreased by weakening the magnetization state of VM-PMs

Summary

Introduction

Flux weakening current (negative d-axis current) causes an increase in copper loss, which leads to a deterioration in motor efficiency This means that it is difficult to achieve high efficiency and high-power density over the entirety of the operating range. The HEFSM typically employs constantly magnetized-permanent magnets (CM-PMs) and field excitation coils (FECs) on the stator body as two different field mmf sources They work as a VFM thanks to an adjustment of the mmf of FECs, and have the following features: (1) ease of cooling of all active components, such as armature windings, FECs, and PMs, because all of them are located at stator body and (2) being mechanically robust for high-speed operation owing to simple and rugged rotor structure. The HEFSM as a VFM has realized better motor efficiency at light-load and high-speed operating regions while keeping the torque and power densities comparable with the IPMSM installed on Toyota third-generation Prius [8]

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