Rare-earth-free propulsion motors for electric vehicles: A technology review

https://doi.org/10.1016/j.rser.2015.12.121Get rights and content

Abstract

Several factors including fossil fuels scarcity, prices volatility, greenhouse gas emissions or current pollution levels in metropolitan areas are forcing the development of greener transportation systems based on more efficient electric and hybrid vehicles. Most of the current hybrid electric vehicles use electric motors containing powerful rare-earth permanent magnets. However, both private companies and estates are aware of possible future shortages, price uncertainty and geographical concentration of some critical rare-earth elements needed to manufacture such magnets. Therefore, there is a growing interest in developing electric motors for vehicular propulsion systems without rare-earth permanent magnets. In this paper this problematic is addressed and the state-of-the-art of the electric motor technologies for vehicular propulsion systems is reviewed, where the features required, design considerations and restrictions are addressed.

Introduction

Fossil fuel price uncertainty, resources scarcity and greenhouse gases emissions (GHG) are forcing the current energy system to be based on more efficient, sustainable and renewable technologies [1], [2]. The transportation sector is one of the main oil users, consuming almost half of the oil resources [3], so it is especially sensible to supply disturbances and price volatility. Road transport accounts for roughly 75% of transport GHG emissions [4]. This is forcing the transportation sector to improve due the need of enhanced efficiency [5], reduce the dependency on foreign resources and lower GHG and noise emissions [3]. The use of green transportation systems is of paramount importance in modern cities due to the current pollution levels, environmental issues and severer emission standards for vehicles [6]. One of the greatest challenges is to develop near zero-emission vehicles [7]. Automotive electrification is experiencing a notorious increase since this technology allows facing most of the aforementioned issues [3], [8]. Therefore power electronics devices, and propulsion and energy storage systems for electric vehicles are receiving much attention [9]. It is worth noting that with the growing installation of fluctuating power generation systems based on renewable energy sources (RES), in both generation and demand sides, more flexibility is required and electric vehicles can help to balance this fluctuation on the demand side [10], [11].

Features required for electric propulsion motors include high efficiency, high torque and power per unit volume, that is, compactness, [12] good dynamic response, simple construction and high reliability (brushless operation). To achieve these challenging requirements, the state-of-the-art technologies for new generation of propulsion systems for electric vehicles with improved features is currently based on different materials, including soft magnetic laminations and rare-earth elements (REEs) [2], among which rare-earth neodymium magnets (NdFeB) [13] highlight. In addition progresses in power and control electronics, and design and analysis systems based on finite element methods [5] highly impact these technologies.

In recent years rare-earth permanent magnet machines have gradually replaced traditional motors and generators in many applications [14], including automotive drives, wind generators or home appliances since they exhibit enhanced efficiency and power density [15], [16], [17]. As an example, modern low-speed direct drive generators for wind turbines have a rare-earth magnet content of around 650 kg/MW [18]. This had led to concerns of rapid depletion of REEs resources [2]. It is widely recognized that there is a geographical concentration of the REEs resources [19], since today China is the predominant supplier [20], with about 96% of the REEs׳ worldwide production [19], [21], [21] although it has less than 40% of the proved reserves [22]. Although new mines are planned, they take several years to be productive [1], [20]. In 2011, REEs products experienced a sudden price rise of about 600 percent due to reductions in export quotas from China [23]. According to [24], the current price of some REEs is too low to reflect their value, does not represent the shortage of the resources and does not compensate the environmental damage. An interesting option consists in recycling and reusing REEs, since it can help in reducing the total amount of primary rare-earth mineral to be used. However, REEs recycling rates are still extremely low, less than 1% [22]. Due to the multiple uses of REEs, their demand is expected to increase considerably in the future [19], so the risk of a limited supply chain is a focus of concern [25]. According to the European Commission [26] and the US Department of Energy (DOE) [27], some REEs are considered critical for the respective economies, since they can place regional industries in a vulnerable situation due to possible shortages or even to an imminent risk of supply interruption [28], and some of them, and especially dysprosium, show symptoms of serious scarcity [18]. Dysprosium and other REEs are often added to the NdFeB formulations to enhance their coercivity [14], [29].

The cost of permanent magnets (PMs) can significantly determine the final cost of PM motors used in electrical propulsion applications [30]. Due to the above-described economic, environmental and geopolitical issues, nowadays there is a growing need to produce efficient electric motors which do not use rare-earth PMs [31]. Therefore the automotive industry is exploring different technologies based on environmentally friendly and available materials.

This paper reviews the state-of-the-art in electric motors technology without rare-earth PMs for vehicular propulsion systems. The paper intends to make a comprehensive review of the most popular technologies under research or under development/application, thus identifying the strong and weak points of each technology and the challenges to be faced. A comparative analysis based on the torque density, machine constant of mechanical power and maximum efficiency of the different technologies is also performed, which is based on data published in recent scientific publications.

Section snippets

Rare-earth materials for permanent magnets

This section reviews the rare-earth materials commonly used for PMs in electrical rotating machines, their main features and environmental issues.

Rare-earth elements (REEs) are more abundant in the earth׳s crust than the name suggests [32], since the name refers to the historical difficulties in identifying and purifying REEs [2], although some heavy REEs are less common [33]. However, REEs mining is only cost-effective in a limited number of locations due to the separation complexity and the

Electric vehicles (Evs) architectures

The concept of EV is not new but came back in the late 1990s [42]. An electric vehicle (EV) is any vehicle whose driving power is partially or totally supplied from an onboard battery pack [11]. There are three main types of EVs, that is, hybrid electric vehicles (HEVs), plug-in electric vehicles (PHEVs) and full electric vehicles (FEVs) [4] whose main configurations are shown in Fig. 1.

EVs with energy storage capacity allow operating in the regenerative braking mode, thus converting some of

Electric motor requirements for Evs

It is well-known that for a given rated power, high speed motors provide a lower shaft torque, thus requiring less iron and copper since they generate a lower magnetic flux and torque. Therefore high speed motors have reduced weight and size compared to low speed machines [44].

Electric machines for EVs propulsion systems require different characteristics than those commonly used in industrial processes. For example, EVs perform frequent starts and stops, need high acceleration rate for fast

Electric motors without Rare-Earth Pms

In this section the electric motors summarized in Fig. 3 are reviewed.

Conclusion

Although many of the current electric motors for HEVs and FEVs are using rare-earth PMs, there are different possibilities to develop rare-earth-free electric motors for such applications. This work has done a deep literature review in this field, which is generating a growing interest due to the cost and supply risks concerns related to rare-earth PMs. The results presented show that some of the rare-earth-free motors reviewed can achieve similar performance in terms of torque density,

Acknowledgments

This work was supported in part by the Spanish Ministry of Science and Technology under the TRA2013-46757-R research Project, and by the Catalan Agència de Gestió d’Ajuts Universitaris i de Recerca, under the AGAUR 2014 SGR-101 Research Project.

References (132)

  • G. Bai et al.

    Study of high-coercivity sintered NdFeB magnets

    J Magn Magn Mater

    (2007)
  • H. Saavedra et al.

    Detection of interturn faults in PMSMs with different winding configurations

    Energy Convers Manag

    (2014)
  • R. Lacal-Arántegui

    Materials use in electricity generators in wind turbines – state-of-the-art and future specifications

    J Clean Prod

    (2015)
  • A. Elshkaki et al.

    Dysprosium, the balance problem, and wind power technology

    Appl Energy

    (2014)
  • K. Smith Stegen

    Heavy rare earths, permanent magnets, and renewable energies: an imminent crisis

    Energy Policy

    (2015)
  • K. Binnemans et al.

    Recycling of rare earths: a critical review

    J Clean Prod

    (2013)
  • E. Machacek et al.

    Alternative value chains for rare earths: the Anglo-deposit developers

    Resour Policy

    (2014)
  • S. Massari et al.

    Rare earth elements as critical raw materials: focus on international markets and future strategies

    Resour Policy

    (2013)
  • R.H.J. Fastenau et al.

    Applications of rare earth permanent magnets

    J Magn Magn Mater

    (1996)
  • Y. Kanazawa et al.

    Rare earth minerals and resources in the world

    J Alloys Compd

    (2006)
  • P. Christmann

    A forward look into rare earth supply and demand: a role for sedimentary phosphate deposits?

    Procedia Eng

    (2014)
  • L. Peng et al.

    Rare earth permanent magnets Sm2(Co, Fe, Cu, Zr)17 for high temperature applications

    J Rare Earths

    (2008)
  • J.M.D. Coey

    Permanent magnets: plugging the gap

    Scr Mater

    (2012)
  • T. a Wellington et al.

    The effects of population growth and advancements in technology on global mineral supply

    Resour Policy

    (2014)
  • C. Grosjean et al.

    Assessment of world lithium resources and consequences of their geographic distribution on the expected development of the electric vehicle industry

    Renew Sustain Energy Rev

    (2012)
  • T.H. Bradley et al.

    Design, demonstrations and sustainability impact assessments for plug-in hybrid electric vehicles

    Renew Sustain Energy Rev

    (2009)
  • J.D. Widmer et al.

    Electric vehicle traction motors without rare earth magnets

    Sustain Mater Technol

    (2015)
  • J.R.R. Ruiz et al.

    Demagnetization diagnosis in permanent magnet synchronous motors under non-stationary speed conditions

    Electr Power Syst Res

    (2010)
  • A. Fasquelle et al.

    Coupled electromagnetic acoustic and thermal-flow modeling of an induction motor of railway traction

    Appl Therm Eng

    (2010)
  • G. Singh

    Multi-phase induction machine drive research – a survey

    Electr Power Syst Res

    (2002)
  • Collocott SJ, Dunlop JB, Gwan PB, Kalan BA, Lovatt HC, Wu W, et al. Applications of rare-earth permanent magnets in...
  • I. Boldea et al.

    Automotive electric propulsion systems with reduced or no permanent magnets: an overview

    IEEE Trans Ind Electron

    (2014)
  • J.C. Urresty et al.

    Detection of demagnetization faults in surface-mounted permanent magnet synchronous motors by means of the zero-sequence voltage component

    IEEE Trans Energy Convers

    (2012)
  • L. Romeral et al.

    Modeling of surface-mounted permanent magnet synchronous motors with stator winding interturn faults

    IEEE Trans Ind Electron

    (2011)
  • J.C. Urresty et al.

    A back-emf based method to detect magnet failures in PMSMs

    IEEE Trans Magn

    (2013)
  • P.C. Dent

    High performance magnet materials: risky supply chain

    Adv Mater Process

    (2009)
  • Information Office of the State Council the People׳s Republic of China Situation and Policies of China׳s Rare Earth...
  • V. Ly et al.

    Low-temperature phase MnBi compound: a potential candidate for rare-earth free permanent magnets

    J Alloys Compd

    (2014)
  • Critical raw materials for the EU: Report of the Ad-hoc Working Group on defining critical raw materials – European...
  • R. Goto et al.

    Microstructure evaluation for Dy-free Nd-Fe-B sintered magnets with high coercivity

    J Appl Phys

    (2012)
  • Highly efficient industrial 11kW permanent magnet synchronous motor without rare-earth metals n.d....
  • J. Wübbeke

    Rare earth elements in China: policies and narratives of reinventing an industry

    Resour Policy

    (2013)
  • Encinas-ferrer C, Valderrey-villar FJ, Hernandez-rodriguez C, Uson-sardaña A. The World Production and Trade of Rare...
  • L.H. Lewis et al.

    Perspectives on permanent magnetic materials for energy conversion and power generation

    Metall Mater Trans A

    (2012)
  • Magnetic Materials Producers Association. MMPA Standard 0100-00. Standard Specifications for Permanent Magnet Materials...
  • Rare Earth Elements: the Global Supply Chain n.d....
  • Xue XD, Cheng K, Cheung NC. Selection of electric motor drives for electric vehicles. In: Proceedings of the Australas...
  • Lazari P, Wang J, Chen L, Chen X. Design optimisation and performance evaluation of a rare-earth-free permanent magnet...
  • Moore S, RahmanK, Ehsani M. Effect on vehicle performance of extending the constant power region of electric drive...
  • Cited by (186)

    View all citing articles on Scopus
    View full text