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