24 n MOTORS September 2023 www.drivesncontrols.com Are rare-earth motors going to get rarer? The burgeoning market for electric vehicles (EVs) has brought the issue of rare-earth materials to the fore. In 2022, 82% of the EV market was relying on electric motors based on rare-earth permanent magnets. China largely controls the supply of the rare-earth materials – such as neodymium, dysprosium, terbium and praesodymium – which are commonly used as additives in the magnets. The most widely used is neodymium. China’s dominance of rare-earth supplies has led to significant price volatility in recent years, with a large spike in prices during 2011/2012, and another big rise between 2021-2022. Car-makers and motor developers have therefore been looking for ways to reduce or, ideally, eliminate the use of these materials. Several ideas have been suggested for eliminating the use of rare-earths in electric motors. The main ones are outlined below, along with their pros and cons, and likelihood of them being adopted. Electric motors consist of a stationary part (the stator) which has coils of metal (typically copper) fed with an electric current to generate a magnetic field. This field turns the rotating part of the motor (the rotor). In a rareearth permanent magnet (PM) motor, the magnets are located on the rotor. In an induction motor (or asynchronous motor), the rotating magnet field produced by the stator induces currents on the rotor which, in turn, produces a magnetic field that is attracted/repelled from the radial field of the stator windings. These motors have copper or aluminium bars or windings on the rotor. They typically provide good peak power and torque density over short periods, but can prove challenging to manage thermally and typically have lower efficiencies than PM options. Induction motors have been used widely in the EV market, being the primary choice of Tesla until it released its Model 3 vehicle, which adopted a PM design. There are still some proponents of induction motors in the automotive industry – including Audi and Mercedes – but induction motors are now largely used as a secondary devices to deliver acceleration boosts because they do not create drag when not in use, thus eliminating the need for a decoupler. Wound-rotor synchronous motors (WRSMs), also known as externally-excited synchronous motors (EESMs), replace the magnets on the rotor with coil windings that can be fed with a DC current to generate a magnetic field. This has the advantage of being able to control both the stator and rotor field. The downsides are the additional manufacturing steps needed to add windings to the rotor and the brushes that are needed to transmit power to the rotor. These motors have historically also displayed poorer power and torque density, although modern versions are comparable with PM motors. Renault was an early proponent of this technology in its Zoe passenger vehicle, but now BMW and Nissan have also adopted WRSMs. The Tier 1 automotive supplier Mahle has The electric vehicle market is dominated by permanent magnet motors that rely on rare-earth materials. A race is on the find alternatives to these controversial materials whose supply (and prices) are controlled by China. This article from the technology consultancy and market researcher IDTechEx assesses the options.
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