High-performance motors dispense with expensive rare-earth magnets

Paul Boughton

Applications ranging from servo motors for industrial automation, through to electric and hybrid vehicles, are often reliant on rare-earth magnets. Jon Severn investigates why the price of these magnets has risen so much and reviews some developments that could help to alleviate the problem.

Rare-earth magnets are typically two to three times stronger than ferrite or ceramic permanent magnets. In electric motors, the use of rare-earth magnets enables greater performance to be obtained from a smaller, lighter motor. Clearly this has its attractions for electric vehicles, where a lighter, more efficient motor reduces the amount of stored energy that has to be transported in the form of petrol, hydrogen or batteries; according to Rare Earth Metals Inc, the Toyota Prius hybrid electric car contains 30kg of rare-earth elements, used in the motors, metal hydride batteries, glass, catalysts and electronics (Fig.1). For industrial applications, the benefits of rare-earth magnets typically relate to the higher performance available from a motor of the same size, or lighter motors for equipment that is in motion. Another related application for rare-earth magnets is in the generators installed in wind turbines, where the long-term efficiency gains make it worth installing higher-cost, higher-performance generators.

The term 'rare-earth magnet' is somewhat misleading, as the magnet is actually an alloy containing relatively small amounts of the rare-earth elements. For example, neodymium magnets, which are the most commonly used type of rare-earth magnet in motors, are made from an alloy of neodymium, iron and boron (Nd2Fe14B). Furthermore, the rare-earth elements - neodymium in this case, or samarium in samarium-cobalt (SmCo5) magnets - are not as rare as the term suggests: samarium is the fortieth most abundant element in the Earth's crust, making it more common than tin, whereas neodymium is almost as abundant as copper. However, rare-earth elements are seldom found in concentrated and economically exploitable forms, hence they are referred to as 'rare'.[Page Break]

Today's main difficulty with rare-earth elements is that China controls approximately 95 per cent of the world's supply. Before it attained this dominant position, China cut its prices, making mines elsewhere in the world uneconomic. As these mines closed, China's dominance increased. Having cornered the market, China has been able to reduce export quotas and force prices to surge (and manufacturers to relocate to China). While it is true that the current high prices are making it economic for some mines in other countries to reopen, China is likely to retain its strong position for the foreseeable future. At the end of 2011 some rare-earth metal prices started to fall, but prices will most probably remain volatile and largely controlled by China.

Such has been the concern that some manufacturers of servo motors have had to charge customers 'servo surcharges' because of the high prices paid for the rare-earth magnets; while some manufacturers charge a flat rate per motor, others levy a charge that relates more closely to the weight of rare-earth magnetic material contained in each motor.[Page Break]

Researching alternatives

Manufacturers of motors and products that incorporate motors, as well as government agencies, have started to take the view that the situation is unsustainable and that alternative technologies need to be developed. In the UK, a group of specialist engineering technology companies has won funding to undertake research into the development of the next generation of electric vehicle drivetrain systems that will reduce significantly the future dependency on rare-earth metals, as most motors for electric and hybrid vehicles currently contain significant amounts of rare-earth metals such as neodymium and dysprosium. Sevcon is leading the collaborative project, with other participants being Cummins Generator Technologies and Newcastle University's Power Electronics and Drives Research Group; the aim is to develop a new type of 'no rare-earth metals' electric traction drive system for use in hybrid and pure electric vehicles. Instead of using conventional motors with rare-earth magnets, the team is to develop a drive system based around advanced high torque density switched reluctance (SR) motors. Over £500000 in matched funding has been received from the government-backed Technology Strategy Board (TSB) and the Department for Business Innovation and Skills (BIS). It is anticipated that the project should result in a drive system being ready for volume production within four years.

The drive system design under development will not only use SR motors to avoid the need for rare-earth magnets, but will also replace traditional electronic control systems with new types based on power electronics. As well as providing sufficient power, the new-generation drive system will be designed to be both cost-competitive and suitable for high-volume manufacture.

Other organisations have also been investigating automotive drive trains that minimise the use of rare-earth magnets. Around one year ago Drive System Design (DSD) and Oxford Yasa Motors introduced the MSYS Multi-Speed Traction System for electric vehicles. This compact, lightweight (45kg), integrated, three-speed system supplies 60kW of continuous power and more than 200Nm of torque. The overall powertrain efficiency is claimed to be better than 91 per cent.[Page Break]

Segmented armature

The MSYS axial flux Yasa (yokeless and segmented armature) motor and multi-speed transmission are said to provide car designers with opportunities to simplify the motor cooling system, electrical architecture and control system. Moreover, the car's drivability is enhanced by the drive system's high torque even from low motor speeds, and the DSD electrically actuated Overlap Shift Technology provides rapid gear changes. While the motor is rated at 60kW continuous, a peak of 85kW or more can be sustained for periods of up to 60seconds (Fig.2).

Oxford Yasa Motors has already built a reputation for its motors, with four Yasa 750 motors - with 750Nm peak torque each - being installed in the Lola-Drayson B12/69EV pure electric Le mans prototype; one of the same motors has also been used in the Ion Horse electric motorcycle prepared for the Isle of Man's TT Zero race, two are in the Westfield iRacer (Fig.3), and two in the Delta E 4 coupé (see panel).

Another company heavily involved in SR motors is Switched Reluctance Drives Ltd (SRDL), which is a subsidiary of Japanese company Nidec Corporation. SRDL says it has pioneered the development, application and commercialisation of SR technology under its SR Drive trademark and is now a world leader in this area. The company was founded in 1980, having grown out of fundamental research work carried out at two UK universities, namely Leeds and Nottingham.

SRDL explains that its SR Drive system comprises a simple variable-speed brushless motor with a dedicated electronic controller. Torque is produced by the magnetic attraction of a steel rotor to stator electromagnets. No permanent magnets are needed - either of the conventional or rare-earth type - and the rotor carries no 'squirrel cage' or windings. Low energy losses in both the rotor and power electronics eases thermal management and enhances reliability and efficiency, while the compact stator windings permit great flexibility in terms of the motor's shape.

In the immediate future there may be no alternative to servo motors and other high-performance motors incorporating rare-earth magnets, other than the motors highlighted above. As manufacturers seek alternatives, however, demand for rare-earth magnets will fall and there may be a reduction in prices - but it would be imprudent to rely on this happening. There is plenty of research underway in the field of motors for electric and hybrid vehicles, but it is less clear what the level of research is in relation to servo motors for applications such as industrial automation. Design engineers therefore need to stay up to date with the latest developments, as it is likely that volatile pricing will continue to be the norm for rare-earth magnets for a few years yet. [Page Break]

Delta E-4 coupé

Delta Motorsport launched the battery-electric Delta E-4 Coupé in May 2011 (Fig.4). Designed from the outset as a pure electric vehicle, the two-door, four-seater's light weight and high-efficiency systems mean it has a range of 320km (200miles) on a single charge, yet it can accelerate from zero to 96km/h (60mph) in less than five seconds.

The key to the E-4 Coupé's impressive performance and range is said to be the close integration of the various technologies. The new carbon composite chassis weighs just 85 kg, which is around one-third that of a comparable steel structure, despite being designed to pass EU crash tests. Each of the two high-performance direct-drive Yasa 750 motors produces 100kW of peak power, while only weighing 23kg. Installing the batteries under the floor aids the sporting handling characteristics and helps to minimise aerodynamic drag.

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