Three ways that traction motor makers race to escape rare earths

Those succeeding in components for these many and varied pure electric and hybrid vehicles must face completely new technologies replacing what is there today, such as supercapacitors replacing some batteries. See the IDTechEx report, Electrochemical Double Layer Capacitors: Supercapacitors 2013-2023 for more on this huge new business.

For example, the new IDTechEx report, Electric Motors for Electric Vehicles 2012-2022 is being constantly updated because the subject is moving so fast, not least in the complete redesign of these motors to escape the tyranny of price hikes for rare earths in the magnets such as dysprosium, neodymium and terbium. The urgency was increased when the Chinese rationed a rare earth in an unrelated political dispute with the Japanese.

There are now three distinct approaches and every serious supplier now has or is preparing at least one such version in its line-up because performance improvements come with the alternatives, not just cost control. Two are synchronous and one is asynchronous and the universities as well as the manufacturers are improving these options very rapidly.

Synchronous motors with no magnets - switched reluctance

The switched reluctance motor is almost as old as the electric motor itself and it is widely used outside the EV industry. In EVs it is variously claimed that it can reduce cost, increase reliability and work in hazardous environments but success has been elusive until recently in EVs because of fine machining, motor acoustic noise and other issues. In 2009, Adura Systems, now Altria Controls, incorporated them in its bus and truck power train and achieved sales in China. Later, Nidec Motor, a major Japanese manufacturer of motors for EVs, bought SR Drives in the UK which is developing SR motors for buses and other heavy uses. In 2011, John Deere fitted switched reluctance motors to two agricultural hybrid electric vehicle models. At EV Japan in January 2012, Nidec exhibited a switched reluctance vehicle traction motor and told us that it has the lowest cost point of any vehicle traction motor and said they were close to several orders.

In all this, the sweet spot seems to be above 20 kW in demanding environments and duty cycles, so, initially at least, switched reluctance motors may be competing more with asynchronous traction motors than with synchronous permanent magnet ones. However, their market share is, as yet, tiny.

Synchronous motors with new magnets

Second comes replacing the magnets with ones that do not contain rare earths. At EV Japan in January 2012, Hokkaido University in Japan, which has also developed switched reluctance motors for electric vehicles, enthusiastically described its 51.5 kW axial gap ferrite magnet synchronous motor with the same output density as a conventional rare-earth permanent-magnet motor for a hybrid vehicle. Efficiency increased at the light loads normally encountered in this application. Higen Motor Co Ltd of Korea displayed a similar ferrite magnet capability for EV traction in EVS60 in Los Angeles this month, theirs being a reluctance version.

Hitachi has a full range of traction motors and a major program to eliminate rare earths. In April 2012, it announced an 11kW highly efficient permanent magnet synchronous motor co-developed with Hitachi Industrial Equipment Systems Co., Ltd., that employs an iron-based amorphous metal in the core and no rare earths. In 2008, they had established basic technology for a rare metal-free motor. In order to further increase capacity and efficiency, technologies such as structural optimization and loss minimization for the core were developed, and leading to a medium capacity 11kW motor. In comparison to conventional motors of the same class, the motor developed is smaller and achieves an energy efficiency of approximately 93％ which fulfills the highest standard of IEC IE4. A part of this work was supported by The New Energy and Industrial Technology Development Organization (NEDO), Japan as part of its support program for the development of practical technology to substitute or reduce rare metals.

A highly resilient motor structure led to higher capacity and efficiency levels. Features of the axial gap technology include increasing the capacity of double-rotor axial gap motors and decreased core loss using stratified iron-based amorphous metal. Motor design optimization technology was also key.

Asynchronous motors

The third option, asynchronous motors, never did have permanent magnets because they are essentially rotating transformers invented by Tesla over 100 years ago. The IDTechEx report, Electric Motors for Electric Vehicles 2012-2022 quantifies how they are now the preferred option for heavy electric vehicles, both pure electric and hybrid, such as buses, trucks, forklifts and earthmoving equipment. The recently announced Solaris bus with its 120 kW four pole asynchronous motor is an example. They are in many golf cars. Indeed, the last year has seen up to 50% of new car-sized vehicles incorporate them, Tesla car company no longer being seen as a maverick in using them in its sports car.

Traditionally, asynchronous motors have had poorer performance than synchronous ones in electric vehicles and they have been larger but more robust - no demagnetisation from overheating for example. However, the gap in performance has narrowed and in some cases has been eliminated. Anyway, size is not always a problem, given the slow progress of in-wheel motors because of cost. On the other hand, copper, while much cheaper than rare earths, is used a great deal in asynchronous motors and it has had its own price hikes.

More to come

The various motor designs are not mutually exclusive and there will clearly be much more to come in terms of both improved and merged motor technologies in electric vehicles. At present, almost all the alternative motors bought are asynchronous but that will not necessarily continue to be the case with the possibility of switched reluctance versions winning on price and synchronous versions with no rare earths sometimes having superior performance in some respects and possibly emerging as one of the better options for the smaller motors.