Full Throttle for New Motors: Electric Vehicles Drive Evolution
Pārpublicēts no Okt 31, 2010, Nikkei Electronics Asia
The new breed of motors is expected to deliver even better performance in terms of higher efficiency, smaller and thinner shape, and lower cost. Universities, automobile manufacturers and motor manufacturers are actively working to find solutions. The developed technology will drive evolution in all types of motors, from vehicles to home appliances
Vehicles have depended on internal combustion engines for drive, but today they are beginning to mount motors. Automobiles are rapidly going electric, with electric vehicles (EV) and hybrid EVs (HEV), and by 2020 the global market is expected to grow to over 10 million units. The same thing is happening in two-wheelers, with electric motorcycles and bicycles becoming more common worldwide. Especially in China, supported by government environmental improvement measures, electric motorcycles are becoming quite common in the cities, and the annual market has surpassed 20 million units.
Electrification is affecting more than vehicles, though... hybrids are becoming popular in the field of construction machinery as well, and according to a source at Hitachi Ltd. of Japan, "The market will grow to about 12,000 units by 2020." The reason that manufacturers are switching from internal combustion engines to motors is obvious: motors deliver better energy efficiency.
Internal combustion engines can, at best, convert only about 30% of the energy they generate into useful kinetic energy. A high-performance motor, on the other hand, can convert almost 90% of input electrical energy into useful kinetic energy. In addition, the motor can drive a generator during deceleration to recover (regenerate) power, which is something no internal combustion engine can do.
At present, problems of battery cost and limitations on how capacity can be put into a vehicle have restricted motors to auxiliary use in most cases, serving to assist internal combustion engines as in the Prius from Toyota Motor Corp. of Japan. Over the long term, however, it seems likely they will eventually usurp the position of inefficient engines, which will vanish from the scene (Fig. 1).
Fig. 1 New Motors Driven by Automotive Applications
Equipment usually driven by internal combustion engines is beginning to switch to electric operation, in search of better fuel efficiency and comfort. This trend will continue to accelerate, opening up an era of new motor designs, and the resulting technology will eventually spread to applications in home appliances, industrial equipment and more.
Home Appliance Energy Conservation Begins in EVs
There is considerable interest in motors for applications other than merely replacing internal combustion engines, and motor technology is being revolutionized in a number of fields where motors are already being used. One excellent example is railway carriages. Railways have mostly used induction motors (IM), but are beginning to switch over to permanent magnet synchronous motors (PMSM) for their higher efficiency. The pace of change is rapid indeed in both new applications like automobiles and existing applications such as rail.
Technologies developed for vehicular use, including new designs and new materials, will eventually spill over into home appliance and equipment fields as well. If these new motor structures and materials achieve widespread adoption, it would mean air conditioners with much smaller condenser units outside, and industrial equipment with much lower power consumption. Toshiba Corp. of Japan has already used a variable-magnetism motor originally developed for use in EVs in a washing machine, cutting power consumption by up to 16% from the prior design.
The switch to motors is expected to cause a variety of effects, especially in saving energy. One estimate puts suggests that motors account for 51% of the total domestic energy consumption of 985 billion kWh (Fig. 2). An improvement in motor-driven equipment efficiency of even 1% would eliminate the need for the electricity generated by 1.5 nuclear power plants.
Fig. 2 High-Efficiency Motors to Slash Energy Consumption
An improvement in overall motor efficiency of 1% would save power equivalent to the output of 1.5 nuclear power plants. Diagram by Nikkei Electronics based on material courtesy Panasonic Motor.
Surging interest in vehicular motors is opening up a "New Motor" era, driven by dramatic advances in motor capacity, energy efficiency and more.
High Efficiency: More than Just Numbers
Existing motor designs, however, simply lack the power needed in the New Motor Era: further technological development is needed before they can satisfy the requirements of vehicular application. It will probably be necessary, for example, to mount motors inside the wheels themselves, making them effectively invisible to the user. It will also have to be possible to drive long distances using motors of even less capacity than now. But is it possible?
There are three major vectors of evolution involved, namely higher efficiency over a wide range of operation, smaller and thinner designs, and lower cost. The most important of these at the moment is boosting efficiency. Vehicular motors currently deliver a high efficiency of 97% within their optimal operating region, falling off to about 90% under other conditions. This is about the same in motors used in home appliances and various equipment. Motors with different characteristics are needed in EVs and HEVs, though: they have to deliver high efficiency even when parameters such as motor rpm or torque vary (Fig. 3).
Fig.3 Required Performance for Automobile Motors
Automobile motors will have to deliver high efficiency in a wide range of driving conditions, from city streets to expressways.
While driving in the city is repeated stop-and-go operation, motors must be able to maintain high-rpm operation continuously on the highway. Changes in conditions such as driving on slopes (up or down), or carrying cargo, will change motor loading, so the dependence of motor efficiency on motor load must be as low as possible.
Unlike home appliances, vehicles cannot make use of the commercial power constantly provided by electric utilities, and must rely on onboard battery capacity. Unless motor efficiency is high, range will be short regardless of driving conditions, and trying to extend range without improving efficiency will simply mean loading in more expensive batteries.
Instead of merely delivering high efficiency at rated operating conditions, motors will have to offer high efficiency across a wide range of use.
Getting the Motors Out of the Chassis
The second point is the need to make motors smaller and thinner. The less volume the motors take up, the larger the cabin becomes, which increases design flexibility. And as motors grow smaller, total vehicle weight drops, contributing to reduced power consumption.
In pursuit of smaller, thinner motors, automobile manufacturers are hoping for the ultimate solution: in-wheel motors. Mounted inside the wheels themselves, these motors are moved completely out of the vehicle body, which of course means more cabin volume and design flexibility for the vehicle.
Preparing for Risks of Rare Earths from China
The final point, lower cost, goes without saying. Especially in HEVs, components like motors, and the batteries and inverters that accompany them, are basically added to a conventional automobile. How far this additional cost can be reduced will be the difference between winning and losing for automobile manufacturers.
Unfortunately, it is getting harder every day to reduce motor manufacturing cost. The high-performance motors used in vehicles require high-performance permanent magnets, and the prices of the rare earths needed—elements like neodymium (Nd) and dysprosium (Dy)—are soaring. The price hikes are especially severe on Dy, with most of the promising mines located in China. It was selling for US$151/kg in Dec. 2009, but had more than doubled by Aug. 2010, to US$320 (Fig. 4).
Fig.4 Motor Industry Faces "China Risk"
The rare earth magnets used in high-performance automotive motors require elements like Nd and Dy, but most production sites are located in China (a). China recently restricted export of rare earths, triggering a price surge (b). Diagrams by Nikkei Electronics based on material courtesy US Geological Survey (a) and articles in Rare Metal News.
The rising prices are driven by the policy of the Chinese government, which cut exports of rare earths in 2010 by 40% from the prior year. 90% of rare earths are produced in China, and if they were no longer exported it would trigger a huge price hike.
In an effort to cover this risk, Japanese trading firms and other organizations are pushing ahead with the development of mines outside China. Even so, it is unlikely that Chinese influence on rare earth prices will weaken significantly, or soon. To avoid being adversely affected by Chinese decisions, industry urgently needs to develop motors that do not use rare earths.
New Materials and Designs Accelerate Adoption in Vehicles
Higher efficiency, smaller and thinner designs, and lower cost... universities, motor manufacturers and materials manufacturers are shifting in to high gear to find the solutions. There are two key points in technology development (Fig. 5), the first of which is revamping motor designs to better fulfill the requirements of automotive application. The second point is finding high-performance, low-cost materials. Between them, these two development targets are accelerating motor evolution.
Fig. 5 Moving to Resolve Key Issues in Vehicular Motors
Efforts are under way in both structure and materials.
As far as motor structure, designs capable of controlling magnetic flux are a very exciting concept. Conventional motors cannot achieve high efficiency over a wide range of conditions because the magnetic flux of internal magnets is fixed. A strong coercive force is needed to obtain high torque at low rpms, but at high rpms a lower coercive force improves efficiency. Universities and motor manufacturers are researching new types of motors now in an effort to resolve the dilemma.
For materials, the key projects are developing magnetic materials with improved heat resistance properties without using Dy, and core materials offering low energy losses even at high rpms.
The Ultimate Solution: In-Wheel Motors
People in the automotive field are extremely interested in in-wheel motors, where motors are actually mounted inside the wheels themselves. In the future the general opinion is that in-wheel motors will be used for drive systems in EVs and HEVs (Fig. 6). Motors will have to be made small enough to fit inside the wheels, which means demand for smaller and lighter motors is very strong.
Fig. 6 Electric Vehicles Evolving toward In-Wheel Motors
In-wheel motors seem likely to show up in not only EVs, but HEVs as well. In particular, when the engines are replaced with fuel cells the vehicle becomes a series hybrid, which can utilize in-wheel motors more easily.
There are a number of advantages to putting the motors inside the wheels. For example, the transmission used to transfer drive force from the engine to the wheels (essential in conventional engine vehicles) is eliminated, which not only improves drive efficiency, but also allows an increase in cabin volume. Another benefit, as a source at Toyota Motor points out, is that the design should make possible a level of performance impossible with engine-driven vehicles.
As automobiles shift to electric there is of course an awareness of the CO2 reduction accompanying the change in the power train from engine to motors, but this is not so appealing to automobile manufacturing by itself. The real prize for them is that if in-wheel motors can provide performance unavailable in gasoline-fueled vehicles, it could provide a major impetus to electrification.
There are three main advantages in in-wheel motors, namely independent control of each wheel, excellent response, and the ability to detect the torque generated at the wheel. These characteristics can be used to improve safety and comfort (Fig. 7). Engined vehicles have used independent braking control to vary the braking force on four wheels, but it has been difficult to implement per-wheel drive control. With in-wheel motors, however, independent control of each wheel becomes simple.
Fig. 7 In-Wheel Motors to Enhance Safety, Comfort
In-wheel motors allow independent drive and steering on all four wheels, providing safety and comfort beyond what conventional vehicles can provide.
Motor response is on the order of 100Hz, which is significantly faster than the engine (1Hz order) or anti-lock braking system (ABS; 10Hz order). The torque generated at each wheel can also be sensed as motor current, making it possible to precisely control drive force at each wheel individually.
Independent control can be expected to provide improved safety, helping prevent sideslips, no-traction spins and locking. Riding comfort would also be improved, because it would become possible to better control pitching caused during sharp acceleration or deceleration, and rolling during cornering. In-motor wheels could be steered independently as well, vastly simplifying parallel parking: yet another function difficult to achieve in engined vehicles.
Extended Range Another Possibility
In addition to improved safety and comfort, the switch to in-wheel motors will also increase the range per charge, which remains a major issue in EVs. Research results indicating the range improvement were presented by the laboratory of Associate Professor, Hiroshi Fujimotoof the University of Tokyo, Japan, which offers two proposals for extending range: in-wheel motors with different characteristics for the front and rear wheels, and optimization of drive balancing in cornering. Using in-wheel motors with different characteristics in front and rear wheels would extend range when used with control to optimize drive balancing (Fig. 8). Tests performed by the laboratory indicate that range could be extended by up to almost 30% per kWh.
Fig. 8 Varying Motor Characteristics to Extend Range
The Fujimoto Laboratory of the University of Tokyo is researching vehicle control using in-wheel motors (a). Motors with different characteristics are used in front and rear wheels, and controlled for maximum total efficiency (b, c). Tests showed efficiency was maximum with a ratio of 0.9 in front and 0.1 in rear (d). Diagram by Nikkei Electronics based on materials courtesy University of Tokyo.
In conventional vehicles the yaw moment required for turning is generated only for the steering angle of the front wheels, with in-wheel motors the steering angle of the front wheel is minimized and the moment provided by the difference in drive force between the rear wheels. This reduces cornering resistance caused by front wheel steering, improving efficiency in turns. Tests on a constant-speed circular course showed that the method extends range of 600m per kWh (300m in simulations).
High Hopes at Toyota Motor
Although in-wheel motors offer a number of advantages, they face difficulties in terms of size, weight and cost. Their shape makes it impossible to use existing front suspensions without modification, and the increase in mass below the springs degrades the ride. Mounting multiple motors demands a variety of changes in chassis design and control methods, not to mention the need to ensure reliability and durability: there is still a host of obstacles to commercial adoption.
EVs from major manufacturers, like the i-MiEV from Mitsubishi Motors Corp. of Japan and the Leaf from Nissan Motor Co., Ltd. of Japan, are single-motor designs. Mitsubishi Motors has built and extensively trialed prototypes with in-wheel motors, including Colt and Lancer models, but decided to leave them out of the i-MiEV.
At the Society of Automotive Engineers of Japan Annual Congress (Spring) held in May 2010, some papers seemed to offer solutions for the above problems. Two presentations on in-wheel motors by Toyota Motor drew praise from the audience, with one motor engineer present saying it revealed just how serious the company is about in-wheel motors.
Toyota eliminated the problems with large, heavy in-wheel motors by developing smaller designs with higher rpm (Fig. 9). Concretely, the motors are not positioned on the same axis as the wheels, but above them, rotating the wheels via a speed reduction mechanism. Motor speed has been boosted to well over 10,000 rpm, and passed through a combination counter/planetary gear speed reduction mechanism at a reduction ratio of 8.5, yielding considerable torque. This design provides the dimensions needed to finally use in-wheel motors with a double wishbone suspension.
Fig. 9 In-Wheel Motors Growing Smaller
Toyota Motor has developed an in-wheel motor than can be mounted in front wheels, where space is at a premium (a). The relationship between mass above and below the springs, which is an indicator of ride comfort, is about the same as in a standard front-wheel drive car (b). The new design is 36% lighter than the 1-motor system now used in EVs (c). Diagram by Nikkei Electronics based on material courtesy Toyota Motor.