新设计、新材料将是下一代电动汽车动力电机的关键。

如今，汽车电气化需求稳步增长，可能创造数十亿甚至上百亿美元的业务，汽车行业对此已经开始为汽车传动系统的转型布局。

福特公司表示，公司计划在 2022 年前推出大约 40 款电动车型，其中包括 16款全新的纯电动汽车。本田预计，到 2030 年，电动汽车将占该公司全球销售额的三分之二。通用汽车公司计划到 2023 年在全球范围内推出 20 款电动车型。传统豪车品牌法拉利也加入了汽车电气化的大军。我们不难从这些全球主要汽车制造商的行动中看出，推进系统的电气化几年后将不再是新事物。此后，各家厂商将专注于发展可以提高推进效率的设计、工艺和制造等关键竞争优势。

举个例子，为了在电推进系统的完整价值链上（即从电池组到动力电子组件再到电机）掌握主动权，博世在 2019 年初接手了其与戴姆勒合资的电机研发公司 EM-motiv 的全部所有权和控制权。博世认为，仅对电动汽车的系统性热管理这一项进行优化，即有潜力将电动汽车的续航里程提高 20%。

博世公司在最近的一次发布中表示，“归根结底，电气化交通出行取得突破的关键在于以低廉的成本提供可以满足需求的续航里程。”

定制设计

电动汽车的评价标准往往集中于电池容量，然而驱动电机也同样重要，重要程度甚至堪比传统汽车的发动机。事实上，电机功率和效率是相互关联的，因此根据不同车辆推进系统定制不同功率和效率的电机至关重要，但也同时需要大量投资。

目前，汽车行业中主要采用“永磁”和“感应”两种主要交流牵引电机，两者各有利弊。许多汽车制造商和供应商都青睐于工作效率更高的永磁电机，其中几家最著名的制造商包括本田、丰田、通用和宝马。

对比来说，交流感应电机的效率较低，但也凭借更高的最高功率输出赢得了一些支持者，比如电动汽车领头企业特斯拉。目前，特斯拉的高性能大尺寸 Model S 和 Model X 系列均采用了交流感应电机，但最近推出的紧凑型 Model 3 却采用了永磁驱动电机。

过去，大多数观点认为感应电机更适合尺寸更大、重量更重或对性能要求更高的电动汽车。事实上，永磁电机的应用也并非仅限于一些尺寸更小且更注重效率的车型。目前，尽管电动汽车初创公司 Rivian 并未透露旗下 R1T 电动皮卡和 R1S 运动多功能车的具体规格，但一位公司发言人向《汽车工程杂志》证实，这两款车型的每个车轮均配备了独立的电机，且这些电机采用了永磁设计。

探索新材料

通常来说，永磁电机生产经常受制于重稀土元素的供应，正因如此，一些高产量制造商一直对永磁电机“敬而远之”。目前，全球重稀土元素供应主要来自中国。据估计，中国的稀土储量占全球的35-40%，比如钕和镝，而这两种稀土元素均对各类磁性产品的生产非常重要。

通常来讲，汽车牵引电机中使用的磁体需要较高的矫顽力，也就是磁体在车辆常见高温环境下保持磁性的能力，而磁体的这种性能主要与其中含有的稀土材料有关。通常来说，磁体中 30% 的原材料都是稀土。

2016 年年中，本田汽车有限公司和大东钢铁有限公司（Daido Steel）共同发布了全球首款专为电动汽车量产而研发的新型磁体材料—热变形钕。该材料将首次用于 2017 款 2017 FreedSport Hybrid 混合动力小货车搭载的新型永磁牵引电机。

具体来说，这种新型热变形钕材料不需要注入镝或铽等“重”稀土元素，即可优化对牵引电机至关重要的一项指标——高耐热性。

本田最新款 Insight 和 Accord 雅阁混合动力车型均采用了旗下第三代双电机（牵引电机和发电机）混合动力设计；据本田透露，这两台电机所用磁体均不使用重稀土金属。据称，Insight 混合动力汽车的牵引电机可提供129 hp 和267 N·m的动力输出。

无独有偶，丰田也曾在去年表示，公司已经开发了一种钕元素含量显著降低的新型耐热电机磁铁。丰田在新闻稿中表示：“这种新型磁体可在高温条件中使用，且其中钕稀土元素的用量明显减少。”

丰田补充道，这种新型磁体并不使用“高耐热钕磁体所必需的”铽或镝元素，“我们采用了成本更低的镧和铈进行替代，从而减少了磁体中钕的用量。”

丰田工程师认为，镧和铈的储量丰富且成本低廉，但同样可以帮助磁体保持高耐热性和高矫顽力。

美国能源部高级研究项目机构 ARPA-e 也启动了“REACT 关键技术中的稀土替代材料计划”（Rare EarthAlternatives in Critical Technologies），以开发成本更低、供应更可靠的稀土替代品。过去十年里，REACT 项目已资助了数个使用非稀土磁体的电动汽车研发项目。

随着电机技术的不断发展，未来的研发重点必定在于提高效率、功率和可靠性。本田、通用汽车（Bolt 电动汽车）等公司已经从为定子绕组配备的方截面导线得到了可靠的结果—方形导线的“排布”更加高效，可以提高给定区域的密度。一些消息来源称，绕组技术也会对电机的功率输出和效率产生重大影响。

安装设计

在纯电动汽车应用中，牵引电机通常用于驱动车轴，另外在某些情况下也会直接驱动某个单独车轮，比如 Rivian 的电动汽车设计。不过，牵引电机在混动汽车中的应用场景则更加多样。

在早期的“轻”电气化设计中，开发人员通常将牵引电机/发电机单元安装在发动机曲柄的前部，也就是所谓的 P0 位置，具体由驱动轴连接。电机可以逐渐移回至传动系统，逐步对发动机曲轴或驱动轮施加更大影响。P3 位置将电机集成到变速器中，而 P4 位置则意味着电机将驱动一根并未与内燃机机械相连的轴。

据了解，Rivian 电动汽车的轮内牵引电机外壳来自 Protean Electric 公司，其 Pd16 和 Pd18 车轮电机系统已经与轮辋集成封装，永磁同步电机则直接集成在外部转子内。电源和控制电子单元也进行了集成。Protean 的车轮电机瞄准自动驾驶大巴的应用场景，Pd18 则用于 Local Motors 的“Olli”自动驾驶穿梭大巴。

作为传统汽车向电动汽车过渡的必要桥梁，电机的重要性不言而喻，因此各大厂商均发力电机设计，推出了功能非常完善的电机。现阶段，很多电机均可以通过不同“P”级，实现不同程度的效率和性能提升，取得比如驱动去耦“航行”、扭矩“填充”等功能，从而缓解发动机增压滞后或换挡卡顿等问题，通过车辆“电子轴”实现车辆的全轮驱动。

New designs and materials are key to the next generation of electric machines for EV propulsion.

With the momentum to expand vehicle electrification increasing steadily, the industry is beginning to arrange the pieces for its multi-billion-dollar transformation of powertrain development.

Ford said it intends to have some 40 electrified models in showrooms by 2022, including 16 all-new battery-electric vehicles. Honda projects electrified vehicles will account for two-thirds of the company’s global sales by 2030. General Motors plans 20 electrified models globally by 2023. Even Ferrari is joining the march. As the list grows, it’s clear that in a few years, propulsion-system electrification no longer will be news per se. The dialogue will then shift to key differentiators in design, engineering and manufacturing that impact efficiency.

Bosch, for instance, in early 2019 assumed full ownership and control of its EM-motiv electric-motor development joint venture with Daimler, as the supplier seeks to manage the full value chain of electric propulsion—from battery pack to power electronics to motors. Optimization of system thermal management alone, the company believes, can increase an electric vehicle’s (EV’s) range by as much as 20%.

“In the end,” said Bosch in a recent release, “affordable [driving] range is the key to helping electromobility achieve a breakthrough.”

Tailored designs

For EVs, the discussion often focuses on battery capacity, but the drive motor is as much a factor as the engine is in a conventional powertrain. Electric-machine power and efficiency are mutually related—and how those characteristics are tailored for automotive propulsion is a matter of widening engineering investment.

The two primary types of alternating-current (AC) traction motors, permanent-magnet and induction, have advantages and limitations for automotive applications. Many automakers and suppliers have favored permanent-magnet motors because they typically are inherently more efficient. Honda, Toyota, GM and BMW, as well as many major suppliers, currently use permanent-magnet motors in production vehicles.

AC induction motors may be preferable if high power output is a factor, but they are less efficient. Tesla, which many consider a bellwether of EV technology and development, uses AC induction motors for its larger and more performance-oriented Model S and Model X vehicles, but elected permanent-magnet drive motors for its most recent (and smaller) Model 3.

Many in the past have viewed induction motors as more aligned with EVs that are either larger and heavier or are focused on high performance, but permanent-magnet motors are not limited to smaller, efficiency-focused vehicles. Although EV startup Rivian has disclosed scant specifics regarding its intriguing new platform for its R1T electric pickup truck and R1S sport-utility, a company spokesperson did confirm to Automotive Engineering that its drive motors—one for each wheel, combined in a unique integrated twin-motor/transmission housing for the front and rear—are permanent-magnet design.

Materials quest

High-volume manufacturers have been wary of permanent-magnet motors because of their traditional reliance on heavy rare-earth elements. The preponderance of these materials currently comes from China, which is estimated to hold 35-40% of the world reserves of rare earths such as neodymium and dysprosium. Both are critical to all manner of magnetic products.

Magnets used in automotive traction motors typically aim for high coercivity, or the ability to maintain magnetization, at the high temperatures that can be common in automotive applications. The rare-earth materials impart added coercivity; often around 30% of the elements used in magnets are rare earths.

In mid-2016, Honda Motor Co. and Daido Steel Ltd. announced the first production application of a new magnet material for EVs. That material was hot-deformed neodymium and it was first used for a new-design permanent-magnet traction motor for the 2017 Freed Sport Hybrid compact minivan.

The hot-deformed neodymium doesn’t require infusion with dysprosium or terbium “heavy” rare earths to achieve the high heat-resistance characteristic vital to traction motors.

Honda’s latest Insight and Accord Hybrid models employ the third generation of the company’s dual- motor (traction motor and generator) hybrid design; the magnets for both motors, the company said, use no heavy rare-earth metals. For the Insight Hybrid, the traction motor develops a claimed 129 hp and 197 lb·ft (267 N·m).

In a similar vein, Toyota said last year it had developed a new neodymium-reduced, heat-resistant magnet for electric motors. “The new magnet uses significantly less neodymium, a rare-earth element, and can be used in high-temperature conditions,” the company said in a release.

The new magnets use no terbium or dysprosium “necessary for highly heat-resistant neodymium magnets,” Toyota said, adding, “A portion of the neodymium has been replaced with lanthanum and cerium, which are low-cost rare earths, reducing the amount of neodymium used in the magnet.”

Use of lanthanum and cerium—both abundant and low-cost rare earths— enables high heat resistance to be maintained and loss of coercivity minimized, Toyota engineers believe.

In the U.S., the Advanced Research Project Agency-Energy (ARPA-e, a part of the U.S. Dept. of Energy) started its REACT (Rare Earth Alternatives in Critical Technologies) program to develop low- cost, reliable alternatives for rare earths. The REACT program in the last decade has helped fund several development efforts for EV motors using non-rare-earth magnets.

Deeper engineering of every aspect of motor design is certain to improve efficiency, power and reliability. Honda, GM (in its Bolt EV) and others have gleaned solid results from using square-cross-section wire for stator windings because it was determined the square wire “nests” more effectively, providing increased density for the given area. And winding technique, some sources say, also can have a significant impact on motor output and efficiency.

Placement options

For pure EVs, traction motors typically drive an axle, or in some cases such as Rivian’s, individual wheels. But for hybridization, there are numerous choices for where in the drivetrain the electric motor can do its work.

Early efforts for “mild” electrification have placed motor/generator units to act on the front of the engine crankshaft, typically linked by a drive belt, for a so-called “P0” location. The electric machine can be progressively moved back in the drivetrain, generally to impart increasing degrees of influence on the engine crankshaft or the drive wheels. A P3 location integrates the electric machine into the transmission, while a P4 location insinuates an electric motor driving an axle not mechanically connected to the combustion engine.

The case for in-wheel traction motors is made by Protean Electric, whose Pd16 and Pd18 wheel-motor systems are packaged with the road wheel rim—the permanent-magnet synchronous machine is contained in the outer rotor. Power and control electronics are also integrated into the units. Protean is aiming its wheel motors at autonomous shuttle applications, and the Pd18 is used in ‘Olli,’ Local Motors’ self-driving shuttle.

As a bridge to EVs, industry sources project increasingly sophisticated designs for incorporating electrification into conventional drivelines. As electric motors progress through the various “P” stages, the corresponding benefits are efficiency- and performance-enhancing features such as drive-decoupling “sailing,” torque “fill” to mask lag in engine boost and smooth gear changes, as well as all-wheel-drive via fully-contained “e-axles.”