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.”
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.
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 newdesign 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 dualmotor (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.
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 gearchanges, as well as all-wheel-drive via fully-contained “e-axles.”
This article originally appeared in Automotive Engineering, an SAE International magazine. Click the link below to subscribe and learn more about Automotive Engineering magazine.
Bill Visnic is an editorial director for mobility media at SAE International. Lindsay Brooke is an editor in chief at SAE International.