The dilemma is simple to explain. World efforts to fight local weather change hinge on pivoting sharply away from fossil fuels. To do this would require electrifying transportation, primarily by shifting from automobiles with combustion engines to ones with electrical drive trains. Such a large shift will inevitably imply far larger use of electrical traction motors, almost all of which depend on magnets that comprise uncommon earth parts, which trigger substantial environmental degradation when their ores are extracted after which processed into industrially helpful kinds. And for automakers outdoors of China, there’s an extra deterrent: Roughly 90 % of processed uncommon earth parts now come from China, so for these corporations, growing dependence on uncommon earths means rising vulnerability in important provide chains.
Towards this backdrop, huge efforts are underway to design and check superior electric-vehicle (EV) motors that don’t use uncommon earth parts (or use comparatively little of them). Authorities companies, corporations, and universities are engaged on this problem, oftentimes in collaborative efforts, in just about all industrialized nations. In america, these initiatives embody long-standing efforts on the nation’s nationwide laboratories to develop everlasting magnets and motor designs that don’t use uncommon earth parts. Additionally, in a collaboration introduced final November,
Common Motors and Stellantis are working with a startup firm, Niron Magnetics, to develop EV motors primarily based on Niron’s uncommon earth–free everlasting magnet. One other automaker, Tesla, shocked observers in March of final yr when a senior official declared that the corporate’s “subsequent drive unit,” which might be primarily based on a everlasting magnet, would however use no “uncommon earth parts in any respect.” In Europe, a consortium known as Passenger contains 20 companions from business and academia engaged on uncommon earth–free everlasting magnets for EVs.
We’ve been working for almost a decade on magnetic and different elements of traction-motor design at
Oak Ridge Nationwide Laboratory (ORNL), in Tennessee, a hub of U.S. analysis on superior motors for EVs. Together with colleagues from the Nationwide Renewable Power Laboratory, Ames Laboratory, and the College of Wisconsin, Madison, we now have been finding out superior motor ideas as a part of the U.S. Division of Power’s U.S. Drive Applied sciences Consortium. The group additionally contains Sandia Nationwide Laboratories, Purdue College, and the Illinois Institute of Expertise.
With all of this exercise, you’d suppose that engineers would have by now developed a complicated understanding of what’s attainable with uncommon earth–free electrical motors. And certainly they’ve. We and different researchers are evaluating promising permanent-magnet supplies that don’t use uncommon earth parts, and we’re evaluating attainable motor-design modifications required to greatest use these supplies. We’re additionally evaluating superior motor designs that don’t use everlasting magnets in any respect. The underside line is that changing uncommon earth–primarily based magnets with non–uncommon earth ones comes at a price: degraded motor efficiency. However improvements in design, manufacturing, and supplies will be capable to offset—perhaps even solely—this hole in efficiency. Already, there are a number of stories of tantalizing outcomes with modern new motors whose efficiency is claimed to be on a par with the perfect permanent-magnet synchronous motors.
Why uncommon earths take advantage of highly effective electrical motors
Uncommon earth parts (which individuals in our line of labor usually check with as REEs) have distinctive properties that make them indispensable to many types of trendy know-how. A few of these parts, reminiscent of neodymium, samarium, dysprosium, and terbium, could be mixed with ferromagnetic parts reminiscent of iron and cobalt to supply crystals that aren’t solely extremely magnetic but in addition strongly resist demagnetization. The metric usually used to gauge these essential qualities of a magnet is known as the
most power product, measured in megagauss-oersteds (MGOe). The strongest and most commercially profitable everlasting magnets but invented, neodymium iron boron, have power merchandise within the vary of 30 to 55 MGOe.
For an electrical motor primarily based on everlasting magnets, the stronger its magnets, the extra environment friendly, compact, and light-weight the motor could be. So the highest-performing EV motors as we speak all use neodymium iron boron magnets. Nonetheless, intelligent motor design can scale back the efficiency hole between motors primarily based on uncommon earth everlasting magnets and ones primarily based on different varieties of magnets. To grasp how, you want to know slightly extra about electrical motors.
The most typical sort of traction motor in electrical automobiles is the interior-mount permanent-magnet synchronous motor. Everlasting magnets contained in the rotor work together with a rotating magnetic discipline created by electromagnet windings within the stator, which surrounds the rotor.Oak Ridge Nationwide Laboratory
There are two fundamental varieties of electrical motors: synchronous and induction. Most trendy electrical automobiles use a kind of synchronous motor that has a rotor outfitted with everlasting magnets. Induction motors use solely electromagnets and are subsequently inherently uncommon earth–free. However they don’t seem to be used as we speak in most EV fashions as a result of their efficiency is mostly not on a par with permanent-magnet synchronous motors, though a number of R&D initiatives in america, Europe, and Asia are attempting to enhance induction motors.
The time period “synchronous motors” refers to the truth that the rotor of the motor (the half that turns) rotates in synchrony with the altering magnetic fields produced by the stator (the half that is still stationary). Within the rotor, everlasting magnets are embedded in a circle across the construction. Within the stator, additionally in a round association, electromagnets are pulsed with electrical energy one after one other to arrange a rotating magnetic discipline. This course of causes the rotor magnets and stator magnets to draw and repel each other sequentially, producing rotation and torque.
Synchronous motors, too, fall into a number of classes. Two essential varieties are
surface-mount permanent-magnet synchronous motors and synchronous reluctance motors. Within the former group, everlasting magnets are mounted on the exterior floor of the rotor, and torque is produced as a result of completely different components of the stator and rotor both appeal to or repel. In a synchronous reluctance motor, however, the rotor doesn’t must have everlasting magnets in any respect. What makes the motor spin is a phenomenon known as magnetic reluctance, which refers to how a lot a cloth opposes magnetic flux passing by it. Ferromagnetic supplies have low values of reluctance and can are inclined to align themselves with robust magnetic fields. This phenomenon is exploited to trigger a ferromagnetic rotor, in a reluctance motor, to spin. (Some reluctance motors additionally make use of everlasting magnets to help that rotation.)
If a motor relies upon primarily on the interplay between the stator and rotor magnetic fields, it’s known as a permanent-magnet
dominated motor. If however it will depend on the torque produced by variations in reluctance, it’s a permanent-magnet assisted motor. The mixed use of each varieties of torque—that produced by the attraction and repulsion of everlasting magnets and that produced by the tendency of magnetic strains of power to stream alongside a path of least reluctance—is the important thing technique being utilized by engineers striving to realize excessive efficiency in a motor that’s much less reliant on REE magnets.
Changing REE-based magnets with non-REE ones comes at a price: degraded motor efficiency. However improvements in motor design, manufacturing, and supplies will be capable to offset—perhaps even solely—this hole in efficiency.
The most typical motor sort in the meanwhile combining the 2 sorts of torque is the
interior-mount permanent-magnet motor, by which the everlasting magnets embedded inside the rotor add to the reluctance torque. Many business EV producers, together with GM, Tesla, and Toyota, now use this kind of rotor design.
The design of the motors for the Toyota Prius underscores the effectiveness of this method. In these motors, the magnet mass decreased considerably over a interval of 13 years, from 1.2 kilograms within the 2004 Prius to about 0.5 kg within the 2017 Prius. A lot the identical occurred with the Chevrolet Bolt motor, which decreased the general utilization of magnet materials by 30 % in contrast with the motor in its predecessor, the Chevrolet Spark.
Wringing probably the most out of everlasting magnets with out uncommon earths
However what about eliminating REEs solely? Right here, there are two prospects: Use REE-free everlasting magnets in a motor designed to take advantage of them, or use a motor that dispenses with everlasting magnets solely, in favor of electromagnets.
To grasp the suitability of a specific REE-free everlasting magnet to be used in a robust traction motor, it’s important to take into account a few extra traits of a everlasting magnet: remanence and coercivity. To start with, recall the metric used to match the energy of various permanent-magnet supplies: most power product. These three parameters—most power product, remanence, and coercivity—largely point out how effectively a permanent-magnet materials will carry out in an electrical motor.
Remanence signifies the quantity of magnetic depth, as measured by the density of the strains of power, left in a everlasting magnet after the magnetic discipline that magnetized this magnet is withdrawn. Remanence is essential as a result of with out it you wouldn’t have a everlasting magnet. And the upper the remanence of the fabric, the stronger the forces of magnetic attraction and repulsion that create torque.
The coercivity of a everlasting magnet is a measure of its potential to withstand demagnetization. The upper the worth of coercivity, the more durable it’s to demagnetize the magnet with an exterior magnetic discipline. For an EV traction motor, an optimum everlasting magnet, reminiscent of neodymium iron boron, has excessive most power product, excessive remanence, and excessive coercivity. No REE-free everlasting magnet has all of those traits. So if you happen to substitute neodymium iron boron magnets with, say, ferrite magnets in a motor, you’ll be able to count on a lower in torque output and in addition should settle for a larger danger that the magnets will demagnetize throughout operation.
An experimental motor constructed by the authors at Oak Ridge Nationwide Laboratory didn’t use any heavy uncommon earth parts. Neodymium iron boron everlasting magnets are mounted on the exterior floor of the rotor. These magnets are represented by the teal-colored ring of blocks surrounding the copper-colored stator windings. To save lots of area, the motor’s inverter and management electronics have been put in contained in the stator.Oak Ridge Nationwide Laboratory
Motor engineers can decrease the distinction by designing a motor that exploits each everlasting magnets and reluctance. However even with a extremely optimized design, a motor primarily based on ferrite magnets will likely be significantly heavier—maybe a 3rd or extra—whether it is to realize the identical efficiency as a motor with uncommon earth magnets.
One method used to wring most efficiency out of ferrite magnets is to pay attention the flux from these magnets to the utmost extent attainable. It’s analogous to passing shifting water by a funnel: The water strikes quicker within the slender opening. Researchers have constructed such machines, known as spoke-ferrite magnet motors, however have discovered them to be about
30 % heavier than comparable motors primarily based on REE magnets. And there’s extra dangerous information: Spoke-type motors could be advanced to fabricate and pose mechanical challenges.
Some designers have proposed utilizing one other form of non-REE magnet, one made out of an aluminum nickel cobalt alloy known as alnico, generally used within the magnets that maintain fridge doorways shut. Though alnico magnets have excessive remanence, their coercivity is kind of low, making them susceptible to demagnetization.
To deal with this challenge, a number of researchers have studied and designed
variable-flux reminiscence motors, which use a magnetizing part of present to help in torque manufacturing, in impact holding the magnets from demagnetizing throughout operation. Moreover, researchers from the Ames Laboratory have proven that alnico magnets can have elevated coercivity whereas sustaining their excessive remanence.
Three parameters—most power product, remanence, and coercivity—largely point out how a everlasting magnet materials will carry out in an electrical motor.
Recently, there’s been loads of consideration targeted on a brand new sort of permanent-magnet materials, iron nitride (FeN). This magnet, produced by Niron Magnetics, has excessive remanence, equal to that of REE-magnets, however like alnico has low coercivity—
a couple of fifth of a comparable neodymium iron boron magnet. Due to these basically completely different properties, FeN magnets require the event of recent rotor designs, which is able to in all probability resemble these of previous alnico motors. Niron is now growing such designs with automotive companions, together with Common Motors.
One more REE-free permanent-magnet materials that comes up in discussions of future motors is manganese bismuth (MnBi), which has been the
topic of collaborative analysis on the College of Pittsburgh, Iowa State College, and Powdermet Inc. Collectively these engineers designed a surface-mount permanent-magnet synchronous motor utilizing MnBi magnets. The remanence and coercivity of those magnets is greater than ferrite magnets however decrease than neodymium iron boron (NdFeB). The researchers discovered {that a} MnBi-magnet motor can produce the identical torque output as a NdFeB-magnet motor however with substantial compromises: a whopping 60 % improve in quantity and a 65 % improve in weight. On the brilliant facet, the researchers steered that changing NdFeB magnets with MnBi magnets might scale back the general price of the motor by 32 %.
One other technique for decreasing uncommon earth content material in motors entails eliminating simply the heavy uncommon earth parts utilized in a few of these magnets. NdFeB magnets, for instance, usually comprise small quantities of the heavy uncommon earth factor dysprosium, used to extend their coercivity at excessive temperatures. (Heavy uncommon earth metals are usually in shorter provide than the sunshine uncommon earths, reminiscent of neodymium.) The rub with not utilizing them is that high-temperature coercivity then suffers.
So the most important problem in designing this type of motor is holding the rotor cool. Final yr, at Oak Ridge Nationwide Laboratory,
we developed a 100-kilowatt traction motor that makes use of no heavy uncommon earth parts in its magnets. One other good characteristic is that its energy electronics are built-in within it. These energy electronics included the inverter, which takes direct-current energy from the battery and feeds the motor with alternating present on the correct frequency to drive the machine.
We confronted a number of elementary challenges in holding the magnets from getting too scorching. You see, everlasting magnets are good conductors. And when {an electrical} conductor strikes in a magnetic discipline, which is what rotor magnets do whereas the motor is working, currents are induced in it. These currents, which don’t contribute to the torque, warmth up the magnets and might demagnetize them. One option to scale back this heating is to interrupt up the trail of the circulating currents by making the magnets from skinny segments which might be electrically insulated from each other. In our motor, every of those segments was only one millimeter thick.
We selected to make use of a grade of NdFeB magnets known as N50 that may function at temperatures as much as 80 °C. Additionally, we wanted to make use of a carbon-fiber-and-epoxy system to bolster the outer diameter of the rotor to let it spin at speeds as excessive as 20,000 rpm. After analyzing our motor prototype, we found it will be essential to power air by the motor to cut back its temperature when working at most pace. Whereas that’s not ultimate, it’s an affordable compromise to keep away from having to make use of heavy REEs within the design.
New approaches for superior motors
Maybe probably the most enticing near-term choice to make highly effective motors that lack REEs solely is to construct synchronous motors which have rotors outfitted with electromagnets (which means coils of wire), both with or with out ferrite magnets included with them. However doing that requires that you simply in some way cross electrical present to these spinning coils.
The normal answer is to make use of carbon brushes to make electrical contact with spinning steel rings, known as slip rings. This system permits you to apply direct present to the rotor to energise its electromagnets. These brushes produce mud, although, and finally put on out, so these motors aren’t appropriate to be used in EVs.
To deal with this challenge, engineers have devised what are known as
rotary transformers or exciters. They make use of an inductive or capacitive system to switch energy wirelessly to the spinning rotor. These motors have a terrific benefit over typical, permanent-magnet synchronous motors, which is that their rotor’s magnetic discipline could be exactly adjusted, just by controlling the present to the rotor’s electromagnets. That in flip permits a method known as discipline weakening, which permits excessive effectivity to be maintained by a variety of working speeds.
In the way in which they produce torque, synchronous electrical motor varieties could be considered current on a continuum between two completely different extremes. On the higher left on this chart is the floor permanent-magnet motor, which produces torque solely from the interplay between everlasting magnets within the rotor and electromagnets within the stator. On the decrease proper is the synchronous reluctance motor, which creates torque by exploiting a completely completely different phenomenon—magnetic reluctance, which refers to how a lot a cloth opposes magnetic flux passing by it. Most motor designs maximize torque by combining these two sorts of torque.Oak Ridge Nationwide Laboratory
A notable current instance is a motor constructed by the automotive provider ZF Group. Final yr the corporate introduced it had produced a synchronous motor by which electromagnets within the rotor are powered by an inductive system that matches contained in the machine’s rotor shaft. The 220-kW motor has power-density and effectivity traits on a par with these of the NdFeB permanent-magnet motors now utilized in EVs,
in keeping with an organization official.
New supplies may also assist bridge the hole between REE-magnet and non-REE-magnet motors. For instance, high-silicon metal, famend for its superior magnetic properties, emerges as a promising candidate for rotor development, providing the potential to enhance the magnetic effectivity of REE-free motors. Concurrently, utilizing
high-conductivity copper alloys or ultraconducting copper strands can significantly scale back electrical losses and enhance general efficiency. Doubling the conductivity of copper, for instance, might scale back the quantity of sure motors by 30 %. The strategic integration of such supplies might dramatically slender the efficiency hole between REE-containing and REE-free motors.
One other good instance of a sophisticated materials that would make an enormous distinction is a
dual-phase magnetic materials developed by GE Aerospace, which could be magnetized both very strongly or in no way in specified areas. By selectively making sure sections of the rotor nonmagnetic, the GE Aerospace group demonstrated that it’s attainable to remove just about all magnetic leakage, which in flip allowed them to forgo utilizing uncommon earth everlasting magnets within the motor.
How engineers will navigate the transition to REE-free motors
The transition towards uncommon earth–free motors for EVs is a serious and pivotal engineering endeavor. Will probably be tough, however analysis is starting to yield intriguing and inspiring outcomes. There’ll quickly be a number of designs obtainable—with, alas, a fancy array of trade-offs. Motor weight, energy density, price, manufacturability, and general efficiency dynamics will all be essential concerns. And success within the market will little question depend upon an equally advanced set of financial elements, so it’s very laborious to foretell which designs will dominate.
What’s turning into clear, although, is that it’s completely possible that REE-free motors will at some point turn into mainstream. That final result would require continued and concerted effort. However we see no motive why engineers can’t navigate the complexities of this transition, guaranteeing that the subsequent era of EVs is extra environmentally benign. Already, at ORNL and elsewhere, AI-enabled motor-design instruments are accelerating the event of those REE-free motors.
Right now, the large-scale use of REE magnets is marked by arguments pitting technological advantages towards environmental and moral concerns. Quickly, these arguments might be a lot much less related.
We’re not there but. As with every main technological transition, the journey to uncommon earth–free motors received’t be brief or straight. However will probably be a journey effectively value taking.
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