It’s a pretty sure bet that you couldn’t get through a typical day without the direct support of dozens of electric motors. They’re in all of your appliances not powered by a hand crank, in the climate-control systems that keep you comfortable, and in the pumps, fans, and window controls of your car. And although there are many different kinds of electric motors, every single one of them, from the 200-kilowatt traction motor in your electric vehicle to the stepper motor in your quartz wristwatch, exploits the exact same physical phenomenon: electromagnetism.

For decades, however, engineers have been tantalized by the virtues of motors based on an entirely different principle: electrostatics. In some applications, these motors could offer an overall boost in efficiency ranging from 30 percent to close to 100 percent, according to experiment-based analysis. And, perhaps even better, they would use only cheap, plentiful materials, rather than the rare-earth elements, special steel alloys, and copious quantities of copper found in conventional motors.

“Electrification has its sustainability challenges,” notes Daniel Ludois, a professor of electrical engineering at the University of Wisconsin in Madison. But “an electrostatic motor doesn’t need windings, doesn’t need magnets, and it doesn’t need any of the critical materials that a conventional machine needs.”

Such advantages prompted Ludois to cofound a company, C-Motive Technologies, to build macro-scale electrostatic motors. “We make our machines out of aluminum and plastic or fiberglass,” he says. Their current prototype is capable of delivering torque as high as 18 newton meters and power at 360 watts (0.5 horsepower)—characteristics they claim are “the highest torque and power measurements for any rotating electrostatic machine.”

The results are reported in a paper, “Synchronous Electrostatic Machines for Direct Drive Industrial Applications,” to be presented at the 2024 IEEE Energy Conversion Congress and Exposition, which will be held from 20 to 24 October in Phoenix, Ariz. In the paper, Ludois and four colleagues describe an electrostatic machine they built, which they describe as the first such machine capable of “driving a load performing industrial work, in this case, a constant-pressure pump system.”

Making Electrostatic Motors Bigger

The machine, which is hundreds of times more powerful than any previous electrostatic motor, is “competitive with or superior to air-cooled magnetic machinery at the fractional [horsepower] scale,” the authors add. The global market for fractional horsepower motors is more than US $8.7 billion, according to consultancy Business Research Insights.

C-Motive’s 360-watt motor has a half dozen each of rotors and stators, shown in yellow in this cutaway illustration.C-Motive Technologies

Achieving macro scale wasn’t easy. Electrostatic motors have been available for years, but today, these are tiny units with power output measured in milliwatts. “Electrostatic motors are amazing once you get below about the millimeter scale, and they get better and better as they get smaller and smaller,” says Philip Krein, a professor of electrical engineering at the University of Illinois Urbana-Champaign. “There’s a crossover at which they are better than magnetic motors.” (Krein does not have any financial connection to C-Motive.)

For larger motors, however, the opposite is true. “At macro scale, electromagnetism wins, is the textbook answer,” notes Ludois. “Well, we’ve decided to challenge that wisdom.”

For this quest he and his team found inspiration in a lesser-known accomplishment of one of the United States’ founding fathers. “The fact is that Benjamin Franklin built and demonstrated a macroscopic electrostatic motor in 1747,” says Krein. “He actually used the motor as a rotisserie to grill a turkey on a riverbank in Philadelphia” (a fact unearthed by the late historian I. Bernard Cohen for his 1990 book Benjamin Franklin’s Science ).

Krein explains that the fundamental challenge in attempting to scale electrostatic motors to the macro world is energy density. “The energy density you can get in air at a reasonable scale with an electric-field system is much, much lower—many orders of magnitude lower—than the density you can get with an electromagnetic system.” Here the phrase “in air” refers to the volume within the motor, called the “air gap,” where the machine’s fields (magnetic for the conventional motor, electric for the electrostatic one) are deployed. It straddles the machine’s key components: the rotor and the stator.

Let’s unpack that. A conventional electric motor works because a rotating magnetic field, set up in a fixed structure called a stator, engages with the magnetic field of another structure called a rotor, causing that rotor to spin. The force involved is called the Lorentz force. But what makes an electrostatic machine go ‘round is an entirely different force, called the Coulomb force. This is the attractive or repulsive physical force between opposite or like electrical charges.

Overcoming the Air Gap Problem

C-Motive’s motor uses nonconductive rotor and stator disks on which have been deposited many thin, closely spaced conductors radiating outward from the disk’s center, like spokes in a bicycle wheel. Precisely timed electrostatic charges applied to these “spokes” create two waves of voltage, one in the stator and another in the rotor. The phase difference between the rotor and stator waves is timed and controlled to maximize the torque in the rotor caused by this sequence of attraction and repulsion among the spokes. To further wring as much torque as possible, the machine has half a dozen each of rotors and stators, alternating and stacked like compact discs on a spindle.

The 360-watt motor is hundreds of times more powerful than previous electrostatic motors, which have power output generally measured in milliwatts.C-Motive Technologies

The machine would be feeble if the dielectric between the charges was air. As a dielectric, air has low permittivity, meaning that an electric field in air can not store much energy. Air also has a relatively low breakdown field strength, meaning that air can support only a fairly weak electric field before it breaks down and conducts current in a blazing arc. So one of the team’s greatest challenges was producing a dielectric fluid that has a much higher permittivity and breakdown field strength than air, and that was also environmentally friendly and nontoxic. To minimize friction, this fluid also had to have very low viscosity, because the rotors would be spinning in it. A dielectric with high permittivity concentrates the electric field between oppositely charged electrodes, enabling greater energy to be stored in the space between them. After screening hundreds of candidates over several years, the C-Motive team succeeded in producing an organic liquid dielectric with low viscosity and a relative permittivity in the low 20s. For comparison, the relative permittivity of air is 1.

Another challenge was supplying the 2,000 volts their machine needs to operate. High voltages are necessary to create the intense electric fields between the rotors and stators. To precisely control these fields, C-Motive was able to take advantage of the availability of inexpensive and stupendously capable power electronics, according to Ludois. For their most recent motor, they developed a drive system based on readily available 4.5-kilovolt insulated-gate bipolar transistors, but the rate of advancement in power semiconductors means they have many attractive choices here, and will have even more in the near future.

Ludois reports that C-Motive is now testing a 750-watt (1 hp) motor in applications with potential customers. Their next machines will be in the range of 750 to 3,750 watts (1 to 5 hp), he adds. These will be powerful enough for an expanded range of applications in industrial automation, manufacturing, and heating, ventilating, and air conditioning.

It’s been a gratifying ride for Ludois. “For me, a point of creative pride is that my team and I are working on something radically different that, I hope, over the long term, will open up other avenues for other folks to contribute.”

The dilemma is easy to describe. Global efforts to combat climate change hinge on pivoting sharply away from fossil fuels. To do that will require electrifying transportation, primarily by shifting from vehicles with combustion engines to ones with electric drive trains. Such a massive shift will inevitably mean far greater use of electric traction motors, nearly all of which rely on magnets that contain rare earth elements, which cause substantial environmental degradation when their ores are extracted and then processed into industrially useful forms. And for automakers outside of China, there is an additional deterrent: Roughly 90 percent of processed rare earth elements now come from China, so for these companies, increasing dependence on rare earths means growing vulnerability in critical supply chains.

Against this backdrop, massive efforts are underway to design and test advanced electric-vehicle (EV) motors that do not use rare earth elements (or use relatively little of them). Government agencies, companies, and universities are working on this challenge, oftentimes in collaborative efforts, in virtually all industrialized countries. In the United States, these initiatives include long-standing efforts at the country’s national laboratories to develop permanent magnets and motor designs that do not use rare earth elements. Also, in a collaboration announced last November, General Motors and Stellantis are working with a startup company, Niron Magnetics, to develop EV motors based on Niron’s rare earth–free permanent magnet. Another automaker, Tesla, shocked observers in March of last year when a senior official declared that the company’s “next drive unit,” which would be based on a permanent magnet, would nevertheless use no “rare earth elements at all.” In Europe, a consortium called Passenger includes 20 partners from industry and academia working on rare earth–free permanent magnets for EVs.

We have been working for nearly a decade on magnetic and other aspects of traction-motor design at Oak Ridge National Laboratory (ORNL), in Tennessee, a hub of U.S. research on advanced motors for EVs. Along with colleagues from the National Renewable Energy Laboratory, Ames Laboratory, and the University of Wisconsin, Madison, we have been studying advanced motor concepts as part of the U.S. Department of Energy’s U.S. Drive Technologies Consortium. The group also includes Sandia National Laboratories, Purdue University, and the Illinois Institute of Technology.

With all of this activity, you would think that engineers would have by now developed a sophisticated understanding of what is possible with rare earth–free electric motors. And indeed they have. We and other researchers are evaluating promising permanent-magnet materials that don’t use rare earth elements, and we are evaluating possible motor-design changes required to best use these materials. We are also evaluating advanced motor designs that do not use permanent magnets at all. The bottom line is that replacing rare earth–based magnets with non–rare earth ones comes at a cost: degraded motor performance. But innovations in design, manufacturing, and materials will be able to offset—maybe even entirely—this gap in performance. Already, there are a few reports of tantalizing results with innovative new motors whose performance is said to be on a par with the best permanent-magnet synchronous motors.

Why rare earths make the most powerful electric motors

Rare earth elements (which people in our line of work often refer to as REEs) have unique properties that make them indispensable to many forms of modern technology. Some of these elements, such as neodymium, samarium, dysprosium, and terbium, can be combined with ferromagnetic elements such as iron and cobalt to produce crystals that are not only highly magnetic but also strongly resist demagnetization. The metric typically used to gauge these important qualities of a magnet is called the maximum energy product, measured in megagauss-oersteds (MGOe). The strongest and most commercially successful permanent magnets yet invented, neodymium iron boron, have energy products in the range of 30 to 55 MGOe.

For an electric motor based on permanent magnets, the stronger its magnets, the more efficient, compact, and lightweight the motor can be. So the highest-performing EV motors today all use neodymium iron boron magnets. Nevertheless, clever motor design can reduce the performance gap between motors based on rare earth permanent magnets and ones based on other types of magnets. To understand how, you need to know a little more about electric motors.

The most common type of traction motor in electric vehicles is the interior-mount permanent-magnet synchronous motor. Permanent magnets inside the rotor interact with a rotating magnetic field created by electromagnet windings in the stator, which surrounds the rotor.Oak Ridge National Laboratory

There are two basic types of electric motors: synchronous and induction. Most modern electric vehicles use a type of synchronous motor that has a rotor equipped with permanent magnets. Induction motors use only electromagnets and are therefore inherently rare earth–free. But they are not used today in most EV models because their performance is generally not on a par with permanent-magnet synchronous motors, although several R&D projects in the United States, Europe, and Asia are trying to improve induction motors.

The term “synchronous motors” refers to the fact that the rotor of the motor (the part that turns) rotates in synchrony with the changing magnetic fields produced by the stator (the part that remains stationary). In the rotor, permanent magnets are embedded in a circle around the structure. In the stator, also in a circular arrangement, electromagnets are pulsed with electricity one after another to set up a rotating magnetic field. This process causes the rotor magnets and stator magnets to attract and repel one another sequentially, producing rotation and torque.

Synchronous motors, too, fall into several categories. Two important types are surface-mount permanent-magnet synchronous motors and synchronous reluctance motors. In the former group, permanent magnets are mounted on the external surface of the rotor, and torque is produced because different parts of the stator and rotor either attract or repel. In a synchronous reluctance motor, on the other hand, the rotor doesn’t need to have permanent magnets at all. What makes the motor spin is a phenomenon called magnetic reluctance, which refers to how much a material opposes magnetic flux passing through it. Ferromagnetic materials have low values of reluctance and will tend to align themselves with strong magnetic fields. This phenomenon is exploited to cause a ferromagnetic rotor, in a reluctance motor, to spin. (Some reluctance motors also employ permanent magnets to assist that rotation.)

If a motor depends mainly on the interaction between the stator and rotor magnetic fields, it is called a permanent-magnet dominated motor. If on the other hand it depends on the torque produced by differences in reluctance, it is a permanent-magnet assisted motor. The combined use of both types of torque—that produced by the attraction and repulsion of permanent magnets and that produced by the tendency of magnetic lines of force to flow along a path of least reluctance—is the key strategy being used by engineers striving to achieve high performance in a motor that is less reliant on REE magnets.

Replacing REE-based magnets with non-REE ones comes at a cost: degraded motor performance. But innovations in motor design, manufacturing, and materials will be able to offset—maybe even entirely—this gap in performance.

The most common motor type at the moment combining the two kinds of torque is the interior-mount permanent-magnet motor, in which the permanent magnets embedded within the rotor add to the reluctance torque. Many commercial EV manufacturers, including GM, Tesla, and Toyota, now use this type of rotor design.

The design of the motors for the Toyota Prius underscores the effectiveness of this approach. In these motors, the magnet mass decreased significantly over a period of 13 years, from 1.2 kilograms in the 2004 Prius to about 0.5 kg in the 2017 Prius. Much the same occurred with the Chevrolet Bolt motor, which reduced the overall usage of magnet material by 30 percent compared with the motor in its predecessor, the Chevrolet Spark.

Wringing the most out of permanent magnets without rare earths

But what about getting rid of REEs entirely? Here, there are two possibilities: Use REE-free permanent magnets in a motor designed to make the most of them, or use a motor that dispenses with permanent magnets entirely, in favor of electromagnets.

To understand the suitability of a particular REE-free permanent magnet for use in a powerful traction motor, you have to consider a couple of additional characteristics of a permanent magnet: remanence and coercivity. To begin with, recall the metric used to compare the strength of different permanent-magnet materials: maximum energy product. These three parameters—maximum energy product, remanence, and coercivity—largely indicate how well a permanent-magnet material will perform in an electric motor.

Remanence indicates the amount of magnetic intensity, as measured by the density of the lines of force, left in a permanent magnet after the magnetic field that magnetized this magnet is withdrawn. Remanence is important because without it you wouldn’t have a permanent magnet. And the higher the remanence of the material, the stronger the forces of magnetic attraction and repulsion that create torque.

The coercivity of a permanent magnet is a measure of its ability to resist demagnetization. The higher the value of coercivity, the harder it is to demagnetize the magnet with an external magnetic field. For an EV traction motor, an optimal permanent magnet, such as neodymium iron boron, has high maximum energy product, high remanence, and high coercivity. No REE-free permanent magnet has all of these characteristics. So if you replace neodymium iron boron magnets with, say, ferrite magnets in a motor, you can expect a decrease in torque output and also must accept a greater risk that the magnets will demagnetize during operation.

An experimental motor built by the authors at Oak Ridge National Laboratory did not use any heavy rare earth elements. Neodymium iron boron permanent magnets are mounted on the external surface of the rotor. These magnets are represented by the teal-colored ring of blocks surrounding the copper-colored stator windings. To save space, the motor’s inverter and control electronics were installed inside the stator.Oak Ridge National Laboratory

Motor engineers can minimize the difference by designing a motor that exploits both permanent magnets and reluctance. But even with a highly optimized design, a motor based on ferrite magnets will be considerably heavier—perhaps a third or more—if it is to achieve the same performance as a motor with rare earth magnets.

One technique used to wring maximum performance out of ferrite magnets is to concentrate the flux from those magnets to the maximum extent possible. It’s analogous to passing moving water through a funnel: The water moves faster in the narrow opening. Researchers have built such machines, called spoke-ferrite magnet motors, but have found them to be about 30 percent heavier than comparable motors based on REE magnets. And there’s more bad news: Spoke-type motors can be complex to manufacture and pose mechanical challenges.

Some designers have proposed using another kind of non-REE magnet, one made from an aluminum nickel cobalt alloy called alnico, commonly used in the magnets that hold refrigerator doors shut. Although alnico magnets have high remanence, their coercivity is quite low, making them prone to demagnetization.

To address this issue, several researchers have studied and designed variable-flux memory motors, which use a magnetizing component of current to aid in torque production, in effect keeping the magnets from demagnetizing during operation. Additionally, researchers from the Ames Laboratory have shown that alnico magnets can have increased coercivity while maintaining their high remanence.

Three parameters—maximum energy product, remanence, and coercivity—largely indicate how a permanent magnet material will perform in an electric motor.

Lately, there’s been a lot of attention focused on a new type of permanent-magnet material, iron nitride (FeN). This magnet, produced by Niron Magnetics, has high remanence, equivalent to that of REE-magnets, but like alnico has low coercivity— about a fifth of a comparable neodymium iron boron magnet. Because of these fundamentally different properties, FeN magnets require the development of new rotor designs, which will probably resemble those of past alnico motors. Niron is now developing such designs with automotive partners, including General Motors.

Yet another REE-free permanent-magnet material that comes up in discussions of future motors is manganese bismuth (MnBi), which has been the subject of collaborative research at the University of Pittsburgh, Iowa State University, and Powdermet Inc. Together these engineers designed a surface-mount permanent-magnet synchronous motor using MnBi magnets. The remanence and coercivity of these magnets is higher than ferrite magnets but lower than neodymium iron boron (NdFeB). The researchers found that a MnBi-magnet motor can produce the same torque output as a NdFeB-magnet motor but with substantial compromises: a whopping 60 percent increase in volume and a 65 percent increase in weight. On the bright side, the researchers suggested that replacing NdFeB magnets with MnBi magnets could reduce the overall cost of the motor by 32 percent.

Another strategy for reducing rare earth content in motors involves eliminating just the heavy rare earth elements used in some of these magnets. NdFeB magnets, for example, typically contain small amounts of the heavy rare earth element dysprosium, used to increase their coercivity at high temperatures. (Heavy rare earth metals are generally in shorter supply than the light rare earths, such as neodymium.) The rub with not using them is that high-temperature coercivity then suffers.

So the major challenge in designing this kind of motor is keeping the rotor cool. Last year, at Oak Ridge National Laboratory, we developed a 100-kilowatt traction motor that uses no heavy rare earth elements in its magnets. Another nice feature is that its power electronics are integrated inside of it. These power electronics included the inverter, which takes direct-current power from the battery and feeds the motor with alternating current at the proper frequency to drive the machine.

We faced several fundamental challenges in keeping the magnets from getting too hot. You see, permanent magnets are good conductors. And when an electrical conductor moves in a magnetic field, which is what rotor magnets do while the motor is operating, currents are induced in it. These currents, which do not contribute to the torque, heat up the magnets and can demagnetize them. One way to reduce this heating is to break up the path of the circulating currents by making the magnets from thin segments that are electrically insulated from one another. In our motor, each of these segments was only 1 millimeter thick.

We chose to use a grade of NdFeB magnets called N50 that can operate at temperatures up to 80 °C. Also, we needed to use a carbon-fiber-and-epoxy system to reinforce the outer diameter of the rotor to let it spin at speeds as high as 20,000 rpm. After analyzing our motor prototype, we discovered it would be necessary to force air through the motor to reduce its temperature when operating at maximum speed. While that’s not ideal, it’s a reasonable compromise to avoid having to use heavy REEs in the design.

New approaches for advanced motors

Perhaps the most attractive near-term option to make powerful motors that lack REEs entirely is to build synchronous motors that have rotors equipped with electromagnets (meaning coils of wire), either with or without ferrite magnets included with them. But doing that requires that you somehow pass electrical current to those spinning coils.

The traditional solution is to use carbon brushes to make electrical contact with spinning metal rings, called slip rings. This technique allows you to apply direct current to the rotor to energize its electromagnets. Those brushes produce dust, though, and eventually wear out, so these motors aren’t suitable for use in EVs.

To address this issue, engineers have devised what are called rotary transformers or exciters. They employ an inductive or capacitive system to transfer power wirelessly to the spinning rotor. These motors have a great advantage over conventional, permanent-magnet synchronous motors, which is that their rotor’s magnetic field can be precisely adjusted, simply by controlling the current to the rotor’s electromagnets. That in turn permits a technique called field weakening, which allows high efficiency to be maintained through a wide range of operating speeds.

In the way they produce torque, synchronous electric motor types can be thought of as existing on a continuum between two different extremes. At the upper left in this chart is the surface permanent-magnet motor, which produces torque solely from the interaction between permanent magnets in the rotor and electromagnets in the stator. At the lower right is the synchronous reluctance motor, which creates torque by exploiting an entirely different phenomenon—magnetic reluctance, which refers to how much a material opposes magnetic flux passing through it. Most motor designs maximize torque by combining these two kinds of torque.Oak Ridge National Laboratory

A notable recent example is a motor built by the automotive supplier ZF Group. Last year the company announced it had produced a synchronous motor in which electromagnets in the rotor are powered by an inductive system that fits inside the machine’s rotor shaft. The 220-kW motor has power-density and efficiency characteristics on a par with those of the NdFeB permanent-magnet motors now used in EVs, according to a company official.

New materials can also help bridge the gap between REE-magnet and non-REE-magnet motors. For example, high-silicon steel, renowned for its superior magnetic properties, emerges as a promising candidate for rotor construction, offering the potential to improve the magnetic efficiency of REE-free motors. Concurrently, using high-conductivity copper alloys or ultraconducting copper strands can greatly reduce electrical losses and improve overall performance. Doubling the conductivity of copper, for example, could reduce the volume of certain motors by 30 percent. The strategic integration of such materials could dramatically narrow the performance gap between REE-containing and REE-free motors.

Another good example of an advanced material that could make a big difference is a dual-phase magnetic material developed by GE Aerospace, which can be magnetized either very strongly or not at all in specified areas. By selectively making certain sections of the rotor nonmagnetic, the GE Aerospace team demonstrated that it is possible to eliminate virtually all magnetic leakage, which in turn allowed them to forgo using rare earth permanent magnets in the motor.

How engineers will navigate the transition to REE-free motors

The transition toward rare earth–free motors for EVs is a major and pivotal engineering endeavor. It will be difficult, but research is beginning to yield intriguing and encouraging results. There will soon be multiple designs available—with, alas, a complex array of trade-offs. Motor weight, power density, cost, manufacturability, and overall performance dynamics will all be important considerations. And success in the marketplace will no doubt depend on an equally complex set of economic factors, so it’s very hard to predict which designs will dominate.

What’s becoming clear, though, is that it’s perfectly feasible that REE-free motors will one day become mainstream. That outcome will require continued and concerted effort. But we see no reason why engineers can’t navigate the complexities of this transition, ensuring that the next generation of EVs is more environmentally benign. Already, at ORNL and elsewhere, AI-enabled motor-design tools are accelerating the development of these REE-free motors.

Today, the large-scale use of REE magnets is marked by arguments pitting technological benefits against environmental and ethical considerations. Soon, those arguments could be much less relevant.

We’re not there yet. As with any major technological transition, the journey to rare earth–free motors won’t be short or straight. But it will be a journey well worth taking.