Among the countless challenges of decarbonizing transportation, one of the most compelling involves electric motors. In laboratories all over the world, researchers are now chasing a breakthrough that could kick into high gear the transition to electric transportation: a rugged, compact, powerful electric motor that has high power density and the ability to withstand high temperatures—and that doesn’t have rare-earth permanent magnets.
It’s a huge challenge currently preoccupying some of the best machine designers on the planet. More than a few of them are at ZF Friedrichshafen AG, one of the world’s largest suppliers of parts to the automotive industry. In fact, ZF astounded analysts late last year when it announced that it had built a 220-kilowatt traction motor that used no rare-earth elements. Moreover, the company announced, their new motor had characteristics comparable to the rare-earth permanent-magnet synchronous motors that now dominate in electric vehicles. Most EVs have rare-earth-magnet-based motors between 150 and 300 kilowatts, and power densities ranging from 1.1 and 3.0 kilowatts per kilogram. Meanwhile, the company says they’ve developed a rare-earth-free motor right in the middle of that range: 220 kW. (The comany has not yet revealed its motor’s specific power score—its kW/kg rating.)
The ZF machine is a type called a separately-excited (or doubly-excited) synchronous motor. It has electromagnets in both the stator and the rotor, so it does away with the rare-earth permanent magnets used in the rotors of nearly all EV motors on the road today. In a separately-excited synchronous motor, alternating current applied to the stator electromagnets sets up a rotating magnetic field. A separate current applied to the rotor electromagnets energizes them, producing a field that locks on to the rotating stator field, producing torque.
“As a matter of fact, 95 percent of the rare earths are mined in China. And this means that if China decides no one else will have rare earths, we can do nothing against it.” —Otmar Scharrer, ZF Friedrichshafen AG
So far, these machines have not been used much in EVs, because they require a separate system to transfer power to the spinning rotor magnets, and there’s no ideal way to do that. Many such motors use sliders and brushes to make electrical contact to a spinning surface, but the brushes produce dust and eventually wear out. Alternatively, the power can be transferred via inductance, but in that case the apparatus is typically cumbersome, making the unit complicated and physically large and heavy.
Now, though, ZF says it has solved these problems with its experimental motor, which it calls I2SM (for In-Rotor Inductive-Excited Synchronous Motor). Besides not using any rare earth elements, the motor offers a few other advantages in comparison with permanent-magnet synchronous motors. These are linked to the fact that this kind of motor technology offers the ability to precisely control the magnetic field in the rotor—something that’s not possible with permanent magnets. That control, in turn, permits varying the field to get much higher efficiency at high speed, for example.
With headquarters in Baden-Württemberg, Germany, ZF Friedrichshafen AG is known for a rich R&D heritage and many commercially successful innovations dating back to 1915, when it began supplying gears and other parts for Zeppelins. Today, the company has some 168,000 employees in 31 countries. Among the customers for its motors and electric drive trains are Mercedes-Benz, BMW, and Jaguar Land Rover. (Late last year, shortly after announcing the I2SM, the company announced the sale of its 3,000,000th motor.)
Has ZF just shown the way forward for rare-earth-free EV motors? To learn more about the I2SM and ZF’s vision of the future of EV traction motors, Spectrum reached out to Otmar Scharrer, ZF’s Senior Vice President, R&D, of Electrified Powertrain Technology. Our interview with him has been edited for concision and clarity.
Otmar Scharrer on...
- The I2SM’s technical bona fides
- The most promising concepts for future motors
- The motor’s coils, efficiency, and cooling
- The prototypes built to date
- The challenges the team overcame
IEEE Spectrum: Why is it important to eliminate or to reduce the use of rare-earth elements in traction motors?
ZF Friedrichshafen AG’s Otmar Scharrer is leading a team discovering ways to build motors that don’t depend on permanent magnets—and China’s rare-earth monopolies. ZF Group
Otmar Scharrer: Well, there are two reasons for that. One is sustainability. We call them “rare earth” because they really are rare in the earth. You need to move a lot of soil to get to these materials. Therefore, they have a relatively high footprint because, usually, they are dug out of the earth in a mine with excavators and huge trucks. That generates some environmental pollution and, of course, a change of the landscape. That is one thing. The other is that they are relatively expensive. And of course, this is something we always address cautiously as a tier one [automotive industry supplier].
And as a matter of fact, 95 percent of the rare earths are produced in China. And this means that if China decides no one else will have rare earths, we can do nothing against it. The recycling circle [for rare earth elements] will not work because there are just not enough electric motors out there. They still have an active lifetime. When you are ramping up, when you have a steep ramp up in terms of volume, you never can satisfy your demands with recycling. Recycling will only work if you have a constant business and you’re just replacing those units which are failing. I’m sure this will come, but we see this much later when the steep ramp-up has ended.
“The power density is the same as for a permanent-magnet machine, because we produce both. And I can tell you that there is no difference.” —Otmar Scharrer, ZF Friedrichshafen AG
You had asked a very good question: How much rare-earth metal does a typical traction motor contain? I had to ask my engineers. This is an interesting question. Most of our electric motors are in the range of 150 to 300 kilowatts. This is the main range of power for passenger cars. And those motors typically have 1.5 kilograms of magnet material. And 0.5 percent to 1 percent out of this material is pure [heavy rare-earth elements]. So this is not too much. It’s only 5 to 15 grams. But, yes, it’s a very difficult-to-get material.
This is the reason for this [permanent-] magnet-free motor. The concept itself is not new. It has been used for years and years, for decades, because usually, power generation is done with this kind of electric machine. So if you have a huge power plant, for example, a gas power plant, then you would typically find such an externally-excited machine as a generator.
We did not use them for passenger cars or for mobile applications because of their weight and size. And some of that weight-and-size problem comes directly from the need to generate a magnetic field in the rotor, to replace the [permanent] magnets. You need to set copper coils under electricity. So you need to carry electric current inside the rotor. This is usually done with sliders. And those sliders generate losses. This is the one thing because you have, typically, carbon brushes touching a metal ring so that you can conduct the electricity.
Those brushes are what make the unit longer, axially, in the direction of the axle?
Scharrer: Exactly. That’s the point. And you need an inverter which is able to excite the electric machine. Normal inverters have three phases, and then you need a fourth phase to electrify the rotor. And this is a second obstacle. Many OEMs or e-mobility companies do not have this technology ready. Surprisingly enough, the first ones who brought this into serious production were [Renault]. It was a very small car, a Renault. [Editor's note: the model was the Zoe, which was manufactured from 2013 until March of this year.]
It had a relatively weak electric motor, just 75 or 80 kilowatts. They decided to do this because in an electric vehicle, there’s a huge advantage with this kind of externally excited machine. You can switch off and switch on the magnetic field. This is a great safety advantage. Why safety? Think about it. If your bicycle has a generator [for a headlight], it works like an electric motor. If you are moving and the generator is spinning, connected to the wheel, then it is generating electricity.
“We have an efficiency of approximately 98 percent, 99 percent. So, very little loss.” —Otmar Scharrer, ZF Friedrichshafen AG
The same is happening in an electric machine in the car. If you are driving on the highway at 75 miles an hour, and then suddenly your whole system breaks down, what would happen? In a permanent magnet motor, you would generate enormous voltage because the rotor magnets are still rotating in the stator field. But in a permanent-magnet-free motor, nothing happens. You are just switched off. So it is self-secure. This is a nice feature.
And the second feature is even better if you drive at high speed. High speed is something like 75, 80, 90 miles an hour. It’s not too common in most countries. But it’s a German phenomenon, very important here.
People like to drive fast. Then you need to address the area of field weakening because [at high speed], the magnetic field would be too strong. You need to weaken the field. And if you don’t have [permanent] magnets, it’s easy: you just adapt the electrically-induced magnetic field to the appropriate value, and you don’t have this field-weakening requirement. And this results in much higher efficiency at high speeds.
You called this field weakening at high speed?
Scharrer: You need to weaken the magnetic field in order to keep the operation stable. And this weakening happens by additional electricity coming from the battery. And therefore, you have a lower efficiency of the electric motor.
What are the most promising concepts for future EV motors?
Scharrer: We believe that our concept is most promising, because as you pointed out a couple of minutes ago, we are growing in actual length when we do an externally excited motor. We thought a lot what we can do to overcome this obstacle. And we came to the conclusion, let’s do it inductively, by electrical inductance. And this has been done by competitors as well, but they simply replaced the slider rings with inductance transmitters.
“We are convinced that we can build the same size, the same power level of electric motors as with the permanent magnets.” —Otmar Scharrer, ZF Friedrichshafen AG
And this did not change the situation. What we did, we were shrinking the inductive unit to the size of the rotor shaft, and then we put it inside the shaft. And therefore, we reduced this 50-to-90-millimeter growth in axial length. And therefore, as a final result, you know the motor shrinks, the housing gets smaller, you have less weight, and you have the same performance density in comparison with a PSM [permanent-magnet synchronous motor] machine.
What is an inductive exciter exactly?
Scharrer: Inductive exciter means nothing else than that you transmit electricity without touching anything. You do it with a magnetic field. And we are doing it inside of the rotor shaft. This is where the energy is transmitted from outside to the shaft [and then to the rotor electromagnets].
So the rotor shaft, is that different from the motor shaft, the actual torque shaft?
Scharrer: It’s the same.
The thing I know with inductance is in a transformer, you have coils next to each other and you can induce a voltage from the energized coil in the other coil.
Scharrer: This is exactly what is happening in our rotor shafts.
So you use coils, specially designed, and you induce voltage from one to the other?
Scharrer: Yes. And we have a very neat, small package, which has a diameter of less than 30 millimeters. If you can shrink it to that value, then you can put it inside the rotor shaft.
So of course, if you have two coils, and they’re spaced next to each other, you have a gap. So that gap enables you to spin, right? Since they’re not touching, they can spin independently. So you had to design something where the field could be transferred. In other words, they could couple even though one of them was spinning.
Scharrer: We have a coil in the rotor shaft, which is rotating with the shaft. And then we have another one that is stationary inside the rotor shaft while the shaft rotates around it. And there is an air gap in between. Everything happens inside the rotor shaft.
What is the efficiency? How much power do you lose?
Scharrer: We have an efficiency of approximately 98 percent, 99 percent. So, very little loss. And for the magnetic field, you don’t need a lot of energy. You need something between 10 and 15 kilowatts for the electric field. So you lose 1 percent of that. This is important because we don’t cool the unit actively and therefore it needs this kind of high efficiency.
The motor isn’t cooled with liquids?
Scharrer: The motor is cooled, but the inductive unit is not cooled.
“A good invention is always easy. If you look as an engineer on good IP, then you say, ‘Okay, that looks nice.’” —Otmar Scharrer, ZF Friedrichshafen AG
What are the largest motors you’ve built or what are the largest motors you think you can build, in kilowatts?
Scharrer: We don’t think that there is a limitation with this technology. We are convinced that we can build the same size, the same power level of electric motors as with the permanent magnets.
You could do 150- or 300-kilowatt motors?
Scharrer: Absolutely.
What have you done so far? What prototypes have you built?
Scharrer: We have a prototype with 220 kilowatts. And we can easily upgrade it to 300, for example. Or we can shrink it to 150. That is always easy.
And what is your specific power of this motor?
Scharrer: You mean kilowatts per kilogram? I can’t tell you, to be quite honest. It’s hard to compare, because it always depends on where the borderline is. You never have a motor by itself. You always need a housing as well. What part of the housing are you including in the calculation? But I can tell you one thing: The power density is the same as for a permanent-magnet machine because we produce both. And I can tell you that there is no difference.
What automakers do you currently have agreements with? Are you providing electric motors for certain automakers? Who are some of your customers now?
Scharrer: We are providing our dedicated hybrid transmissions to BMW, to Jaguar Land Rover, and our electric-axle drives to Mercedes-Benz. The [Mercedes] EQA, -B, and -C are equipped by us, for example, those three cars. And we are, of course, in development with a lot of other applications. And I think you understand that I cannot talk about that.
So for BMW, Land Rover, Mercedes-Benz, you’re providing electric motors and drivetrain components?
Scharrer: BMW and Land Rover. We provide dedicated hybrid transmissions. We provide an eight-speed automatic transmission with a hybrid electric motor up to 160 kilowatts. It’s one of the best hybrid transmissions because you can drive fully electrically with 160 kilowatts, which is quite something.
“We achieved the same values, for power density and other characteristics, for as for a [permanent] magnet motor. And this is really a breakthrough because according to our best knowledge, this never happened before.” —Otmar Scharrer, ZF Friedrichshafen AG
What were the major challenges you had to overcome, to transmit the power inside the rotor shaft?
Scharrer: The major challenge is, always, it needs to be very small. At the same time, it needs to be super reliable, and it needs to be easy.
A good invention is always easy. When you see it, if you look as an engineer on good IP [intellectual property], then you say, “Okay, that looks nice”—it’s quite obvious that it’s a good idea. If the idea is complex and it needs to be explained and you don’t understand it, then usually this is not a good idea to be implemented. And this one is very easy. Straightforward. It’s a good idea: Shrink it, put it into the rotor shaft.
So you mean very easy to explain?
Scharrer: Yes. Easy to explain because it’s obviously an interesting idea. You just say, “Let’s use part of the rotor shaft for the transmission of the electricity into the rotor shaft, and then we can cut the additional length out of the magnet-free motor.” Okay. That’s a good answer.
We have a lot of IP here. This is important because if you have the idea, I mean, the idea is the main thing.
What were the specific savings in weight and rotor shaft and so on?
Scharrer: Well, again, I would just answer in a very general way. We achieved the same values, for power density and other characteristics, for as for a [permanent] magnet motor. And this is really a breakthrough because according to our best knowledge, this never happened before.
Do you think the motor will be available before the end of this year or perhaps next year?
Scharrer: You mean available for a serious application?
Yes. If Volkswagen came to you and said, “Look, we want to use this in our next car,” could you do that before the end of this year, or would it have to be 2025?
Scharrer: It would have to be 2025. I mean, technically, the electric motor is very far along. It is already in an A-sample status, which means we are...
What kind of status?
Scharrer: A-sample. In the automotive industry, you have A, B, or C. For A-sample, you have all the functions, and you have all the features of the product, and those are secured. And then B- is, you are not producing any longer in the prototype shop, but you are producing close to a possibly serious production line. C-sample means you are producing on serious fixtures and tools, but not on a [mass-production] line. And so this is an A-sample, meaning it is about one and a half years away from a conventional SOP ["Start of Production"] with our customer. So we could be very fast.
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