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Introduction
Automotive industry is electrifying, and fast. Most of the traditional makes have fully electric versions of their cars, and many of them have completely re-designed their cars to fit for the electric drive trains. Also, the electric era has brought completely new car manufacturers. Everything is happening fast, and different technologies are being used. This blog text opens up the motor technology used in fully electric, battery-powered cars currently on the market.
Differences between traditional and electric cars
Let us start with fundamentals. There are a some key differences between the traditional, internal combustion engine powered cars and battery-powered electric cars.
Traditional cars have only one engine, usually placed in the front, and coupled to a transmission with up to 9 gears. The power can be brought to the front wheels, rear wheels, or all four wheels depending on the car model, but all these three options would have the engine placed in more or less the same way. The engine design has to take into account many different functions, like fuel type and vehicle power demand. A hybrid car has a secondary electric motor, but the primary engine design is still not much changed from the traditional car.
The cars for different purpose used to have very different engines. For example, an american pick-up truck typically has an enormous V8, which is thirsty and noisy, and gives the impression of a car with lots of power for heavy duty service. Meanwhile, a european car has a small, often turbo-charged four-cylinder engine providing some performance with low fuel consumption, balancing with the impressions of a car designed for everyday life, and a car convenient to drive on narrow and hilly roads. The two engine types mentioned are very different. But in the electric era, both of these cars could be equipped with motors of the same principal design. A pick-up truck would only have larger variants of these motors, or alternatively just more motor units installed. For example, based on performance values, a rear motor in a heavy duty Rivian R1T pick-up truck is almost identical to the rear motors in family cars like Hyundai Ioniq 5 or Volkswagen ID.4.
Electric cars have motors coupled to the axles through single- or two-speed gearboxes. Usually the motor is favoured on the rear axle, so the basic car option has rear-wheel drive. More advanced high-performance options have motors on both shafts, providing four-wheel drive. This logic is used for example in Tesla Model 3, BMW i4, and Ford Mustang Mach-e. There are some exceptions, however. Renault Megane E-Tech and Volvo C40 Recharge have front-wheel drive as they have fitted the motors on the front axle in their basic versions.
Probably the best motivation for the electric era is that the electric cars have more torque than the traditional cars. Electric motor is able to deliver the peak torque instantly, even at low motor speed. Electric motor can be accelerated smoothly without any gear changes. These provide superior acceleration compared to internal combustion engines. Therefore, because the engine characteristics are so different, comparing the performance figures of petrol-engined cars and electric cars, is not straightforward. A modern day fully electric family car (such as Tesla Model 3, BMW i4, or Polestar 2) has equivalent acceleration characteristics to an internal combustion engine powered sports car. However, the handling characteristics will still make a big difference between the two.
Key engine specifications
A car is basically a traction motor application, similar to a train: It needs high torque on low speed to get the vehicle moving, and less torque in higher speeds to overcome the drag and resistance of spinning wheels. But the car motor needs also considerable torque during the breaking to recharge the batteries. Further, the driver’s driving style and traffic conditions make the acceleration/deceleration routines very random, compared to a train on a certain section of track. The motor needs high efficiency throughout the entire speed range. This is required for the vehicle range, not for low operating costs as usually with electrical machines.
The motor speed range is very wide. The car speed ranges from 0 to 200 km/h. This corresponds to the motor speed range of roughly 0 – 15’000 rpm. The field weakening point (or rated point) is normally placed at around 5’000 rpm (e.g. Tesla Model 3 rear motor at 5’075 rpm). Beyond this point, the motor torque is de-rated. At the maximum speed the motor torque is less than one third of rated. This means that a car with multiple motors could deactivate one of them at high speeds, or when cruising at constant speed.
In addition, the motors need to be light and physically small. High power density is required. This is slightly different to the train application, which does not really mind the dead weight, although the physical size is often limited if the motor is placed in the wheel bogie. If a car has multiple motors, the size and weight constraints are emphasized. One of our customers summarised the electric vehicle manufacturer requirements as: ”They want a megawatt out from a fist-sized chunk”.
Machine types
Motors in different fully electric cars are listed in Table 1. The AWD car models and high performance options are favoured to mark the limits of technology. The data in Table 1 is collected from Wikipedia [1], www.evspecification.com [2], and from the manufacturers web pages.
Table 1: Electric motors used in different cars.
There are differences. Audi e-tron GT, Ford Mustang Mach-e GT and Mercedes-Benz EQS 580 all have similar torque, slightly above 800 Nm. But their horsepower (and kilowatt) ratings differ considerably: 637 hp, 480 hp, and 516 hp, respectively. Further, the vehicle acceleration values from 0 – 100 km/h also differ: 3.3 s, 3.8 s, and 4.3 s, respectively. In addition, Tesla Model 3 Performance has more power, but less torque than the Jaquar I-Pace. Differences are not large. But the Tesla Model 3 Performance accelerates from 0 to 100 km/h in 3.3 s, and the Jaquar I-Pace in 4.8 s. Both cars are fast, but one would not expect the acceleration to differ this much after seeing the power and torque values on paper.
These examples only illustrate how differently the motors are driven. It is not straightforward to set up the motor layout. The engineers must balance between the acceleration, top speed, and range. For example, Tesla sacrifices 50 km (almost 10%) of the Model 3 range with the performance package that provides the maximum acceleration. Furthermore, the traditional manufacturers have to take the other car models in their fleet into account. For example, if BMW trimmed the i4 for the very maximum performance values, in the same way as Tesla does for the Model 3 Performance, the car would probably be faster than their famous and much more expensive flagship model M5.
Table 1 highlights that the manufacturers prefer permanent magnet synchronous machines. Tesla and Audi appear to be moving away from induction motors into PMSMs. Renault and BMW have taken another approach with electrically excited synchronous machines.
Permanent magnet synchronous machine, PMSM
PMSMs have usually the highest power density, or power-to-weight ratio. They are simple and straightforward to manufacture. In general, they have high efficiency, because no auxiliary system is needed for magnetisation. However, the efficiency drops at speeds beyond the rated point. The vehicle application requires a long field weakening range, and by nature the PMSM does not work in field weakening. The permanent magnet field is fixed and cannot be adjusted (so cannot be weakened). At high speed the motor has excessive iron losses and low efficiency, although these effects can be significantly influenced by the control method of the inverter drive. Further, since the motor is always magnetized, it cannot be completely deactivated at low vehicle power demand.
Another disadvantage of the PMSM is related to the procurement of the magnets. The magnets in the vehicles are of the NdFeB type, the strongest of the common magnet types. Useful resources of Neodymium and Dysprosium are mainly found in Chinese bedrock, so their market price can be influenced by the Chinese foreign and trade policies. In addition, high general demand of these types of magnets make their market price unpredictable. If every car has to be fully electric and powered by PMSMs, the world would simply run out of magnets.
In 2011 Neodymium and Dysprosium prices suddenly climbed causing real sourcing headaches for permanent magnet machine manufacturers. This was merely due to political reasons. I can recall one project at the time, when we had a strict specification and needed a magnet grade with high Dysprosium content. We received an offer from a magnet supplier, which was ridiculously expensive and valid until the end of the day. This is a situation where no car manufacturer wants to end up with.
Electrically excited synchronous machine, SM
The electrically excited synchronous machines do not have magnets. Their field is formed by DC coils in the rotor. This machine type has been used in large power plants for more than 100 years. The power plant generator stator is connected directly to the grid, and the machine is controlled by adjusting the magnetizing DC current. Now in electric vehicles, the stator is driven by the inverter in the same way (although with different control methods) as with the PMSMs and induction machines. But in addition, also the magnetizing current can be adjusted.
The benefit of the SM is its full controllability. The magnetization can be adjusted based on the vehicle load. For the maximum torque, the motor can be magnetised beyond its rated conditions, and at low power demand the magnetic field can be decreased. Also, the motor can be deactivated when no power is required. At the rated point PMSM provides higher efficiency, but at operating points further away from the that, i.e. when not driving with full throttle, the SM efficiency might be better.
But the disadvantage is the slip ring and brush arrangement used to feed the current into the rotor. Large machines of this type use a separate magnetising machines to electromagnetically transfer the power into the rotor, and a diode bridge on the rotor to convert the power into DC. The former takes considerable space at the end of the shaft and the latter is notorious for breaking into pieces and causing unplanned power plant outages. Slip ring and brush solution is simple and compact, but it contains wearable parts, so potentially the electric cars with this motor type (e.g. BMW i4) need more consideration for maintenance. On the other hand, the brushes are proven technology in alternators and they can be installed at the end of the shaft, so the brush module should be exchangeable.
Induction machines, IM
For general industrial applications, this is by far the most common machine type. It is inexpensive, robust, and reliable, it does not need exotic materials, and there is lots of experience in using it. So, it is no surprise that the car industry started the electric era with this one.
However, when higher performance is required, the induction machine gets less attractive. First, it is almost impossible to calculate or model accurately without extensive test results and empirical correction coefficients. The machine is highly non-linear by nature. Second, to meet the electric car motor specifications the rotor cage must be made of copper instead of cheap and fast aluminium casting. If higher speed is required the copper cage joints have to be made more carefully and quality control starts to have a price impact. The induction machine is also heavier than the above-mentioned motor types. And finally, when the power density is increased the machine starts to get really really hot, and requires complicated cooling system.
In Tesla Model 3 and Volkswagen ID.4 the induction machines are used on the front axle as secondary motors to the PMSMs on the rear axle. They can be deactivated at low power demand.
More design details
Motor details are compiled in Table 2. The information is collected from advertisement illustrations by different manufacturers and various tear-down shows (mostly videos by Sandy Munro) revealing what is really inside the frame. However, the tear-down analyses do not reveal the motor dimensions. The reader is advised to see the list of references at the end of the article.
Table 2: Design details of electric car motors.
A consensus seems to be 48 stator slots, hairpin winding, and 8 poles with interior permanent magnets arranged in some sort of a V-configuration. Surprisingly, all motors are 3-phase radial-flux motors. The list also contains no rotors with surface-mounted magnets, which configuration is used in racing cars and for example in Rimac Nevera supercar.
Tesla has a different magnet configuration with a V-shape with all the side iron bridges removed, and the entire rotor covered by carbon fibre sleeve, which provides the mechanical strength. A nice analysis of the design can be found in [14]. To me this seems like a strange design. Many industrial motor manufacturers would not allow such a configuration, because it is probably rather difficult and expensive to make, and because the demagnetisation risk is increased if the rotor is covered with a thermal insulation blanket, which the carbon fibre sleeve essentially is. Tesla wanted to remove the iron bridges around the magnets to improve the motor inductance ratio, and further to increase the peak torque. But they could have achieved this also by making the motor six-phase or 8-pole, or just a little longer.
It seems however, that Tesla has lately replaced their so-called ”Carbon wrapped motor”. The Tesla motor design in Model Y seems to contain a hairpin-wound stator and a rotor without the carbon fibre sleeve.
Random-wound windings are still mentioned in Table 2, but apparently the manufacturers are moving away from them. Random-wound winding is made by hand out of round copper wires. It is laborious to make, but the end result is very compact, and there is much freedom for the winding design. Another disadvantage is that the winding heads heat up easily, since the conductors are densely packed together. Hairpin winding on the other hand is fully automated. It is made of rectangular copper wires prefabricated into U-shapes, which look like hairpins. Once inserted into slots they are joined to the adjacent hairpins on one end of the stator. The method contains some compromises for the winding design, and due to relatively large copper conductors, it also contains high skin effect at high frequencies, i.e. more losses at high speed. The benefits of the hairpin winding are related to the use of rectangular wires. They provide higher copper filling factor, but also better heat transfer, because the geometry is fixed, the end-winding loops are not stacked together, and the insulation system can use the high-performance materials, such as PEEK and coating resins with improved thermal conductivity.
Lucid uses a winding configuration they call “a wave” and they have managed to make it out of rectangular copper without intermediate joints. This winding has smaller copper strands and shorter winding heads than the hairpin windings. It should be noted here that the term ”wave winding” normally means that the winding circulates the stator periphery instead of making loops around the poles. The hairpin winding also has a wave form, and some old hydro generators use the wave winding, but they contain plenty of intermediate joints, just like other winding types. Lucid’s ”wave” is free of joints, referring also to how it is manufactured.
Cooling systems
Electric car motors are rather heavily loaded and very power dense, so they need efficient cooling. The cooling methods and presumably also their effectiveness differs from manufacturer to manufacturer. Technical details are not easy to understand from the illustrations, but there seems to be two prime methods for cooling. Porsche and Volkswagen seem to be using water jacket cooling in the stator housing. The cooling fluid is water-glycol mixture, just like in internal combustion engines. Hyundai, Lucid, Rivian, and Tesla have oil drizzle cooling for the stator frame and winding heads. The cooling oil is sprayed through some nozzles directly to the end-windings, then collected and circulated through a filter, a pump, and a heat exchanger. But basically, the similarities end here. The practical execution, and presumably the cooling effectiveness as well, is very different in all these cars.
The latter method, oil drizzle cooling, is probably more efficient allowing higher current density in the stator winding, and further higher power density. This is provided by spraying the coolant directly on the copper conductors. Water jacket cooling is further away from the heat source. It should be noted that water (or water-glycol mixture) cannot be sprayed directly to the coils, because the insulating materials do not tolerate hot water. But they are fully compatible with mineral oil.
Axial-flux motors remain absent
It is a surprise that none of the cars listed here has axial-flux motors. Not even the Mercedes, who recently purchased the axial-flux motor manufacturer YASA. Mercedes cars sold in the present day are still based on their old drive train concept. They expect to start using the axial-flux machines from 2025 but not in every model.
Axial-flux motors, or radial-axial-flux motors are used in hybrid supercars by Koeniggsegg and Ferrari for instance. The construction contains two rotors and one stator in the middle. A third rotor can be added on the outside of the stator to operate on radial flux. This motor configuration provides very high torque for a small motor size. It has notably higher frequency than the motors listed in Table 2, even though the rated speed is less. High frequency is not problematic for the stator, since the stator iron mass is much less than in radial-flux machines. But the high frequency causes overheating in the magnets, unless they are segmented. The construction is expensive with huge amount of customised magnets and the stator pieces made of powder material (soft magnetic composite) instead of lamination sheets.
Nevertheless, axial-flux motors cannot just replace the existing motors. If a car manufacturer wishes to use them, it would need to completely redesign its drive train. The physical constraints are changed. The inverter cannot be assembled on top of a disc-shaped axial-flux machine as it is assembled on top of a cylinder-shaped radial-flux motor in many present day cars, including Mercedes. Also, the higher number of poles with different speed and frequency profiles probably require a re-design for the mechanical gear.
What I would have done
Had I been given a task to design a motor for these cars, I would have probably also started with radial-flux permanent magnet machines. I know how they are designed, they are easy to design, and they are very powerful motors. At first, I would have also skipped the surface-mounted magnet configuration. It might be powerful at the rated point, but probably has lower efficiency in higher speed. I would have tried the option used by Volkswagen, Ford, and Hyundai, an 8-pole machine with interior magnet configuration. This type has fine torque production and efficiency, and with the given physical dimensions, the geometry can be set up nicely. But problems arise at high speeds, and I would then pay more attention to the losses in the stator winding.
I would have used a straight V magnet configuration with just two magnets per pole, like Tesla but without the carbon fibre sleeve. Volkswagen has three magnets per pole. Ford, Hyundai, Lucid, and Rivian have four magnets per pole in a double-V configuration. However, this is basically a matter of design philosophy. Additional magnets may enable higher efficiency, lower content of harmonics (lower vibration levels), or just simply higher airgap flux density. But additional magnets bring additional leakage, and there are also other design aspects to consider to achieve the above-mentioned effects. Larger amount of magnets also tends to be more expensive, even though the total weight of the magnets stays the same.
Regarding the stator design, the wave winding configuration used by Lucid, without intermediate joints and with smaller rectangular copper strands, appears the most attractive. I think the hairpin windings have too cumbersome joining method, because the welds in the winding head need considerable axial space. Cutting out this space would enable a longer and hence more powerful motor into the same housing.
But still, considering the irregular load profile of an electric car, the full controllability of an electrically excited synchronous motor, and the constantly increasing sourcing risk of the magnets, I am fascinated by the solution by BMW and Renault. It seems to me that this machine configuration might finally, when fully optimised, deliver the highest total efficiency. It will be slightly heavier and larger than the PMSM, but it is a robust and extremely powerful motor with predictable and stable cost profile. The only downside appears to be the brush modules.
I would also push for higher power by making the motor 6-phase, even though it requires 6-phase capability also from the inverter, i.e. two smaller inverters connected in parallel with a phase shift. This approach is used by Mercedes in their Mercedes-AMG EQS 53 performance car, although only on the rear axle. The front motor of this car is still 3-phase. 6-phase motors can be expected to spread in the future, especially in high-powered cars. The approach of adding the number of phases is also used in electric aircraft motors, which represent arguably the highest power density of all electrical machines today.
Final words
It seems the permanent magnet synchronous machines are the preferred choices for electric cars of today. Tesla and Volkswagen still have some induction motors used as secondary motors, and Audi’s out-of-date drive train still uses induction motors. However, the preferences may change in the future. The cars manufactured in China are likely to use NdFeB permanent magnets, but the others may look for alternatives.
The electric car ”engines” look impressive, solid works of engineering, and designed mostly in the right way. I remember the first images of hybrid car electric motors from 20 years ago, which looked essentially like idiotic inside jokes. But now it seems the car manufacturers have hired qualified people to design the motors.
The motor technology reviewed here suggests that the electric drive trains are here to stay. After seeing how the electric drive train performs, it is difficult to believe that the old internal combustion engines would suddenly bounce back and re-conquer the market. But still, there is surprisingly much misleading and completely wrong information about these motors around in the internet. Car journalists, blog writers, and even the car manufacturers themselves struggle with terminology related to electrical machines. This is a symptom of traditional industry moving into a new direction, and constantly learning new things. And that there is still room for improvement.
References
- Wikipedia in English language, Available at: https://en.wikipedia.org/wiki/Main_Page
- EVspecifications, Available at: https://www.evspecifications.com/
- Robin Thomas, Hugo Husson, Lauric Garbuio, Laurent Gerbaud. Comparative study of the Tesla Model S and Audi e-Tron Induction Motors. 17-th International Conference on Electrical Machines, Drives and Power Systems (ELMA), Jul 2021, Sofia (on line), Bulgaria. hal-03304650
- MotorTrend, No Magnets, Big Power: BMW’s Fifth-Generation Electric Motor, Available at: https://www.motortrend.com/news/bmw-ix-m60-brushed-electric-motor-tech-deep-dive/
- Munro Live, Ford Mach-E Rear Motor Teardown and Analysis, Available at: https://www.youtube.com/watch?v=mHVV52lPyIs
- Munro Live, Hyundai/Kia E-GMP Electric Drive Units – Clever, Cost Efficient, and Future-Proofed, %Available at: https://www.youtube.com/watch?v=xm4VDfdKUm0
- Lucid Tech Talk, Episode 4: Drive Unit: Motor, Available at: https://lucidmotors.com/tech-talks
- Mercedes: All-electric Driving Performance in a luxurious ambience: The new Mercedes-AMG EQS 53 4MATIC+ with battery-electric drive, Available at: https://media.mercedes-benz.com/article/33afcb1a-8f3f-47a6-a211-6d8995ff5c23
- Inside EVs: Mercedes-Benz Reveals Technical Specs For EQS Electric Sedan, Available at: https://insideevs.com/news/498761/mercedes-technical-specs-eqs/
- Porsche: The powertrain: Pure performance, Available at: https://newsroom.porsche.com/en/products/taycan/powertrain-18555.html
- Porsche: High voltage: Porsche electric motors explained, Available at: https://newsroom.porsche.com/en/2021/innovation/porsche-electric-motors-taycan-high-voltage-christophorus-398-23809.html
- Munro Live: A Multitude of Motors | Rivian R1T Electric Drive Unit, Available at: https://www.youtube.com/watch?v=Ldw5w3x5kUc
- Munro Live: Tesla Model S Plaid Motor EXTRAVAGANZA, Available at: https://www.youtube.com/watch?v=4lGVimLK58g
- Antti Lehikoinen, Tesla’s ”Carbon-wrapped” motor, Available at: https://www.anttilehikoinen.fi/technology/electrical-engineering/teslas-carbon-wrapped-motor/
- Autoevolution, Tesla’s New Hairpin Stator Revealed in Drive Unit Teardown, Sandy Munro Missed the Moment, Available at: https://www.autoevolution.com/news/tesla-s-new-hairpin-stator-revealed-in-drive-unit-teardown-sandy-munro-missed-the-moment-212457.html#agal_0
- Volkswagen: Volkswagen offers additional all-wheel drive version for the ID.4 , Available at: https://www.volkswagen-newsroom.com/en/press-releases/volkswagen-offers-additional-all-wheel-drive-version-for-the-id4-8001
- Munro Live: Volkswagen ID.4: Electric Motor Teardown and Analysis, Available at: https://www.youtube.com/watch?v=3Bab6CttkEY
