The auto industry reinvents the electric motor — Part 2 of 2
Spurred by the need to roll out electric vehicles, auto manufacturers and suppliers are developing electric motors with greater power densities and higher efficiencies, while making cost trade-offs.

Double-layer ‘V’ arrangement of the permanent magnets inside the rotor of the Chevrolet Bolt.
Just as with the stators presented in the part 1 of this article, rotors have undergone a series of innovations that includes using neodymium magnets in greater numbers and arranging them in order to recover reluctance torque. In addition, the cooling of some machines is becoming more and more targeted and goes so far as to absorb heat generated by the rotor. Inverters are built into the motor and coaxial drives are being marketed.

Coaxial drive on the rear-axle powertrain of the Audi e-tron.
Although the vast family of electric motors makes use of a wide range of technologies, the scope of this article is limited to those used in the automotive industry.
Permanent-magnet rotors

Permanent-magnet motor controlling a Schaeffler active anti-roll bar.
The rotor’s magnetic field is generated either by magnets of varying strength or by a coil. Most hybrid or electric cars use magnets that often have a very strong magnetic field. The advantage of this technique is a high power-to-weight ratio and high efficiency.
Its main disadvantage lies in its cost, which is high and fluctuates when magnets containing rare-earth elements such as neodymium are used (see ‘Permanent-magnet rotor technology’ below). In addition, the magnets’ permanent magnetic field generates resistance torque when the motor is no longer powered at high speed (‘freewheeling’) due to eddy currents and hysteresis in the stator.
Wound rotors

Wound rotor on the Renault R240 motor in the Zoe.
A handful of manufacturers, such as Renault, have opted not to rely on neodymium or other rare-earth elements, choosing instead to use a winding on the rotor to create the magnetic field. The coil is powered by an inverter and current passes through two brushes that rub against two rings. A wound rotor does not create resistance torque at high freewheeling speeds.
However, this solution requires a longer rotor for an equivalent power, which also increases its weight. Powering the coil with a dedicated inverter reduces the cost savings and recovers a portion of energy.
Squirrel-cage rotors

Squirrel-cage rotor of an induction motor.
Squirrel-cage rotors are used on induction motors. They consist of a core covered with conductive bars running along its entire length and which are shorted together to carry induced currents. The bars are often skewed at a slight angle with the core’s longitudinal axis so as to reduce torque ripple generated by variations in the magnetic field depending on their position facing the stator coils. Torque ripple is most noticeable at very low speeds and causes harmonics, engagement, and noise issues.
These magnetless rotors do not generate resistance torque during freewheeling. Their main disadvantage lies in their high energy losses (20–35% of total losses). Among mass-market vehicle manufacturers, only Tesla and VW (ID.3 front-wheel drive) use squirrel-cage rotors.
Permanent-magnet rotor technology

BorgWarner rotor with helically aligned magnets to reduce torque ripple.
The magnets (note their slight V-shape) in the rotor of the Honda CVR Hybrid motor are not made of rare-earth elements.
Like the conductive bars on some squirrel-cage rotors, the magnets are sometimes aligned along the rotor’s length and are slightly offset with one other (helical alignment) to reduce torque ripple.
To have a straight alignment conducive to torque, an electronic control can compensate for these oscillations, in particular by modifying the shape of the signal.
Most electric vehicles use permanent-magnet motors. The power density and efficiency of these motors depend largely on the strength of the magnetic field generated by the magnets.

The second-generation Chevrolet Volt Hybrid uses a 50 kW coaxial electric motor with a ferrite rotor and a 100 kW coaxial electric motor with neodymium-iron-boron magnets.
Ferrite magnets are the cheapest, but their magnetic flux density is relatively low (0.2–0.45 tesla). While suitable for alternators in ICE vehicles, they are not well-suited to electric vehicle motors, which must deliver a significantly higher power density.
The most common solution at present is the use of neodymium-iron-boron (Nd2Fe14B) magnets, which provide a magnetic flux density of 1.08–1.46 tesla. On average, they consist of an alloy of 61% iron, 35% neodymium, 1.8% dysprosium, 1.4% boron, and 0.5% aluminium.
Not-so-rare-earth elements
Rare-earth elements are a set of 17 metals with similar properties, particularly in terms of their electromagnetic capacity. This name ‘rare earth’ is surprising because these strategic minerals are actually plentiful in the Earth’s crust. For example, a 2018 study published in the journal Scientific Reports estimated that there is enough dysprosium to meet global demand for 730 years.
Challenge posed by neodymium magnet rotors

The magnetism of ferrite rotors in an alternator can be improved by inserting neodymium-iron-boron magnets.
One of the technical challenges of neodymium-iron-boron magnets is their low coercivity at high temperatures. Coercivity is the intensity of an applied magnetic field required to completely demagnetise a ferromagnetic material. Coercivity varies with temperature and two temperature values are critical: the maximum operating temperature (a magnet will lose its magnetisation above this temperature but gain it back at a lower temperature) and the Curie temperature, i.e. the temperature at which a magnet will become irreversibly demagnetised.
To push back the limits of coercivity and meet the requirements of electric motors, neodymium-iron-boron magnets are doped with dysprosium or, more rarely, terbium. Doing so increases their maximum operating temperature 80 °C — a critical limit nonetheless — and their Curie temperature to 310 °C (in comparison, the temperature limits of an undoped ferrite magnet are 280 °C and 460 °C, respectively). Because these magnets are sensitive to corrosion and acids, they are coated with a layer of nickel, chromium, or silver.

Enercon machine for wind turbines. Measuring 13 m in diameter and with 4 MW of capacity, it contains 500 kg of rare-earth magnets.
Neodymium (Nd) is a silver-grey metal that is both ductile and malleable at room temperature (Vickers hardness of 343 MPa versus 608 MPa for iron). Dysprosium (Dy) is also a silver-grey metal, but with a higher hardness value (540 MPa). Neodymium was discovered by the Austrian chemist Carl Auer von Welsbach, in 1885, and dysprosium was discovered by the French chemist Paul Émile Lecoq de Boisbaudran the following year. Both of these rare-earth elements, which belong to the lanthanide series, are strategic raw materials. Most of the world’s neodymium and dysprosium come from China. Dysprosium accounts for half the price of a magnet but only makes up 3.5% of its total weight.
The motors in today’s 80–200 kW vehicles contain 1–2 kg of neodymium. For comparison, an offshore wind turbine contains 150 kg of neodymium and as much as 24 kg per megawatt of power. The amount of dysprosium can be lowered by enhancing or better targeting cooling and through grain boundary diffusion technology. For example, Chevrolet reduced the amount of dysprosium from 280 g to 40 g between the first and second generation (2011 and 2015) of its Volt.
Work is being carried out to reduce the amount of rare-earth elements used or to replace them by cerium, lanthanum, or praseodymium, which also belong to the lanthanide series but are more widely available, cheaper, and better distributed geographically, particularly in California. To learn more, read our December 2016 Focus tech article on the ROMEO project.
Permanent-magnet induction and reluctance rotors

Evolution of the magnetic flux of motors in the four generations of the Toyota Prius.
A hybrid design that is gaining use is the permanent-magnet rotor, which captures magnetic flux created by reluctance, which produces a field between coils and passes through the rotor core. It is important that the geometry of this type of rotor (geometric anisotropy) allows the magnetic field to pass between the magnets.

Principle of increasing torque by recovering reluctance torque.
This reluctance effect generates a torque which combines with the torque of the magnetic field between the coils and magnets, increasing the power density.

Changes in magnet arrangement

V-shaped magnets on the rotor of the Schaeffler motor for the front axle of the Audi e-tron.
Arranging magnets in a ‘V’ pattern facilitates reluctance torque and reduces torque ripple, thereby lowering noise and vibration.

Double-layer ‘V’ arrangement of the magnets inside the rotor of the Chevy Volt’s motor.
Other configurations are also used, such as double-layer ‘V’ arrangements, triangular arrangements, and type I or double I arrangements where the magnets are perpendicular to the rotor’s radius but set back from its periphery.

Double I magnets.
Magnets placed in a triangular arrangement (Motor Design).
Toyota reports that it has increased the ratio of reluctance torque to total torque from 38% to 72% and halved the weight of rare-earth magnets between the first and fourth generations of its Prius.
The speeds of electric motors will be higher in the future. This will make it possible to decrease their weight, footprint, and cost (fewer magnets) at iso-power. The challenge lies in rotor balancing and the strength of the magnets during centrifugation. A solution used in other applications is to cover the rotor with a carbon fibre tube. Several projects with motor speeds of 18,000–24,000 rpm are currently under way.

Developed in 2013 by TEOS Powertrain Engineering, this motor–generator rotor for a turbo-compound engine is encapsulated in a carbon fibre sheath to be able to withstand speeds of up to 100,000 rpm and deliver 100 kW.
https://www.youtube.com/watch?v=V72cmDjuKHA&feature=emb_logo
Air or water cooling

The VW ID.3 powertrain is cooled by a water circuit that runs through the inverter and the motor housing.
The motor of the Renault Zoe is one of the few motors that are cooled by air. Filtered air is drawn in by a fan, passes through the rotor and stator in a guided flow, and is discharged downwards through a large-diameter hose (approx. 8–10 cm). This solution is cheaper than a water circuit and the air can also be used to cool the rotor. The downside is a lower cooling capacity (air heat capacity of 1 kJ/kg/K versus 4.2 kJ/kg/K for water) and the machine is no longer completely sealed. Its inverter is liquid cooled.

Filtered air intake on the motor of the Renault Zoe.
BMW i3 electric motor housing with separately moulded coaxial stator jacket (Nemak).
The vast majority of electric machines are cooled by a water circuit. This is especially true in the case of built-in inverters because they allow the same circuit to be used. Since a vehicle’s power electronics cannot withstand the same operating temperature as the motor, water first passes through a pipe running along on one or both sides of the inverter (and especially around the semiconductors) and then flows around the machine.
Heat, mainly generated by Joule losses in the stator coils, is dissipated via the core in the housing.

Liquid cooling channels machined into the housing of the ZF motor used by the Formula E Venturi team during the 2017 season.
Oil cooling

The rotor in the front motor of the Audi e-tron is cooled by the circulation of oil from a coaxial inner tube.
However, water cooling has some limitations for high power density motors. For example, the stator coils require enhanced cooling but the gaps between its conductors are reduced to increase the fill factor (see Part 1 of this article). A study by Institut Vedecom estimated that 1 W/cm2 is required to cool the surface of the hairpins stators used on such motors.

Cooling-oil circuit of the electric motor in the fourth-generation Toyota Prius.
The rotor must also be cooled, especially to reduce the risk of the magnets becoming demagnetised (coercivity).

BorgWarner hybrid module with oil-cooled rotor and stator.
Changes in the P2 hybrid module (placed between engine and transmission) developed by BorgWarner show the various stages of cooling. In the first generation, only the stator was cooled by water circulating in the housing. BorgWarner then used the oil needed for the hybrid system’s clutches to also absorb heat from the stator coils.
In the latest generation of the module, oil flows through the rotor at 10 l/min to cool it and then recover heat energy from the stator coils, and the housing's water cooling system has been eliminated.
A coaxial drive complicates the supply of oil to the rotor’s centre (see ‘Two transmission architectures’ below).
Inverter integration

Connection using three busbars between the inverter and the motor of the VW ID.3.
A distinctive feature of the latest generation of electric vehicles is the inclusion of an inverter built into the motor. Not only are the volume and weight of the housings and fasteners reduced but the three cables between the inverter and the three-phase stator are replaced by busbars, which are more reliable and facilitate assembly.
One of the challenges is the inverter’s thermal insulation, as the three metal busbars conduct heat from the motor. One solution to overcome this is to increase the heat transfer area between the busbars and the water cooling circuit. The latest inverters feature coolant channels in the top and bottom of their housings.

Valeo Siemens eAutomotive inverter and motor connected by three cables.

Audi e-tron with the inverter and front motor connected by three busbars.
As the motor is cylindrical in nature, the inverter’s various components have been reorganised to fill the space above the stator with the lowest possible footprint. According to Nissan, integrating the inverter into the motor enabled it to reduce the motor weight and volume of its first-generation Leaf models by 11.7 kg and 5.1 l, respectively, between 2005 (phase 1) and 2011 (phase 2).
The power-to-weight ratio and power density of inverters have continued to increase. This is mainly due to improvements in electronic components, such as MOSFETs and silicon carbide semiconductors, which are more compact, offer better performance, and thus reduce the need for cooling.
Toyota announced that between the first and fourth generations of its Prius, the volume of the power module (consisting of two inverters and a voltage converter) was halved from 17.4 litres to 8.4 litres while the power density was multiplied by a factor of 2.5. The weight was also cut in half, from 24.5 kg to 12 kg.
Two transmission designs

Parallel-axis front drive and coaxial rear drive on the Audi e-tron.
Most electric vehicles use parallel transmissions similar to those on ICE vehicles. The motor output shaft drives one or two parallel shafts to drive the differential, which distributes torque to the two wheel shafts. Since electric vehicles are quieter than conventional ones, their transmission parts must be manufactured with greater precision to avoid generating additional noise. The gear reduction ratio ranges between 7 and 13 depending on the vehicle.

VW e-Golf powertrain with two-stage gearbox.
“A two-stage gearbox gives the best efficiency-space compromise for such a ratio and such an offset between the machine and the differential”, Edouard Valenciennes, R&D Engineer at Renault told us in 2015 when he presented the new-generation motor for the Zoe, which has a ratio of 9.3:1.
Volkswagen also explained that its reason for choosing a two-stage gearbox for its ID.3 was a smaller drive system.

Powertrain with single-stage gearbox (left) on the front axle of the Audi e-tron.
The coaxial arrangement developed by Schaeffler is more compact and innovative. The motor drives a planetary gearbox (reduction ratio of 11 to 18 depending on demand) and then a spur gear differential. The advantage of these two features is that they limit the length of the powertrain in order to have wheel shafts long enough to accommodate suspension travel.

Schaeffler coaxial drive.
One of the differential’s two output shafts directly drives a wheel shaft while the other is coupled to a an intermediate shaft that runs through the motor’s centre and transmits torque to the wheel shaft on the opposite side. This solution, which does not reduce the steering angle, is used on both axles of the Jaguar I-Pace, the rear axle of the Audi e-tron, and the front axles of the Porsche Taycan and VW ID.3. However, supplying cooling oil inside the rotor is more complex to implement.

GKN’s two-speed eTwinsterX module with torque vectoring technology.
Larger but still with two coaxial output shafts, GKN’s eTwinsterX system offers two additional functions: a two-speed transmission and torque vectoring. A torque vectoring system transfers torque independently to the left and right wheels to generate yawing moment and better control understeer and oversteer.
Single or multiple gear ratio transmission?

11.5:1 single-ratio transmission on the Volkswagen ID.3.
The vast majority of powertrains in electric cars are single gear ratio transmissions. This solution is acceptable because electric motors deliver their maximum torque at 0 rpm and have a wide speed range (between 8,000 and 16,000 rpm maximum depending on the applications) whereas the maximum speed is frequently lower than that of ICE vehicles.
One car with one of the highest gear reductions is the Volkswagen ID.3. This is because of the high maximum speed of its motor (16,000 rpm) and its top speed of 160 km/h, or a speed of 10 km/h at 1,000 rpm (gear ratio 11.5:1). In contrast, the Jaguar I-Pace has a speed of 15.3 km/h per 1,000 rpm (200 km/h per 13,000 rpm) and a gear reduction ratio of 9.06:1.
Thanks to its powerful rear motor that runs at a record speed of 19,000 rpm, the Tesla Model 3 Performance can reach its top speed of 261 km/h with a final drive ratio of 9:1.

Two-speed rear transmission on the Porsche Taycan. The planetary gear inside the multi-plate clutch acts as the first gear reducer.
The Porsche Taycan is the only electric car with a two-speed transmission, which is located only on the rear-axle motor. The ratio of the first gear is 16:1 and ratio of the second gear is 8.05:1. The engagement of the dogs is synchronised by a multi-plate clutch. The rear motor revs up to a maximum of 16,000 rpm at 130 km/h in first gear and reaches the same rpm at maximum speed of 260 km/h in second gear.
The novelty of the Exagon Furtive-eGT concept car was that it was powered by a pair of electric motors, each with two speeds but different first gears. The combination of the two powertrains thus offered a three-speed transmission without interrupting the tractive force during shifting.

Acronyms


Chevy Bolt powertrain
Written by: Yvonnick Gazeau
Sources: Audi, BorgWarner, Chevrolet, GKN, GM, Honda, Porsche, Renault, Schaeffler, Tesla, Toyota, Valeo, VW, ZF