With the increase of greenhouse gases and, therefore, the elevated average temperature of the earth, the focus has been on sustainable technologies. Transportation, one of the significant contributors of greenhouse gases (i.e., 29%), is shifting its focus towards all-electric vehicles. Although Electric Vehicles (EVs) came into the picture in the late 19th and early 20th century, they were revived in the USA and Europe in the 1990s and early 2000s. In India, annual EV sales reached 4,55,773 in 2022 from just 56,551 in 2017, as shown in Figure 1.
Source: “Annual India EV report card FY2022” by JMK research and analytics
The advent of reliable power electronics and cheap and better batteries made the transition more inevitable. An electric vehicle requires three core systems, apart from many other parts, to work. These are batteries, power-electronic converters, and electric motors, of which the electric motor is the power horse for the vehicle.
Therefore, choosing traction motors for an electric vehicle’s powertrain system is a crucial process that needs careful consideration. The automotive industry is still looking for the best electric-propulsion solution for EVs. But what makes a motor “Best” for traction applications, especially 4W and below?
EV motor requirements
The primary requirements of electric motors for EV propulsion are:
- A high torque at low speeds and high power at high speeds for cruising.
- A wide speed range, including constant-torque and constant-power regions.
- A high efficiency over the wide speed and torque ranges and Controllability.
Selecting a motor is an essential aspect of EV design to satisfy the above requirements. At present, the most popular machines used in EV systems are induction motors and permanent magnet motors, while DC motors have become obsolete due to low power density and failures in the brushes and commutator segments.
Figure 2 shows the typical torque-speed characteristics of a vehicle. A vehicle engine is supposed to provide high torque at low speed and significant torque to meet resistive forces at high speed. Conventional IC engines achieve these using gears (i.e., Low gear offers high torque and vice versa). Output torque in electric motors can be controlled electronically; multiple gearboxes are eliminated and replaced with a single gear.
Therefore, the torque-speed characteristic of the motor becomes a crucial parameter for motor selection. As the speed of the vehicle increases, the vehicle torque becomes equal to the resistive torque acting on the vehicle, and the equilibrium condition is reached. Vehicle speed in this condition is the maximum speed of the vehicle.
On the other hand, the generalized torque-speed characteristic of the electric motor is shown in Figure 3. Electric motors operate in 3 modes: constant torque, constant power, and constant speed. The constant power region is crucial in deciding the motor’s power rating and high-speed performance. Now, without diving deep into the technicality of the vehicle dynamics, let’s focus on the popular motors used in EVs and their limitations with respect to application.
Popular Motor Technologies
- Permanent-Magnet Synchronous Motor (PMSM): The most suitable contender to compete for electric vehicle powertrains is PMSM. These motors offer several benefits, such as i) high power density, ii) better efficiency, usually above 95%, and iii) effective heat dissipation to the environment.
However, because of their relatively limited field-weakening capability – caused by the PM field – these motors have a narrow constant-power region by nature (the fixed PM restricts their extended speed range). PM motors are preferred by major BEV manufacturers due to their high power and torque density.
However, problems are associated with rare-earth magnets used in PM motors, i.e., mining waste and the cost of magnets. A tonne of radioactive residue, one tonne of waste gas (approx. 11000 cubic meters), 75 cubic meters of wastewater, and 13 kg of dust are produced for every tonne of rare earth that is extracted during the mining process. Additionally, PMSMs have shorter constant power regions and reduced efficiency in higher-speed regions.
PMs may be broadly categorized as interior magnet (IPMSM) or surface-mounted (SPMSM), based on how they are arranged in the rotor. However, interior magnet design produces more significant air-gap flux density and is used extensively in battery electric vehicles. Tata Nexon EV, a top-selling BEV in India, uses 106.4 kW IPMSM motors for propulsion.
- Brushless DC Motors (BLDC Motors): The concept for creating BLDC motors was established more than 50 years ago by T. G. Wilson and P.H. Trickey. They conducted several solid-state commutation experiments to operate Direct Current (DC) motors, which paved the way for developing a BLDC motor. BLDC motors are expected to have better torque-to-weight ratios, reduced operational noise, and higher efficiency than other motors, especially induction or conventional DC motors.
Global sales of BLDC motors are projected to grow from an anticipated 9.6 billion USD in 2020 to 15.2 billion USD by 2025. It is anticipated that the Indian BLDC motor market will expand at a Compound Annual Growth Rate (CAGR) above 6.4%, culminating in a valuation of US$ 1.85 billion by 2029.
BLDC is extensively used in e-rikshaws, primarily due to its performance and cost, with the typical range of 800W to 1500W. Most of the BLDC motors are imported from China. However, Indian companies are coming forward with their version of improved BLDC, manufactured in-house. Table 1 shows popular Indian electric 2-wheelers using BLDC motors with their power and voltage ratings.
- Induction Motor (IM): Nikola Tesla and Galileo Ferraris created the first AC commutator-free polyphase induction motors separately. The cage-rotor induction motor was created in 1889 by Mikhail Dolivo-Dobrovolsky.
IMs have well-established production processes, are comparatively inexpensive, and are sturdy. Direct torque control or vector control can both provide good dynamic torque control performance. Compared to synchronous machines, three-phase IMs have a starting torque. Vehicle applications are assured of a strong starting performance. While not as efficient as PMSM, IMs can achieve excellent efficiency. Doing this decreases the drive system losses and raises overall vehicle efficiency. Figure 4 shows a squirrel cage induction motor.
Source: Wikipedia
Conventional IMs usually have a constant power range of two to three times the essential speed. This may be increased to 4-5 times the necessary speed for traction machines, though, which is usually preferred. For traction drives, operating at a high efficiency is crucial. The ideal flux level for best efficiency is inversely correlated with speed and directly changes with torque.
Many electric vehicles have used induction motors, such as the Tesla Model 3 and Model S, BMW i3, Mercedes-Benz B-Class-Electric, Toyota RAV-4, and Nissan Leaf.
Emerging Motor Technologies
- Switched Reluctance Motor (SRM): Back in 1842, the first use of SRM was for powering a locomotive. In 1969, Nasar proposed a variable reluctance motor for variable speed applications, which reignited interest in SRMs. This revival of reluctance motors was due to the advent of high-power semiconductor devices, cheap microcomputers, and energy saving achieved using variable-speed drives.
The benefits of switched reluctance are its innate simplicity of construction, durability, and high-speed operations. SRMs are considered strong candidates for e-bikes and 4W-EV powertrains, but apart from prototypes, there isn’t a single electric passenger car with an SRM drive on the market now.
Considering a traction characteristic, SRM performs better at high speeds and, therefore, has a higher CPSR ratio. CPSR ratios are the most crucial factor determining a motor’s power rating for a given acceleration. A CPSR is the ratio of critical speed to base speed (see Figure 3). Figure 5 shows a comparison of SRM with induction motor and PMSM. In high-speed operations, the SRM outperforms PMSM and has an edge over the induction motor in terms of efficiency (The shaded portion indicated an efficiency of over 85%).
A Canadian-based startup, Enedym Inc., is betting heavily on switched reluctance motors and has raised $15 million for its magnet-free motor. The startup is led by Prof. Ali Emadi of McMaster University, situated in Ontario, Canada. Figure 6 below shows a model of Enedym SRM from the India Auto Expo 2023. They have partnered with Sona Comstar and Napino Auto and Electronics Ltd in India.
Source: Autoexpo India 2023
- Axial Flux Motor: In applications where high torque densities and a short axial length compared to the outer diameter are required, axial flux machines present a strong substitute for traditional radial flux machines. They became popular for applications requiring high power density and compact size. Every axial flux PM motor structure uses less volume than the radial flux PM motor machine for every power rating, and the difference gets larger as power increases.
Also, the moment of inertia of the rotor is low, thus making it suitable for applications requiring quick acceleration and deceleration, i.e., racing cars and aircraft. Aspect ratio (axial length/diameter) plays an important role in defining the torque density of the motor. It is observed that axial flux motors are preferred for high torque density applications where a low aspect ratio (<0.3) is demanded.
Axial flux motors are suitable for automotive applications, especially for in-wheel or near-wheel direct-drive EVs. Axial flux motors’ compact and flat design makes cooling more effective. Good thermal management contributes to maintaining ideal operating temperatures, increasing the motor’s lifetime and dependability.
However, axial flux motor technology is not mature enough, especially in manufacturing and materials. Axial flux machine manufacturing involves more complex engineering and careful considerations to attain cost parity, unlike radial flux motor manufacturing, which may be created using a basic lamination stacking approach. Another challenge is maintaining a uniform air gap between the stator and rotor due to higher magnetic forces.
Figure 7 shows an axial flux motor (P400) by YASA, a UK-based startup. This drive, being compact, can produce a staggering torque of 200 Nm. Its rated power is 60kW with a maximum speed of 8000 rpm. Automobile giant Mercedes acquired YASA in 2021.
Source: YASA website
- Flux-Switching Permanent Magnet Motors: Flux-switching motors (FSMs) were developed in the 19th century, but their significance grew after 2000. Flux-Switching Permanent Magnet (FSPM) machines have drawn increasing attention. This is because these kinds of machines have several benefits. These benefits include a passive and, therefore, sturdy rotor structure appropriate for high-speed applications, high torque density because of the flux-focusing effects, and advantageous thermal management because of the placement of PMs on the stator. FSM is more efficient, uses less PM, has a higher torque density, and can weaken flux more than other PM machines. Also, the Permanent Magnet (PM) is used in the stators. Thus, the complexity of manufacturing a rotor with PM is avoided.
Figure 8 shows the structure of a three-phase flux-switching permanent magnet motor (FSPMM), where A1 and A2 represent phase `A.’ Similarly, B1, B2, C1, and C2 represent Phases B and C, respectively. Using stator coils and stator magnets, a flux path is created. Torque is produced using a combination of reluctance and PM torque.
FSM suffers from some disadvantages: (i) Low reluctance torque. (ii) Low overloading capacity compared to IPMSM. (iii) lack of technological maturity. Despite these demerits, FSPMM is still considered a potential motor for electric vehicle applications.
Conclusion
This article has highlighted popular EV motors and their characteristics. Also, potential motors have been discussed, as well as their pros and cons. The global EV motor market is valued at $25 Bn in 2022 and is projected to reach $99 Bn by 2032 at a CAGR of 15.9%.
With India in the middle of the EV wave, the main challenges lie in reducing the import of rare-earth magnets in PM motors. Research should focus on looking for materials that can be sourced in India and reduce the use of permanent magnets if they are not eliminated.
A switched reluctance motor can potentially replace PMSM, completely free from magnets. Axial flux and flux switching motors use magnets, but researchers are trying to replace the rare-earth permanent magnet with ferrite magnets. Other motors that hold merit are PM-assisted SynRM and wound field synchronous motors.
Shubham Dabral, MIE(I), MTech (IIT-BHU) is a Lecturer (Sr.) in the Department of Technical Education, Govt. of M.P. and Research scholar at IIT-Delhi.
