Electric vehicle (EV) traction motors are highly efficient, making designing more efficient ones a daunting challenge. Adding to the challenges, EV traction motors have different performance criteria compared to industrial motors.
This article reviews the different load profiles experienced by industrial and EV traction motors. It also looks at some of the specific design challenges that need to be overcome in the next generation of EV traction motors and considers how today’s rare earth permanent magnet synchronous motors (PMSMs), induction motors (IMs), and switched reluctance motors (SRMs) compare. It concludes with a brief discussion of the emerging axial flux technology.
Industrial processes typically involve constant loads or highly defined changes in the loading of the motor. The traction motor in an EV must be responsive to requirements for speed changes, increased torque when climbing slopes, and the abrupt application of the brakes.
The load requirements for an EV traction motor can be divided into three sections based on speed: constant torque, constant power, and reduced power (Figure 1).
EV traction motors must deliver a rapid torque response at low speeds and consistent power at higher speeds. They must also provide high efficiency over the entire speed range — including constant torque and power — supporting an overload capacity of twice the rated torque for short periods.
However, there’s a maximum speed at which an EV traction motor can operate efficiently, which defines the constant power-to-base speed ratio (CPSR). The base speed is the speed at which the motor transitions from constant torque to constant power operation (see Figure 1).
The CPSR of modern traction motors ranges from 2.5 to 4 and is determined by the type of motor, the materials, gear ratio, and other powertrain elements.
Design challenges
Designers of next-generation traction motors have goals for CPSR of 7 to 10. Increases in CPSR will reduce overall powertrain costs while maintaining performance. Improved simulation and modeling of traction motors are needed to support the development of advanced designs that increase efficiency while reducing or eliminating dependence on rare earths.
Another area where industrial motors and EV traction motors differ is the importance of mitigating the noise, vibration, and harshness (NVH) levels in EVs. Industrial applications are unconcerned about NVH. This is another area where advanced simulation and modeling tools are important.
Sources of NVH can be complex and include rotor eccentricity, harmonics from pulse width modulation (PWM) control, and interactions with motor mounts, transmissions, and other drivetrain elements.
Today’s motor choices
Rare earth PMSM, IM, and SRM are common traction motor choices. The rare earth PMSM offers high efficiency, high torque, and the highest cost. It’s used in high-performance EVs that are less cost-sensitive. If rare-earth-free PM motors can be designed with adequate performance, they will become serious competitors to current-rate earth PMSMs.
The IM offers a lower cost and lower efficiency option for EVs. However, the opportunities to improve IM performance are more limited, and developments in rare-earth-free PM motors and SRMs may reduce the competitiveness of this design.
Compared with the other two options presented here, the SRM has a significant material cost advantage. However, it suffers from low torque density and NHV challenges. Research is focused on improving the torque density of SRMs and new control strategies have been proposed for reducing noise and torque ripple. None of these developments changes the cost advantages of SRMs, and if they can be successfully commercialized, SRMs are expected to become an attractive option for low-cost, high-efficiency EV traction motors that deliver moderate torque densities.
Axial flux
PMSMs, IMs, and SRMs are all examples of radial flux motors where the magnetic flux paths are radial from the center to the edge. Axial flux motors, also called axial gap or pancake motors, are under development that align the gap between the rotor and stator, along with the direction of magnetic flux, parallel to the axis of rotation.
Axial flux motors are thinner and flatter than rotary flux designs. They’re also easier to cool (Figure 2). Simplified cooling means that they run at lower temperatures and can produce higher continuous power without overhearing. They offer good efficiencies, but typically have lower torque capacities than radial motors.
Summary
Designing improved EV traction motors has several challenges. A key element to solving many of those challenges is improved simulation software and modeling tools.
Although PMSM motors offer the highest performance and IMs offer the lowest cost, those tradeoffs are expected to change in the future. IM motors may capture more of the market and SRM motors may become practical for EVs. In addition, axial designs are being developed to replace today’s radial motor architectures.
References
- A Multi-Criteria Analysis and Trends of Electric Motors for Electric Vehicles, MDPI World Electric Vehicle Journal
- Analysis and Design of a High-Performance Traction Motor for Heavy-Duty Vehicles, MDPI energies
- Challenges Faced by Electric Vehicle Motors and Their Solutions, IEEE Access
- Traction motor design, Siemens
- Traction motors for electric vehicles: Maximization of mechanical efficiency – A review, Applied Energy
- Why Aren’t All Electric Vehicle Motors Axial Flux (Yet)?, Traxial
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